• Percy LehmannEmail author
Living reference work entry


Photobiology investigates the complex effects of solar radiation on biological systems. The skin plays a very special role as a barrier organ to our environment. A high percentage of dermatoses are developed directly or indirectly in relation to solar radiation. This has led to the development of the field of photodermatology, which has, in recent years, gained significant importance as a field of basic science, as well as a clinical specialty involving photodiagnostics (light diagnostics) and phototherapy (light therapy). Acute and chronic skin reactions related to solar radiation range from sunburn, phototoxicity, and photoallergic diseases, to benign and malignant chronic light damage of the skin (elastosis, basal cell carcinomas, spinocellular carcinomas, and sometimes even melanomas).

1 Introduction

Photobiology investigates the effects of solar radiation on biological systems. The skin plays a very special role as a barrier organ to the environment. There is a high percentage of dermatoses that develop directly or indirectly in relation to solar radiation. This has led to the development of the field of photodermatology, which has gained significant importance, in recent years, as a field of basic science, as well as a clinical specialty involving photodiagnostics (light diagnostics) and phototherapy (light therapy). Acute and chronic skin reactions associated with solar radiation range from sunburn, phototoxic, and photoallergic diseases, to benign and malignant chronic light damage to the skin (elastosis, basal cell carcinomas, spinocellular carcinomas, and sometimes melanomas).

2 Physical Basics

Light is a part of the electromagnetic spectrum (Table 1). The areas of this spectrum relevant for photobiological reactions in dermatology involve optical radiation, and include ultraviolet rays, visible light, and infrared (IR) radiation. In the solar spectrum, UV rays make up about 10%, visible light about 50%, and infrared radiation about 40%. However, these figures depend on latitude, seasonal position of the sun, time of day, and cloudiness.
Table 1

Electromagnetic spectrum

Radiation type

Wavelength [nm]

Gamma rays




Ultraviolet radiation:












Visible light


Infrared radiation


Radio waves


While the effects of UV radiation on the skin have been well documented, the biological effects of visible light have been studied to a lesser extent. In recent years, it has been possible to demonstrate the mostly damaging effects of IR radiation on the skin, which are very similar to the effects of UV radiation, but have other mechanisms.

2.1 Invisible UV Radiation

Invisible UV radiation is divided into three areas. The subdivision is based on biological-physical laws, such as the ability to form erythema or melanin pigment, and on conventional agreement.

2.1.1 UVC

UVC does not occur on the earth’s surface, because this short-wave UV radiation emitted by the sun is absorbed by the atmosphere, in particular by the ozone layer. However, it is emitted by some artificial lamps, such as xenon lamps and mercury vapor lamps. UVC can be blocked by filters. Because it has a lethal effect on unicellular organisms, it is used for some technical purposes, such as bacterial disinfection of the air. UVC triggers an erythema, which becomes visible on the skin after about 6 h. A UVC-related skin tan is weak. In contrast, UVC rays irritate in particular the conjunctiva, so that protective glasses should be worn when handling such radiation. Window glass blocks these rays.

2.1.2 UVB

UVB occurs in natural sunlight and reaches the earth’s surface. It is also part of certain artificial light sources used for diagnostic and treatment purposes. Mercury vapor lamps have strong emission lines at 297, 303, and 313 nm. UVB irritates the conjunctiva, but slightly less so than UVC. However, protective glasses should be worn when handling UVB-emitting artificial lamps, as well as during exposure to strong sunlight. UVB is filtered through window glass. Sunburn caused by UVB behind window glass is therefore not possible. However, it can penetrate quartz glass and water; sunburn while swimming is thus possible.

Biological effects of UVB are erythema (sunburn), which occurs 12–24 h after exposure and is mediated by prostaglandin synthesis of keratinocytes, and pigmentation (tan), which is added 48–72 h after irradiation, as late pigmentation.

UVB photoisomerizes 7-dehydrocholesterol to the biologically effective precursors of vitamin D3.

The negative effects include acute and chronic damage to the skin. The acute or chronic effects induced by UVB on the cells of the epidermis, connective tissue, and blood vessels are DNA, RNA, protein, and cell membrane changes. Particularly mutagenic effects on DNA lead to carcinogenesis.

Histologically, UVB rays from more than a minimal erythema dose (MED) lead to a characteristic phototoxic change in the epidermal keratinocytes, and depending on the dose, intra- and intercellular edema, as well as dyskeratotic cells (sunburn cells). The synthesis of tumor necrosis factor-α (TNF-α) by keratinocytes probably plays a role here. The vessels in the upper corium are dilated, and there is a slight perivascular inflammatory infiltrate.

2.1.3 UVA

UVA occurs in natural sunlight, reaches the earth’s surface, and is less toxic than UVC and UVB. In higher doses, it causes an immediate erythema. It triggers instant pigmentation and induces the synthesis of melanin, causing late pigmentation. The UVA erythema is biologically different from sunburn, as no damage is caused to the keratinocytes (sunburn cells). The intensity of UVA in solar radiation is about 500–1,000 times higher than that of UVB, so that UVA erythema and pigmentation are also caused by sun exposure under natural conditions. UVA radiation can be further divided into UVA1 (340–400 nm) and UVA2 (320–340 nm). UVA2 radiation can produce a small amount of UVB-like effects. These include mutagenic effects on DNA and increased erythema efficacy. UVA is also a component of numerous diagnostic and therapeutic irradiation devices. Mercury vapor lamps have a strong emission line at 365 nm, and fluorescent lamps, as used in irradiation equipment for phototherapy and photochemotherapy, can have their main emission spectrum in the UVA range.

In small doses, UVA does not irritate the conjunctiva, but does so in combination with photosensitizing drugs. UVA penetrates window glass, so UVA-induced photodermatosis can also be triggered behind building or car windows.

In contrast to UVB rays, histologically UVA in cans of up to 100 J/cm2 does not lead to phototoxic changes in the epidermis.

However, the vessels of the upper plexus in the dermis are dilated, and a sparse lymphohistiocytic infiltrate is formed, which may also contain eosinophilic and neutrophilic granulocytes.

2.2 Visible Light and Infrared Radiation

The biological effects of visible light on the skin are weak. Skin changes may be triggered by visible light only in patients with extreme photosensitization in the context of light urticaria or chronic actinic dermatitis (eczema).

About half of the radiation energy that reaches the skin through solar radiation is caused by IR radiation. It has long been known that IR in high doses leads to heat damage of the skin. In recent years, it has become increasingly clear that IRA radiation (IRA: 760–1400 nm), which is responsible for about one-third of solar energy, can have damaging effects on the skin, similar to UV. Today, the spectral range of IR radiation is divided into three wavelength ranges:
  • IRA: 760–1,400 nm

  • IRB: 1,400–3,000 nm

  • IRC: 3,000 nm to 1 mm.

IRB and IRC, which penetrate into the upper dermis, are responsible for classic heat induction. IRA, on the other hand, penetrates the entire dermis right through to the upper subcutis. IRA represents a relevant noxious agent for the skin. It leads to a change in gene expression as a significant contributor to photo ageing by disturbing the balance of collagen synthesis and collagen degradation. Collagen degradation is induced by increased matrix metalloprotease expression (MMP-1) after IRA irradiation.

In animal experiments, the damage of the connective tissue by UVB could be intensified by additional IR radiation. Clinical observations are also known to indicate a possible co-carcinogenic effect of IR. For example, frequent sitting in front of an open fire seems to promote the development of spinocellular carcinomas on the skin of the lower legs. Spinocellular carcinomas that develop at the contact points with warming bags placed on the skin have also been described (such as used in various tribes in Asia).

2.3 Dosimetry

Similar to X-ray therapy, defined units exist for UV rays:
  • Watt [W]: power or intensity of the radiator

  • Wattsecond [Ws] = joule [J]

  • Joule [J]: energy quantity

The power of a radiator is thus given in watts [W] or milliwatts [mW]; the intensity is expressed as power per unit area, as W/m2 [mW/cm2].

The unit of the dose, i.e., the radiation energy [J], is also given per unit area (skin surface).

$$ \frac{W\times s}{{\mathrm{cm}}^2}=\frac{J}{{\mathrm{cm}}^2} $$

Thus, for light tests or light therapy, the dose unit is J/cm2. The indication of the irradiation time alone, on the other hand, is not sufficient as a dose indication.

2.3.1 Dosimeters

UVA and UVB measuring instruments are available as hand-held instruments. They are mostly adapted to specific irradiation devices and relatively inaccurate for absolute measurements at different radiation sources. A much more accurate measuring instrument in the UV range, with measuring devices for UVA and UVB, is the centra (osram) instrument. Another more accurate measuring instrument is the bolometer (thermopile), which, in combination with a display device, enables information to be given in W or J. Distance law and time factors must be taken into account.

3 Skin Types (Fitzpatrick 1975)

Regardless of the amount of energy irradiated by natural sunlight or artificial radiation sources, pathobiological reactions such as erythema, sunburn, pigmentation, light callosity, phototoxic reactions, and also light-related long-term effects such as actinic keratoses, basaliomas, carcinomas, and melanomas depend on the respective skin type. The classification into six skin types is based on clinical observation (Table 2).
Table 2

Classification of skin types and their reactions to the first 30 min of sun exposure in summer. (From Fitzpatrick 1975)

















Dark ethnic groups, Mediterranean residents, Mexicans, Indians, etc.





aAfter extreme UV exposure, sunburn and additional pigmentation are also possible for these skin types

People with skin types I and II often have light skin, blue eyes, and blond or red-blond hair, and freckles, but some also have dark brown hair, and brown or green eyes. These people are particularly at risk with regard to the development of chronic light damage to the skin.

4 Pigmentation

Stimulation of the melanocytes leads to pigmentation (tanning: suntan), which is divided into two types:
  • Immediate pigmentation

  • Late pigmentation.

4.1 Immediate Pigmentation (Meirowsky 1909; Haußer 1938)


Direct pigmentation; Immediate pigment darkening (IPD)

Clinical Features

Immediately after or during irradiation in the range of 300–450 nm, an ash-gray or brownish pigmentation occurs (Meirowsky phenomenon). The color of this pigment differs significantly from the copper and coffee-brown pigmentation caused by UVB (sun tan). The maximum effective wavelength for immediate pigmentation is 340 nm. Usually, 10–30 J/cm2 (330–460 nm) is required to trigger this phenomenon. The threshold dose is lower when more pigmentation is already present in the skin. Immediate pigmentation occurs after extensive sunbathing during the UVA-rich afternoon hours, as well as after the application of larger doses of UVA as part of phototherapy, or after the use of a sunbed for cosmetic tanning of the skin. The immediate pigmentation fades after some hours. High UVA doses applied once, or smaller UVA doses applied repeatedly, also lead to delayed-type pigmentation. Immediate pigmentation is based on photo-oxidation of non-colored melanin precursors.

4.2 Late Pigmentation


Indirect pigmentation; Delayed pigmentation; Sun tan

Clinical Features

It occurs about 24–72 h after UV exposure to artificial light sources or natural sunlight. Wavelengths around 297 nm have the strongest pigmentation capacity. However, the action spectrum for melanogenesis ranges from 250 to 400 nm. In the UVA range, the pigmentation effect is about 100 to 1,000 times lower than at 297 nm. However, this is largely compensated for by the much higher proportion of UVA in sunlight, so that under natural conditions, a melanin synthesis is also induced by UVA. Depending on the degree of pigmentation, the sun tan remains for days to weeks. The quantitative extent of melanin formation depends on genetic and hormonal factors. The melanin-producing cells (melanocytes) are located in the basal cell region of the epidermis. Approximately every fifth to eighth cell in the basal layer is a melanocyte. Under the influence of pigment-inducing UV radiation, melanosome complexes are increasingly formed and dispersed within the melanocytes, into the perikaryon region, and also into the distal sections of the melanocytes.

The number of melanocytes in humans shows topographical differences, but no ethnic differences. Skin pigmentation depends on the number and activity of epidermal melanosomes. In an epidermal basal cell, there are about 400 melanosomes in a person with skin types V–VI, whereas in a light-skinned Central European, there are only about 100. In Europeans, mongoloid ethnic groups, and American Indians, the melanosomes are about 0.6 μm to 0.3 mm in size (see chapter “Disorders of Melanin Pigmentation”).

The facultative pigmentation depends on the ability of the person concerned to pigment beyond the constitutional pigment content of sun or artificial light. In some animals, pigmentation is largely subject to hormonal influences.

UVA- and UVB-induced pigmentation of the skin differs with regard to the pigment distribution within the epidermis. In UVB-induced melanogenesis, more melanosomes are deposited in the keratinocytes at all levels of the epidermis and discharged via the horny layer, so that melanin is available as a protective pigment over the entire thickness layer of the epidermis, including the horny layer. After UVA irradiation, the newly formed melanin is mainly restricted to the basal layers of the epidermis. The protective effect against toxic UVB radiation is therefore considerably lower, since the keratinocytes remain unprotected in the higher epidermal layers.

4.3 “Lichtschwiele” (Miescher 1930)

The German term “Lichtschwiele” (literally, light callus) describes the ability of UV-induced thickened stratum corneum to block the effects of light. Exposure to the sun leads to the development of protective mechanisms that shield the skin from further radiation. The basis of this protection is the ability to stimulate the epidermis to produce melanin and acanthosis, as well as hyperkeratosis after UV radiation (Lichtschwiele, light callosity). The effective barrier is the stratum corneum. Repeated erythema-producing UVB irradiations can produce a maximum-size light callus within 2–3 weeks, which can then no longer be increased.

UVA radiation is not capable of exerting such an effect. Although it produces a cosmetically appealing tan, the protective effect is comparatively low due to the lack of light callosity and the pigment distribution being limited to the basal layers of the epidermis.

The Lichtschwiele remains for weeks, and dissipates again in the reduced-sun seasons. In addition to protection against erythema-generating UV radiation, the induction of Lichtschwiele also provides protection against non-specific environmental noxious agents (irritants). This may also be a basis for the beneficial effect of phototherapy in dermatoses that have a lowered threshold to environmental irritation, such as atopic eczema.

5 Photodiagnostics

5.1 Radiation Sources

A whole range of devices are used for diagnostic and treatment purposes in dermatology. Based on new findings on the action spectrum of phototherapy, devices with specifically effective emission spectra have been developed (selective phototherapy). An example of this is phototherapy for psoriasis, in which the most effective radiation is in the long-wave UVB range (311–313 nm).

The application of high UVA doses with conventional fluorescent lamps is difficult. To apply about 20–40 J/cm2, one needs about 20–60 min of irradiation time. High-pressure metal halide lamps have ensured progress, allowing high doses of UVA to be applied without UVB and C contamination, so that no unwanted UVB erythema is induced.

Monochromators such as prism or grating monochromators optionally deliver very narrow spectra, depending on the type of light source. The burner in a monochromator can be a high-pressure mercury or xenon lamp. Monochromators are suitable for the determination of action spectra, for example photosensitizing drugs or light urticaria. Disadvantages are the long irradiation time and the small irradiation field. Grid monochromators (Bausch and Lomb, USA; Dermolum HI; Müller, Moosinning) have proven effective for diagnostic purposes.

5.2 Photodiagnostic Test Procedures

5.2.1 Lichttreppe (Wucherpfennig 1931)

Lichttreppe (literally, steps of light) is another German term that has been widely adopted in photobiology. It has proved practical for indicating the dose that triggers a clearly visible and distinguishable erythema in the UVB range, 24 h after irradiation. This is the minimal erythema dose (MED), and is the lowest dose of UVB that produces a uniform redness with sharp limitation on the skin. The MED is determined by a series of graded radiation doses (Lichttreppe). Factors 1.25 or 1.4 are common for the ratio of successive doses of a Lichttreppe. The reading takes place after 12–24 h. The MED depends on the patient’s skin type and body region. The test is carried out on skin that is not exposed to light or tanned, for example on the buttocks. The amount of energy triggered by 1 MED also depends on the wavelength. A MED in the range of UVB (300 ± 5 nm) is triggered, on average, by 0.038–0.053 J/cm2; for a MED in the UVC range (250 ± 5 nm), 0.02 J/cm is required, and in the UVA range about 1,000 times more, between 20 and 50 J/cm2. If no further details are available, the MED refers to the UVB range. On a sunny, cloudless summer day, approximately 20-fold MED can reach the skin.

The minimum phototoxicity dose (MPD) is the lowest dose of UVA which, in combination with a light sensitizing substance (8-methoxypsoralen), produces a straight, visible, uniform redness, that is sharply demarcated. The subsequent pigmentation in the irradiation fields can also be used to read this MPD. The determination of the MPD is similar to MED, with series of graded radiation doses and readings after 48–72 h, because only then does the phototoxic erythema reach its peak. MPD is used during the initiation of photochemotherapy, for example, in PUVA treatment (Psoralen+ UVA-radiation), and provides an indication of the initial UVA dose. It is also tested on non-exposed and non-tanned skin (buttocks). The MPD is expressed in J/cm2, and is usually in the range of 0.2–2 J/cm2 for PUVA treatment of skin types I-III.

UVA Lichttreppe
UVA Lichttreppe can, for example, be carried out with a metal halide source (Dr. Hönle high-pressure emitter, Sellas Sellamed, Mutzhas UVASUN, et al.), which emits in the spectrum of 340–460 nm, without a measurable UVB component. The UVA Lichttreppe can determine two further parameters, in addition to MED-UVA:
  • Immediate pigment darkening (IPD)

  • Late pigmentation (minimal tanning dose, MPD).

The parameters are read immediately (IPD) and again 24 h after irradiation (MTD, MED-UVA).
Diagnostic Significance of UVB and UVA Lichttreppen

The MED-UVB is a measure of a patient’s sensitivity to erythema-inducing radiation from the UVB range. However, this value does not always correspond to the expected sunburn sensitivity of a patient according to the anamnestic investigations of the skin type, since the MED-UVB correlates only moderately with the individual skin types. The pigmentation ability of the skin can be assessed effectively by IPD or MTD. Depending on the skin type, MED-UVA is also subject to strong fluctuations.

Standard values for the Lichttreppen cannot be given here, since the threshold doses depend very strongly on the radiation sources used, the dosimetry applied, the test location, and the skin type. Before corresponding photoprovocations, pathological reactions in the Lichttreppen can already give initial indications of a particular photodermatosis. Patients with chronic actinic dermatitis show reduced MED-UVB and sometimes even reduced MED-UVA. Eczematous skin reactions develop in the test areas within a few days.

In patients with solar urticaria, hives can, within minutes, form in the irradiation fields of the UVA and UVB Lichttreppen if the action spectrum of the disease is in the UVB and UVA range. The significance of the MED-UVB and -UVA for the diagnosis of photodermatoses is not strong, especially since the threshold values for the UVB erythema and the UVA pigmentation are within the normal range for the vast majority of photodermatoses.

5.2.2 Photoprovocation Tests

In contrast to the Lichttreppe, photoprovocation tests are of great importance in the diagnosis of photodermatoses. Test protocols have been developed for most photodermatoses in order to reproduce the dermatosis in defined skin test areas by single provocation or multiple tests.

Some dermatoses are caused solely by radiation, for example solar urticaria or polymorphic light dermatosis. In addition, there are dermatoses that are only triggered by the combination of photosensitizer and radiation, such as photoallergic or phototoxic dermatitis. Accordingly, the former are tested with UV radiation alone, while the latter are tested with both sensitizer and UV radiation. The aim of the tests is the experimental triggering of pathognomonic skin changes.

Minimum Test Dose

The minimum dose required to induce the clinical picture in the test area under daily living conditions or in the laboratory is expressed in J/cm2 according to the wavelength, if possible. The triggering dose varies considerably, from <0.1 J/cm2 for urticaria solaris, and up to >40 J/cm2 UVA for polymorphic light dermatosis.

Photo Patch Test (Exposed Epicutaneous Test)

The photo allergens in question are applied to the skin of the back in duplicate, as is done in a normal epicutaneous test, under standardized conditions (finn chambers). The most common photo allergens are summarized in photo patch test blocks. Recommendations have been made by international working groups. After 24 h, a patch test series is opened and irradiated with 5 or 10 J/cm2 UVA. Test response readings are taken immediately before and after irradiation, and on consecutive days, up to 72 h after irradiation. The control patch test series remains closed for 24 h or 48 h, and is evaluated immediately after closure, and then daily, until 72 h after application. In particular, the control site must remain protected from light throughout testing.

6 Light Provoked Skin Reactions

These include photobiological reactions on normal skin and photodermatoses, which require a pathological reaction of the skin upon irradiation, as well as light-induced intensification of other skin diseases in the sense of aggravation. Among photodermatoses, primary photodermatoses, in which electromagnetic radiation is the most important pathogenetic factor, can be separated from secondary photodermatoses, which include genodermatoses or metabolic diseases with increased sensitivity to light. The waveband that leads to a specific reaction is called the action spectrum. It is important to determine the action spectrum in order to be able to carry out appropriate treatment or prophylactic measures.

Sunburn can be avoided by using UVB absorbing and reflecting sunscreens. Phototoxic and photoallergic reactions, as well as a predominant number of primary photodermatoses, are caused by long-wave UVA, so that broadband sunscreens with absorption in the UVA range must be applied. The erythropoietic protoporphyria shows an action spectrum in visible light (400–410 nm) referred to as the Soret band (Soret 1883); in this case, light protection through use of opaque make-up is required. This also applies to solar urticaria.

6.1 Light-Provoked Reactions on Normal Skin

These essentially include adaptive processes for light adaptation of the skin, and acute as well as chronic toxic influences on the skin organ. They represent a major medical problem, because, in addition to sunburn and sun-induced aging of the skin, they also include sun-induced cancer.

The provoked reactions on normal skin are summarized in Table 3. With regard to chronic light damage such as skin aging (see chapter “Introduction to the Skin and Dermatology”), precanceroses, and malignancies (see Chapter “Malignant Epithelial Tumors”), reference is made to the corresponding chapters.
Table 3

Sun-provoked reactions on normal skin


Skin reactions



Immediate pigmentation

Delayed pigmentation





Extrinsic ageing

Solar elastosis

Farmer’s skin

Seaman’s skin

Favre-Racouchot disease


Basal cell carcinoma

Actinic keratosis

Squamous cell carcinoma

Lentigo maligna melanoma

Radiodermatitis and X-ray dermatitis induced by ionizing rays, such as X-rays and cobalt rays, are described in the chapter “Disorders Caused by Ionizing Radiation”.

In the following, the immunosuppressive effects of UV radiation and sunburn as the most common UV-induced dermatoses are discussed briefly.

6.1.1 Acute Light-Provoked Reactions on Normal Skin

UV-Induced Immunosuppression

Photoimmunology has developed into a biomedical specialty. Studies on the antigenicity of UV-induced tumors have been influential. Experiments on syngeneic mice showed that pre-irradiation with UVB leads to a specific immune tolerance against UV-induced skin tumors, so that these tumors can continue to grow in pre-irradiated host animals after transplantation, while they are rejected by unirradiated animals. It is also not possible to generate contact sensitization by allergen exposure on pre-irradiated skin areas. Instead, there is a tolerance for the antigen. The specific immune tolerance against tumor-associated antigens and contact allergens is mediated by T-suppressor cells.

By pre-irradiating the animals, a systemic immune tolerance against tumor antigens and chemical allergens can also be induced. Furthermore, the cell-mediated immune response can be inhibited against organ transplants as well as certain microorganisms. In addition to UVB, UVA also acts as an immunomodulator in combination with photosensitizers (psoralen), for example as systemic photochemotherapy. The local immunosuppressive effect is caused, inter alia, by modifying the function of the Langerhans cells in the epidermis. The Langerhans cells show a loss of their antigen-presenting function after UVB exposure and after PUVA treatment. This effect is reversible within 3–4 weeks.

Dermatitis Solaris




Sunburn is caused by the strongest erythema-generating wavelengths of UVB radiation. It is a frequent reaction of the skin and depends directly on the genetic skin type. The degree of expression is influenced by adaptive mechanisms and environmental influences (time of day, season, weather conditions, duration of light exposure). The erythema is mediated by prostaglandins.


Sunburn is caused by too intensive irradiation with sunlight or a UVB-containing light source in people who have a normal light sensitivity of the skin (skin types I–VI). Solar radiation is particularly rich in UV rays at sea and in the high mountains. UV-absorbing dust and haze particles are missing. With increasing height, the radiated layer thickness of the atmosphere decreases. In addition, the erythema-producing UV spectrum is reflected by snow, water, and sand. A sunburn usually consists of multiple MED. With a cloudless sky, in midsummer, at around midday, a MED is roughly reached in about 20 min, so that a day at the beach can produce more than a 20-fold MED of exposure and sunburn. The wavelength of the strongest erythema-producing rays is between 295 and 315 nm. The main cause of erythema is vasodilation in the papillary dermis. The UV-damaged keratinocytes are apoptotic cells. The induction of apoptosis by UV radiation is a crucial etiopathogenic factor for the development of clinical symptoms such as sunburn, but probably also other UV-related dermatoses.

Clinical Features

Sunburn occurs acutely after 4–6 h, reaches its peak after 12–24 h, and subsides again after 72 h. The maximum UV erythema after irradiation with artificial UV sources is usually seen after 24 h. A toxic contact dermatitis with metachronous polymorphism occurs, limited to the site of irradiation (Fig. 1). Initially, an intense reddening of the skin with edematous swelling and a feeling of heat develops, and this is followed by blisters. A weeping and crusty stage follows, which turns into scaling, and finally into healing. With less sunburn, the inflammatory redness is followed only by scaling. This is followed by pigmentation of the skin. In very strong sunlight combined with heat accumulation, general discomfort, with fever, nausea, vomiting, headache, and circulatory collapse can result. Immediate pigmentation plays no role in acute sunburn. The late pigmentation, the sun tan, is caused by the formation of new melanin.
Fig. 1

Dermatitis solaris

Differential Diagnosis

Sunburn may occasionally be superimposed by a phototoxic drug reaction. Tetracyclines and psoralens (8-MOP, 5-MOP, or trimethylpsoralen) can lead to massive phototoxic reactions after local or oral administration, including subungual hemorrhages and phototoxic onycholysis on fingers and toes.


Depending on the given dose, damage to epithelial keratinocytes occurs 12–72 h after exposure to UV radiation. Eosinophilic dyskeratotic cells with pyknotically shrunken nuclei and a pale, empty-looking cytoplasm are found in the upper and middle stratum spinosum, and on a lesser scale in the lower stratum spinosum. The focal cell necrosis can change into extensive epithelial necrosis and lead to blisters when there is exposure to intensive UV light. The blood vessels in the upper corium are dilated. There is a slight perivascular lymphohistiocytic infiltrate.



The treatment corresponds to that for toxic contact dermatitis. In the case of initial sunburn, the external application of powder or glucocorticoids in the form of creams, foam, or milky preparations, as well as moist compresses, has proved successful. Application of lotio zinci relieves itching and produces cooling.


Only severe sunburn reactions require systemic treatment with glucocorticoids or nonsteroidal anti-inflammatory drugs (NSAIDs).

Keratoconjunctivitis Photoelectrica

Very strong sunlight can lead to snow blindness (keratoconjunctivitis photogenica). UVB, in particular short-wave UVB rays, and the UVC emitted only by artificial lamps (room germination lamps, electric welding equipment) have a particularly irritating effect on the subjunctives.

6.1.2 Chronic Light-Provoked Reactions on Normal Skin

Besides the development of precanceroses and carcinomas as well as the lentigo maligna melanoma and perhaps also other forms of melanoma, light-induced skin ageing (photoaging) results from chronic light damage. Both UVA and UVB radiation are responsible for photoaging of the dermis. Although this extrinsic aging is superimposed by the intrinsic chronological aging processes, these processes can be distinguished from each other. Light damage develops only in chronically light-exposed skin areas, especially in light-skinned, light-sensitive people. Such changes are rarely found in dark-skinned people.

The target cells of chronic light damage are melanocytes, keratinocytes, and fibroblasts. Damage to the melanocytes leads to patchy hyperpigmentation and depigmentation, and probably also to the development of lentigines seniles.

Elastosis Actinica


Elastosis senilis; Elastosis solaris; Solar or senile elastosis; Basophilic collagen degeneration


The disease is relatively common, but seldom occurs in the pigment-protected skin of people with dark pigmentation. The fact that actinic elastosis occurs exclusively in chronically sun-exposed skin areas also reflects the importance of chronic sun exposure in pathogenesis. The skin of light-skinned people (skin types I and II) is particularly at risk.


Chronic exposure to the sun is always a factor. It is assumed that both UVA and UVB, as well as infrared, are responsible for the development of actinic elastosis. UV radiation can directly release reactive oxygen radicals in the skin and weaken antioxidant protective mechanisms. Furthermore, after UV radiation, inflammatory cells migrate into the connective tissue of the skin. By releasing reactive oxygen radicals, these can contribute to damage to the connective tissue of the skin and other organs. Direct damage to structural proteins is distinguished from disturbances of the connective tissue metabolism. The superoxide anion and the hydroxyl radical destroy the collagen molecule with different degradation products. UVA and UVB radiation can directly release singlet oxygen, which, in turn, leads to an increased synthesis of collagen-degrading metalloproteases.

Clinical Features

Preferably on the temples, forehead, and neck, and more rarely on the cheeks, net-like, finely striped or more diffuse ivory-colored, occasionally slightly prominent inclusions appear. Perioral and periorbital skin wrinkles are often associated with elastosis.


Under atrophic epidermis there is a narrow strip of elastically free connective tissue due to the decline of the subepidermal elastic plexus. This leads to the accumulation of coarse, partly clod-like, homogeneous connective tissue fibers, which stain like elastic fibers and exhibit a strong basophilia in the hematoxylin-eosin preparation; hence the earlier term basophilic degeneration of the connective tissue. Biochemically and histochemically, the fibrous or clumpy material behaves like elastic fibers. Electron microscopic examinations indicate that it can develop either de novo or from collagen.


The disease is chronic. Further exposure to the sun is expected to lead to an increase in changes. Increased sensitivity to light, as is the case of porphyria, also increases the tendency to actinic elastosis. In pronounced cases and with younger patients, appropriate examinations (involving porphyrins) should be carried out. There is no increased risk of skin cancer.


Prophylactic sun protection, also in the UVA range, is used. Low concentrations of topical retinoids are recommended. Laser skin resurfacing with ablative laser systems is an established method of reducing these signs of skin aging and for improvement of esthetic appearance.

Cutis Rhomboidalis Nuchae (Jadassohn 1925)

This is a very typical change occurring in outdoor workers, such as farmers, sailors, and sportsmen, and is therefore also called “farmer’s neck” or “sailor’s neck.”. It can also be observed in patients with increased sensitivity to light, especially in patients with porphyria, such as porphyria cutanea tarda. In women, cutis rhomboidalis nuchae is very rare, because the neck is usually protected from exposure by the hair.

The skin on the neck, and less so in the lateral dorsal parts of the neck, as well as occasionally in the V-neck of the shirt, is thickened, yellowish in color, and characterized by deep furrows, in a rhomboid pattern (Fig. 2). Occasionally, comedones, sometimes with multiple small hairs, can also be found (trichostasis spinulosa).
Fig. 2

Cutis rhomboidalis nuchae

Elastoma Diffusum (Dubreuilh 1913)

Patients may present with widespread but usually sharply bordered, yellow, thickened plaques on the forehead, cheeks, or lateral portion of the neck. Frequently, comedones may be seen, overlapping with Favre-Racouchot disease. Sometimes there is a single plaque or nodule. Microscopically, there is a massive elastosis, without any tumor cells.

Nodular Elastosis with Cysts and Comedones (Favre and Racouchot 1951)


Favre-Racouchot’s disease (Nodular elastosis with cysts and comedones)

Clinical Features

The preferred sites for this conspicuous variant of senile-actinic elastosis are malar ridges, lateral aspects of the cheeks, the inferior periorbital region, the temples, and, less often, the nose and ears (Fig. 3). Older men are the most likely to be affected. In addition to the elastosic lesions described above, numerous comedones as well as smaller or larger yellowish-white or yellowish follicular cysts, some of which have comedo-like blackheads, are characteristic. Usually no blackhead, but rather a crumbly horn-like mass (follicle keratosis), can be expressed. Grouped, standing comedo-like follicle keratoses also occur (Fig. 3).
Fig. 3

Nodular elastosis with comedones and cysts


In addition to massive dermal elastosis, atrophic sebaceous glands and hair follicles with horn-filled follicular pseudocysts and cysts are found.


Follicular keratoses are softened and expressed using topical treatment with retinoid preparations. Sufficient light protection is required. In severe cases, dermabrasion or ablative laser treatment is also indicated.

Lemon Skin (Milian 1921)


These skin changes also appear after chronic exposure to the sun and are characteristic. In some cases, a family occurrence has been described.

Clinical Features

The facial skin appears thickened, is generally a diffuse yellow color, and shows more wrinkles.

Elastotic Bands (Raimer et al. 1986)

This special form shows cordlike thickenings of the skin.

Erythrosis Interfollicularis Colli (Leder 1944)


Erythromelanosis interfollicularis colli


This harmless but cosmetically disturbing irreversible change is often found in people who are exposed to strong light exposure at work or during their leisure time.

Clinical Features

On both sides of the neck, below the ears, often also on the chest, and up to the edge of the shirt cutout, there is a sharply contrasting, even teleangiectatic erythema, with a recess the size of a pinhead (Fig. 4) in the normal-colored follicle. The submental region and retroauricular region are always free of the erythema.
Fig. 4

Erythrosis interfollicularis colli

Sometimes interfollicular redness is associated with hyperpigmentation: erythromelanosis interfollicularis colli.


The affected body must be covered. Laser treatment (diode, KTP, dye laser) requires experience. Light protection is used prophylactically.

6.1.3 Other Forms of Light Damage

The star-like pseudo scars and the purpura senilis (see chapter “Disorders of Hemostasis”) suggest light damage, although both occur almost exclusively on the forearms, and only very rarely on the face.

Light Reactions of Diseased Skin

In contrast to the actual light-induced dermatoses (photodermatoses), light-provocable reactions on diseased skin represent dermatoses of other pathogenesis, which can be provoked or aggravated by light. This is especially true for some forms of cutaneous lupus erythematosus. In particular, lupus erythematosus of the tumidus type as well as subacute cutaneous lupus erythematosus are light sensitive. Systemic lupus erythematosus is also exacerbated by sun exposure. A detailed consultation with the patients is particularly important here. The photosensitivity of lupus erythematosus has been more effectively analyzed by standardized photo tests, revealing the light-sensitive subtypes of its complex clinical picture.

Jessner Kanof lymphocytic infiltration, as well as some dermal forms of mucinosis (reticular erythematous mucinosis, papular mucinosis, plaque-like mucinosis), also show photo-aggravation. Herpes simplex (glacial fire) is also known to occur after exposure to sunlight; UV-induced immunosuppression is assumed to be responsible for the occurrence of the disease. Furthermore, dyskeratosis follicularis and bullous dermatoses are known to worsen due to sun exposure. In some skin diseases, an isomorphic irritant effect (Koebner phenomenon) occurs after greater exposure to light, for example in lichen ruber or psoriasis vulgaris. While in most cases atopic eczema is favorably influenced by UV radiation, about 5% of atopic patients experience exacerbation of the clinical picture after exposure to sunlight (see overview). Photoprovoked atopic eczema is a disease that has so far been insufficiently investigated; it also considerably restricts the quality of life of patients.

Some Dermatoses Provoked by Light

  • Dyskeratosis follicularis (Darier’s disease)

  • Disseminated superficial actinic porokeratosis

  • Perioral dermatitis

  • Lichen ruber

  • Herpes simplex

  • Lupus erythematosus

  • Jessner-Kanof lymphocytic infiltration

  • Mucinoses (reticular erythematous mucinosis, papular mucinosis, plaque-like mucinosis)

  • Rosacea

  • Seborrheic eczema

  • Atopic eczema

  • Psoriasis (rare)

  • Duhring’s disease

  • Hailey-Hailey disease

  • Pemphigus foliaceus

  • Pemphigus erythematosus

  • Bullous pemphigoid.

A number of skin diseases respond well to UV exposure: atopic eczema, psoriasis vulgaris, or acne vulgaris usually show a clear improvement of the skin findings during the sunny season, or under appropriate phototherapy. Photo-therapeutically treatable skin diseases are:
  • Acne

  • Psoriasis vulgaris

  • Parapsoriasis diseases

  • Mycosis fungoides, Sézary syndrome

  • Atopic eczema

  • Pruritus, prurigo

  • Photodermatoses

  • Mastocytoses

  • Scleroderma.

The response of some skin diseases to UV rays is used therapeutically.

7 Primary Photodermatoses

Idiopathic photodermatoses such as solar urticaria (sun urticaria), polymorphic light dermatosis, hydroa vacciniforme, and actinic prurigo occur in otherwise healthy patients; their etiology is largely unexplained. Another group of primary photodermatoses is caused by chemical photosensitization in the course of phototoxic or photoallergic dermatitis. Finally, the group of chronic persistent photosensitivity (chronic actinic dermatitis) can be distinguished (Table 4).
Table 4

Classification of primary photodermatoses

Clinical diagnosis

Clinical picture


Action spectrum

Idiopathic photodermatoses

Solar urticaria



UVA, UVB, UVC, visible light

Polymorphic light dermatosis

Single-standing, monomorphic papules, papulovesicles, or plaques

Very rare


Hydroa vacciniforme

Vesicles, hemorrhagic crusts, varioliform scars

Very rare


Actinic prurigo

Prurigo papules, plaques, lichenification

Very rare


With known photosensitizer

Phototoxic reaction

Erythema, blisters, exaggerated sunburn



Photocontact allergy

Dermatitis, eczema



Systemic photoallergy

Dermatitis, lichenoid papules



Persistent photosensitivity (chronic actinic dermatitis)

Persistent light reaction

Lichenified eczema


UVB, UVA, visible light

Actinic reticuloid

Lichenified eczema, lymphoma-like infiltrates


UVB, UVA, visible light

Photosensitive eczema

Chronic eczema



Chronic photosensitive dermatitis

Chronic eczema



Photoaggravated atopic dermatitis

Chronic eczema

Relatively rare


7.1 Idiopathic Photodermatoses

7.1.1 Solar Urticaria (Merklen 1904)


Light urticaria; Sun urticaria; Photoallergic urticaria


The etiology of this rare disease is unclear. The action spectrum ranges from long-wave UVC to IR. Most patients react to visible light, and some develop an urticarial reaction at the site where they are injected with their own previously irradiated plasma or serum. Previous classifications are based on criteria such as action spectrum, transfer tests (Prausnitz-Küstner), and histological changes. A newer classification suggests two types of light urticaria. In general, it is postulated that radiation is absorbed by a chromophore (the precursor stage of a suspected photoallergen). The molecule is altered by the absorbed energy, forming the photoallergen, which elicits a primarily IgE-mediated immune response. Antigen-specific IgE is bound to mast cells, so that subsequent exposures to the same irradiation may induce additional amounts of the photoallergen, which bind to mast cells and produce urticaria in a typical type I Gell and Coombs reaction. This type I solar urticaria is characterized by the fact that the patient has a specific precursor substance. In type II light urticaria, specific IgE is produced against a photo product normally produced in any skin during irradiation. Although histamine is an important mediator for light urticaria, the urticarial reaction is insufficiently blocked by antihistamines.

Clinical Features

Burning, erythema, and, after a few minutes, itchy wheals usually occur in adults immediately after irradiation (sun, artificial radiators) on all parts of the body, but predominantly on otherwise light-protected areas. The urticarial reaction lasts for minutes to hours. Depending on the dosage, there may be large hives, edema, cardiovascular complaints, hypotension, tachycardia, or even shock symptoms. A warning should therefore be given against whole-body irradiation for diagnostic purposes.

Fixed solar urticaria is a special form of solar urticaria in which the hives develop only in certain areas of the body. The rest of the integument remains free of hives, even after irradiation. Another special form is delayed solar urticaria. Here, the first hives appear within hours of the irradiation.

Differential Diagnosis

Erythropoietic protoporphyria, urticarial phototoxic reaction when taking photosensitizing drugs, physical urticaria (heat, cold), as well as polymorphic light dermatosis must be ruled out.


The upper corium is edematous. In some patients, 6–35 h after the occurrence of hives, lymphocytic and granulocytic perivascular infiltrates with eosinophils and nuclear debris, as well as deposits of complement factors, are also observed.

Diagnostic Procedure

This condition is tested with beams of different wavelengths (UVC, UVB, UVA, visible light, and IR) to determine the action spectrum and threshold for the triggering of the wheals (MUD: minimal urticaria dose). In addition, in vitro pre-irradiation of the individual’s own plasma or serum is possible.


The disease is chronic, often persists for years, and has an uncertain tendency to heal.



Many antihistamines have proved to be insufficiently effective in most cases. Higher doses or combinations of different antihistamines have been shown to be helpful in some patients. Increasing reports describe good success of the monoclonal IgE antibody omalizumab for otherwise treatment-refractory courses.


Phototherapy, photochemotherapy, or plasmapheresis are used.


Repeated exposure to sunlight and/or UV radiation causes the skin to become exhausted (acclimatization to light hardening, tachyphylaxis), and then it no longer reacts with wheals. However, this stage of tolerance lasts only 2–3 days. A light acclimatization is thus usually not sufficient for the permanent treatment of a solar urticaria. Rash hyposensitization under inpatient conditions, in which light acclimatization can be achieved within a few days, has proven successful in initiating light therapy.


This has established itself as an effective method for solar urticaria. Before starting oral photochemotherapy with UVA and 8-methoxypsoralen, which is performed analogously to the treatment of psoriasis, it is recommended that tolerance is generated by repeated provocative irradiations over the entire integument. The PUVA treatment is then initiated, overlapping with this light acclimatization. It must be continued during the sunny months of the year as a maintenance treatment, but may be suspended during the winter months, depending on the patient’s sensitivity to light.


If a serum factor or plasma factor is detected, plasmapheresis can achieve at least a temporary improvement of the light urticaria.

7.1.2 Polymorphic Light Eruption (Rasch 1900; Hausmann and Haxthausen 1929)


Prurigo aestivalis; Poly morphic light eruption (PMLE); Summer prurigo (Hutchinson 1878); Lupus erythematosus light dermatosis; Eczema solar


Polymorphic light eruption is a frequently occurring, etiologically unexplained, very itchy skin alteration caused by exposure to the sun, especially in temperate zones. Morphologically, a distinction is made between papular, papulovesicular, and plaque-like variants. The reaction in individual patients is usually monomorphic.


Polymorphic light eruption is a relatively common disease in the Northern latitudes, and occurs predominantly in the months of March to June, as well as among tourists who travel to sunny regions in the off-season. Its prevalence is estimated at 20%. It can occur at any age, even in childhood. Among the European population, it occurs predominantly in women (9:1), while in California the sex-distribution is generally 1:1. Familial accumulation and an association with minor criteria of atopic diathesis are described. Although polymorphic light dermatosis also occurs in patients with dark or black skin, it is most common in the light-skinned population. The family-occurring variant, generally the actinic prurigo form, is a special form found amongst Native Americans.


The etiology is unknown. Pathogenesis is assumed to be a delayed type, cell-mediated immunological response. The clinical course as well as the type of efflorescence and the histopathological picture support this view. The triggering allergen has not yet been identified. The disease is dependent on genetic susceptibility, and there is also an environmental component, such as the type of exposure. Experimental investigations have shown heat-shock proteins to be probable antigenic photo products. The induction of proinflammatory cytokines by UVA seems to be an essential factor for the pathogenesis of polymorphic light dermatosis. Additionally, it has been proposed that protein modification during apoptotic cell clearance could lead to a potential auto-antigen formation.

UVA radiation induces substances that upregulate inflammatory processes, essentially prostaglandin E2, reactive oxygen species, tumor necrosis factor-2, interleukin-1 and -8, and ICAM-1. A potential genetic polymorphism in the regulation of these molecules could be co-responsible for the development of polymorphic light dermatosis. Although the expression of adhesion molecules may be cytokine-mediated, UVA directly induces the AP-2 transcription factor through a single oxygen-dependent mechanism that ultimately leads to the activation of intercellular adhesion molecule 1. Decisive pathogenetic relevance is attributed to this process.

The patients show a normal erythema threshold as well as pigmentation reactions. The action spectrum is covered by UVA in the majority of patients. Few patients react to both UVA and UVB, and only to UVB.

Clinical Features

The skin changes develop a few hours to a few days after intensive exposure to the sun, as a delayed reaction. Only sun-exposed skin is affected. At first, blotchy erythema accompanied by itching occurs. Several morphological variants are described (Figs. 5 and 6).
Fig. 5

Polymorphic light eruption

Fig. 6

Polymorphic light eruption: sternal region

Eczematous images and scars do not belong to polymorphic light dermatosis, but can occur secondarily, as complications due to eczema or excoriation. Despite the different morphological variants, polymorphic light dermatosis is mostly monomorphic in individual patients. Papules, papulovesicles, or plaques develop. The following special forms are differentiated:
  • Papular type

  • Hemorrhagic type

  • Plaque type

  • Erythema multiform-like type

  • Papulovesicular type

  • Ictus (insect sting) type

  • Vesiculobullous type.

Preferred locations are the neckline, lateral areas of the upper arms, back of the hand, thighs, and lateral parts of the face. The skin changes recede spontaneously within several days of light absence. They leave no residuals.

Juvenile Spring Eruption (Burckhardt 1942)

This is a variant of the PLE that mainly affects young men or children in spring. The auricles are most frequently affected, and the hands or nose (Fig. 7) less frequently. Occasionally, outbreaks of this otherwise rare variant have been described in soldiers or campers after sun exposure in combination with cool weather. Some investigators equate this dermatosis, known as juvenile spring eruption, with spring perniosis (Keining 1940).
Fig. 7

Juvenile spring eruption

A recent investigation has shown an association with parvovirus B19 infection. This observation could explain the outbreak after a combination of cold weather and sudden sun exposure. Parvovirus B19 DNA was detected in lesional skin, while parvovirus B19 IgM was found in the serum of affected individuals, indicating a recent infection. Interestingly, parvovirus B19 infections are most frequent in winter and spring, which is the characteristic season for juvenile spring eruption.

Differential Diagnosis

It varies according to the type of polymorphic light dermatosis. In the case of the papular or papulovesicular type, the following are to be mentioned: photoallergic eczema, atopic eczema, icterus, prurigo simplex acuta, or subacuta and hemorrhagic vasculitis; in the plaque type: light urticaria, erythropoietic protoporphyria, and erythema exsudativum multiforme. Photosensitive forms of cutaneous lupus erythematosus can be very difficult to distinguish, especially the tumidus type. Lupus erythematodes, however, shows a latency period of 1–3 weeks after intense exposure to the sun, and a slow healing within weeks after complete sun protection. Histological and immunological examinations, as well as the search for organ manifestations of lupus erythematosus, are crucial for differential diagnosis.

Diagnostic Procedure

Monomorphic eruption in sun-exposed skin areas of the neckline, arms, thighs, and face, predominantly at the beginning of the sunny season and in young women, is suspicious. The disease has a characteristic temporal course: it begins a few hours to a few days after exposure, and within days, shows spontaneous remission, without leaving residuals. The anamnesis is typical. By experimental provocations in a test area, with repeated irradiations of 60–100 J/cm2 UVA or 1.5 MED UVB, the genuine skin changes can be reproduced at the predilection sites (Table 5). Histological examinations of genuine or provoked herds confirm the diagnosis.
Table 5

Photoprovocation of polymorphic light dermatosis

Test protocol

Test site

Stretch sides of arms, possibly also shoulder and upper back

Test area

5 cm × 8 cm

Radiation source

Metal halide lamps (UVASUN 340–400 nm); fluorescent lamps (Philips TL 20 W/12, 285–350 nm)


3 to 4 times 60–100 J/cm2 UVA; 3 to 4 times 1.0 to 1.5 times MED UVB


Before and immediately after each irradiation and 24 h after the last irradiation. To differentiate an LE observation, up to 3 weeks


Common to all morphological variants is a cuff-shaped perivascular lymphocytic infiltrate extending over the entire dermis. In addition, subepidermal edema and usually a low vacuolization of the basal cells occurs. Exocytosis and spongiosis are differently pronounced; in vesiculobullous variants there are even blisters. Epidermal necrosis is typical for the ictus type.


The disease is chronic and recurrent after excessive exposure to the sun. In the course of the sunny season, many patients show a habituation effect, so that, finally, more intensive sunbathing is tolerated.


It is uncomplicated, because spontaneous remission occurs when further exposure is avoided. It can be externally accelerated by glucocorticoids in creams or lotions, and by zinc shaking.

Topical Prophylaxis

Sunscreens with broadband filters are helpful. They can be supplemented by general sun protection measures such as appropriate clothing and sensible behavior. Many patients achieve a gentle acclimatization to light in this way. By topical application of DNA repair enzymes, which are also incorporated into light protection filters, an additional effect can be achieved by elimination of the antigenic trigger. Further experimental approaches pursue additional pigmentation by afamelanotides as well as immune modulation by vitamin D3 as prophylaxis.


Most patients get used to light during the summer. This can be anticipated by phototherapy, before the sunny season. For this purpose, whole-body irradiations with broadband UVB (290–320 nm), narrow-spectrum UVB (311–313 nm), or UVA-1 (>340 nm) are suitable. Occasionally, light attacks of polymorphic light dermatosis are provoked by phototherapy. In these cases, temporary external applications of glucocorticoids and treatment pauses are helpful.


PUVA is extremely effective, but should be reserved for very light-sensitive patients (Table 6).
Table 6

Stepwise treatment of polymorphic light dermatosis


Step-by-step treatment

Light forms

General light protection

UV adaptation

Broadband light protection filter

Topical application of antioxidants

Medium forms

Additional: phototherapy (broadband UVB, narrow spectrum UVB, UVA-1)

Heavy forms

PUVA-treatment instead of UVA-/-B-phototherapy

Systemic Prophylaxis

Prophylaxis through internal medication is disappointing. Anecdotal recommendations have included β-carotene, chloroquine, nicotinamide, Escherichia coli-extract, antihistamines, and calcium. None of these therapies could be confirmed in controlled trials. Newer approaches use extracts from Polypodium leucotomos, which has antioxidant and anti-inflammatory properties, for oral prophylaxis. Smaller studies have shown positive effects.

7.1.3 Hydroa Vacciniforme (Bazin 1860)


The disease is very rare. The first manifestation usually occurs before the age of 10 years, and girls are probably affected more frequently than boys. Disease relapses are provoked by excessive sun exposure during the light-rich season. Two forms may be differentiated: a very severe form, which occurs mainly in Latin American juveniles and may be associated with systemic disease leading to lymphoma, and less severe forms in Caucasians, which subside after puberty and are not associated with systemic disease.


The exact etiopathogenesis is not known. A similarity to polymorphic light eruption is assumed. However, the course is more severe, because UV radiation leads to blisters with scarring, and frequent eye involvement, with conjunctivitis and keratitis, is observed. The efflorescences can be reproduced experimentally with UVA. An association of hydroa vacciniforme with latent Epstein-Barr virus infection has been reported. The virus was detected both in genuine skin lesions and in photoprovoked lesions. It is postulated that reactivation of the virus leads to an outbreak of the disease. In this context, the combination with Epstein-Barr virus-associated lymphoproliferative diseases has also been reported.

In case series, it could be demonstrated that hydroa vacciniforme-lymphoproliferative disorder is an Epstein-Barr virus-related lymphoproliferation, with earlier disease onset and lower RBV DNA levels in blood, and a lower risk of developing a systemic disease, when compared with non-white people.

A similar but clinically more severe disorder has been described in children from Latin America and Asia. These patients have more severe skin lesions, large necrotizing ulcers, and severe scarring, accompanied by systemic symptoms, like fever, lymphadenopathy, and hepatosplenomegaly. For these patients, the term hydroa vacciniforme-like lymphoma was suggested and included in the recent WHO classification of lymphomas.

Clinical Features

Hydroa vacciniforme offers a very interesting clinical picture. It is limited to uncovered body areas. The spring sun causes inflammatory reddening of the ears, nose, cheeks, fingers, back of the hands, and forearms; it is circumscribed, and blisters with serous or hemorrhagic content form (Fig. 8). These dry out, forming a blackish scab that is repelled and leaves behind bowl-shaped, varioliform, often depigmented scars. Hyper- and hypopigmentation are added, resulting in a skin texture of varied appearance.
Fig. 8

Hydroa vacciniforme. (a) Severe incrustation on nose, cheeks, and lip. (b) The same patient, 2 years later, with still active efflorescences and scarring. (c) Characteristic papules, crusts, and scarring of the light-exposed forearm in another patient

There are mild, severe, and very severe cases that can be accompanied by fever and reduced general condition, mainly in juvenile Latin American and Asian patients. In such severe cases, a thorough investigation is required for Epsrein-Barr virus infection in lesional skin, as well as in serum. Furthermore, monoclonal rearrangements of the T-cell receptor genes should be investigated to encompass the diagnosis of a hydroa-vacciniforme-like lymphoma. In addition to scarring, there may be mutilations on the nose, auricles, and fingers, with considerable disfigurement. Corneal scars due to eye involvement have also occurred.

Differential Diagnosis

Erythropoietic and hepatic porphyrias (porphyrins in blood and urine, erythrocyte fluorescence) can be considered. With hydroa vacciniforme, the porphyrin metabolism is normal. Acute phototoxic reactions and the vesiculobullous form of polymorphic light dermatosis must also be distinguished.


Focal epidermal necrosis, intraepithelial vesicles, and subepidermal vesicles filled with leukocytes are seen, as well as perivascular oriented necrotizing inflammation.


The disease recurs every spring and often subsides spontaneously in adulthood.


A causal and effective treatment is not known. Direct and indirect sunlight should be avoided; UV protection glasses should be worn if necessary.


The blisters and hemorrhagic crusts are treated symptomatically with ointments and general wound healing measures. Light protection in the UVB range is ineffective. Covering with powerful broadband light protection agents, which also absorb in the UVA range, or total covering of the skin with make-up or skin-colored lotion, however, are effective.


PUVA treatment in spring before the beginning of the sunny season can have a prophylactic effect. An experiment with pyridoxine 600 mg/day and β-carotene, in severe cases with glucocorticoids, can be tried.

7.1.4 Actinic Prurigo (Lopez-Gonzáles 1961)


This idiopathic photodermatosis is rare in the White population. Larger series from England and Scandinavia have been described. It initially appears in childhood, with 80% of the patients being under the age of 10 years. Women are predominantly affected, and about 50% of the patients have an atopic diathesis. A family variant occurs among Native American people and in people from Latin America. It is referred to as hereditary polymorphic light dermatosis or family actinic prurigo.


The cause is unknown. The action spectrum for the provocation of pruriginous skin changes lies in the UVB and UVA range, with the latter predominating. The pathomechanism is largely unknown. HLA typing in Native Americans shows a preference for B40 and Cw3, as well as A3, A24, and Cw4, which underlines the heredity nature of the disease in this population.

Clinical Features

Pruriginous skin changes, such as lichenified foci, cushion-like infiltrations, and prurigonic nodules on chronically light-exposed skin parts, such as the face (often centrofacial), neck, ears, back of the hands, and forearms, are dominant (Fig. 9). They can also be found, to a lesser extent, on covered parts of the body, especially the back. Often an exudative exfoliative cheilitis of the lower lip exists, especially in Indian patients. Itching is the predominant symptom, and sensitivity to light is very pronounced. Immediately after exposure to the sun, an edematous erythema develops, which slowly disappears and gradually changes into an eczematous, then pruriginous phase.
Fig. 9

Actinic prurigo

In childhood, the light-exposed areas are preferably affected, and the course is predominantly seasonal. Later, covered parts of the body are increasingly affected, and the disease takes on a perennial character. Actinic prurigo persists into adulthood; in some patients (25%) there is an improvement during adolescence.

Differential Diagnosis

Photo-aggravated atopic eczema, polymorphic light eruption, and chronic actinic dermatitis (persistent light reaction) can be distinguished. The pruriginous aspect, the strong sensitivity to light, and the typical course are indicative.


The papular or plaque-like foci show discrete acanthosis, exocytosis, and spongiosis in the epidermis, and a lymphohistiocytic perivascular infiltrate in the dermis, occasionally with eosinophilic granulocytes.


The disease usually occurs before the age of 10 years, and persists into adulthood.


It is symptomatic. One characteristic of the disease is the extremely difficult treatment. The drug of choice for systemic treatment is thalidomide. There is also long-term experience with this drug in the treatment of actinic prurigo. In addition, no local or systemic medication has been able to achieve a significant improvement. Even acclimatization to light through phototherapy or PUVA treatment often has no influence on the clinical picture.

7.2 Photodermatoses Caused by Exogenous Photosensitization

These skin reactions are divided into two large groups: phototoxic and photoallergic reactions (Table 7 and Fig. 10).
Table 7

Clinical characteristics of phototoxic and photoallergic reactions







Latency between first exposure and skin reaction



Radiation dose (mostly UV)

Mostly high

Mostly low

Action spectrum

Narrow, mostly UVA radiation

Broad, mostly UVA


Exaggerated sunburn, erythema, blisters, postinflammatory pigmentation

Polymorph: erythema, papulovesicles, blisters, lichenification

Distant flares


Spreading to nonirradiated skin areas,

previous test areas may flare

Fig. 10

Pathogenesis of phototoxic and photoallergic skin reactions

7.2.1 Phototoxic Reactions


This form is more common than photoallergic reactions. Clinically important are the phytophotodermatitis (meadow grass dermatitis) and phototoxic reactions due to medication. Phototoxicity is also used therapeutically (PUVA).


While sunburn is a purely quantitative overdose of radiation, it requires a phototoxic reaction of a photosensitizer in the presence of UV radiation to trigger it (Fig. 10). Photosensitizing substances can be produced endogenously (porphyrins) or administered parenterally (drugs). Radiation doses that are tolerated without reaction under normal skin sensitivity lead to acute inflammatory skin reactions similar to sunburn in combination with photosensitizing substances. Here, a molecule absorbs a photon, developing into a short-lived, energy-rich singlet state. By releasing heat or transferring energy to other molecules, the photosensitizer is deactivated, and achieves a lower energy state. At this stage, reactions with biological systems such as cell membranes, lysosomes, lipids, proteins, and DNA occur. These reactions take place directly, i.e., without oxygen, or indirectly mediated by oxygen.

Phototoxic reactions after local application of hard coal tar, 8-methoxypsoralen, 5-methoxypsoralen, trimethylpsoralen, or similar furanocoumarins are known, for example from the following plants: hogweed or the cartilage carrot (principle of action in PUVA treatment) (Figs. 11 and 12), acridine dyes (trypaflavin, rivanol, flavidin), or eosin. Patients treated with psoralens should therefore have little exposure to sunlight. Phototoxic reactions are known in workers in the petroleum and tar processing industries.
Fig. 11

Phototoxic contact dermatitis (8-methoxypsoralen and exposure)

Fig. 12

Phototoxic contact dermatitis on hemlock

Drugs can trigger both phototoxic and photoallergic reactions which are not always easy to distinguish, and can also be caused by the same active ingredients. Therefore, the generic term photosensitizers, which does not distinguish between phototoxic and photoallergic, could be used for these substances. The development of b-raf inhibitors for metastatic malignant melanoma is of great importance. Of this group, vemurafenib is characterized by high phototoxicity. The most common photosensitizing drugs come from the following groups:
  • Diuretics: hydrochlorothiazide, furosemide, amiloride, triamterene, xipamide

  • Nonsteroidal anti-inflammatory drugs: naproxen, ketoprofen, tiaprofen acid, ibuprofen, diclofenac, piroxicam

  • Antibiotics: sulfamethoxazole/trimethoprim, ciprofloxacin, ofloxacin, oxytetracycline, tetracycline, doxycycline, minocycline

  • Antipsychotics: chlorpromazine, promethazine, thioridazine, haloperidol

  • Antidepressants: amitriptyline, trimipramine, nortriptyline, desipramine

  • Cardiac: amiodarone, nifedipine, quinidine, enalapril, hydralazine, simvastatin

  • Cytotoxic substances: fluorouracil, vinblastine, dacarbazine, procarbazine, vemurafenib.

This list does not claim to be complete, especially since many photoreactive substances mentioned in the literature are based on individual cases, and in reality, hardly cause any relevant phototoxicities.

Clinical Features

The clinical picture is similar to sunburn, and shows acute toxic dermatitis in light-exposed skin areas, with redness, edema, or blisters, and, subsequently, often strong pigmentation. Phototoxic onycholysis, which mainly affects the distal areas of the nail bed, is caused particularly by individual tetracycline derivatives. Phototoxic reactions after amiodarone can be accompanied by slate-grey, mostly irreversible pigmentation of the light-exposed areas. As special forms, berloque-dermatitis and dermatitis bullosa pratensis (meadow grass dermatitis) are dealt with in the following section.

Differential Diagnosis

Dermatitis solaris and photoallergic reactions (Table 7) are to be distinguished.

Diagnostic Procedure

Anamnesis and typical findings are diagnostic. Where appropriate, the photosensitizer must be detected by internal photoprovocation or a photopatch test.


The photosensitizer must be avoided.


The symptoms are treated with glucocorticoid cream and lotio zinci. In the case of large-area blisters, treatment is the same as for second-degree burns (see chapter “Disorders Caused by Physical and Chemical Damage”).

Berloque-Dermatitis (Freund 1916; Rosenthal 1925)


Berloque-dermatitis is more common in women than in men.


Numerous fragrances in perfumed toilet waters, cologne, soaps, creams, and lotions, from the group of bergamot oils or similar essential oils, have phototoxic effects. In combination with solar radiation (UVA), phototoxic reactions occur. Heavy perspiration and moist skin promote the development.

Clinical Features

After exposure to the sun, inflammatory reddening and blisters appear on the wetted or treated skin, after hours or days. The healing takes place with long-lasting hyperpigmentation. At low radiation doses, the vesicular inflammatory stage may also fail to occur, and hyperpigmentation may slowly develop directly. The site of the artificial appearances is usually the face, neck, chest, and back. Brown-red or deep brown stripes, which correspond to the placing of the perfume droplet or fragrance-containing care products (Fig. 13), are found.
Fig. 13


After long-term use of cosmetics, such as shaving cream, aftershave products, or moisturizing cream, diffuse chloasma-like hyperpigmentation occurs, especially on the forehead and zygomatic arch. For as long as the causal connection is not recognized, there is the possibility of a relapse.

Differential Diagnosis

In the case of diffuse pigmentation, other forms of hyperpigmentation and chloasma must be considered.


Phototoxically damaged cells are found in the epithelium. In the basal cell region, pigmentation is increased; in the upper corium, there is pigment incontinence, with melanin uptake in macrophages.


All perfumed and phototoxic medications or cosmetics must be discontinued, and a fragrance-free, light protection should be applied consistently.


A peeling treatment with retinoids can be carried out. The combination of vitamin A acid 0.1%, hydroquinone 5.0%, and hydrocortisone 1% also has a depigmenting effect. Occasionally, however, the hyperpigmentations persist.

Dermatitis Bullosa Pratensis (Oppenheim and Fessler 1928)


Dermatitis pratensis (pratum: meadow); Grass dermatitis; Phytophotodermatitis


Photosensitizing substances, mostly furanocoumarins of meadow grass, hemlock, cartilage carrot, or fig trees are responsible. In combination with the UVA of sunlight, this causes an acute bullous and subsequently strongly hyperpigmenting dermatitis. An essential precondition seems to be the lying down in grass with still moist skin after bathing, thus facilitating skin contact with the photosensitizer. Dermatitis bullosa pratensis is common during the summer months, especially in children.

Clinical Features

Stripes or strokes, and bizarrely configured, erythematobullous, itchy, or burning changes are characteristic, only at the points of contact, especially on the legs, face, neck, and forearms. There are no spreading phenomena. Anamnestically, sun exposure in fields after bathing, hiking, or gardening is indicated; there is always contact with furocoumarin-containing grasses or plants. Later, it develops into a strong hyperpigmentation.

Differential Diagnosis

Photoallergic contact dermatitis with rhusan antigen (rhus toxicodendron; poison ivy; toxic ivy; poison oak; toxic oak) is rare in our country and very common in North America; toxic dermatitis of other origins should also be ruled out.


Phototoxically damaged cells, intraepithelial, and subepithelial blisters, as well as epithelial necroses are found in the epidermis. This is followed by hyperpigmentation in the basal cell region, with pigment incontinence.


All photosensitizing plants must be avoided.


The symptoms are treated with glucocorticoid cream and lotio zinci; in the case of widespread blisters, the treatment is the same as for second-degree burns.

7.2.2 Photoallergic Reactions

In contrast to the obligatory phototoxic substances, which affect everyone in the same way, if they reach the skin and have sufficient UV radiation, photoallergic reactions only occur if a specific sensitization has been acquired. This only applies to a small number of people in the population.

The photosensitizing substances can lead to the sensitization of a patient by epicutaneous contact (photoallergic contact dermatitis) or by oral or parenteral absorption (hematogenic photoallergy). After photocontact sensitization, many patients react to the allergen, no matter how it reaches or enters the body (Table 8).
Table 8

Important photo allergens (selection)


Substance groups



Halogenated salicylanilides

 tetrachlorosalicylanilide (TCSA)

 tribromo salicylanilide (TBSA)

Soaps, toiletries, disinfectants, (dermatotherapeutics) (historical, halogenated salicylanilides are no longer allowed)






Antimycotic (no longer on the market)


Antimycotic (no longer on the market)

Ambrette musk

Fragrance in toiletries, largely replaced


Nonsteroidal external antirheumatic agent

Para-aminobenzoic acid

Sunscreens (UVB)


Sunscreens (UVA, UVB)


Sunscreens (UVA, UVB)

p-Methoxycinnamate isoamyl ester

Sunscreens (UVB)


Sunscreens (UVA, UVB)


Sunscreens (UVA, UVB)




Tiaprofenic acid

Antirheumatic agent (surgam)


Phenothiazine (atosil)


Only in veterinary medicine (farmers, etc.)


Diuretics (amiloride, disalunil, diu-melusin, esidrix, and combinations)


Antiarrhythmic agent (quinidine-sulfate)


Lipid reducer

Some substances are both contact allergens and photocontact allergens, making testing complicated. In rare cases, a drug causes a contact allergy, a photocontact allergy, and a phototoxic reaction (for example: chlorpromazine, tiaprofenic acid, 8-methoxypsoralen).


The allergic reaction only occurs in the direct interaction of allergens and radiation. The absorption spectrum of the allergen and the action spectrum may be identical, but they are often different, so that it is assumed that the incident radiation changes the photoallergen. The action spectrum is almost always in the UVA range, and only very rarely, as with some sulfonamides, also in the UVB range.

Typical triggers of a hematogenic photoallergy are phenothiazines, sulfonamides, hydrochlorothiazide, and quinidine derivatives. As in the case of a contact allergy, a photo contact allergy persists for a lifetime.

Clinical Features

The acute clinical picture usually occurs in adults, and is limited to parts of the skin exposed to light; in photoallergic contact dermatitis, it occurs where contact with the triggering photoallergen has also occurred. It shows characteristics of allergic contact dermatitis, with relatively sharply defined erythema, as well as papulovesicles and (rarely) blisters (Fig. 14). The submental triangle is often free (low incidence of light). The diseased skin sections are separated from the body parts protected from light by clothing. With low light exposure (autumn, winter) or low intake of the photoallergen, the skin changes are slightly pronounced and blurred.
Fig. 14

Photoallergic contact dermatitis due to chlorpromazine

With continued allergen intake, the clinical picture changes into a chronic form (chronic photoallergic contact dermatitis). The skin is slightly inflammatory-red, but lichenified and scaly. There are no eczematous lesions at uncovered and unexposed parts of the body, as long as the clothing provides sufficient light protection; however, there are scattered stoves (eczema scattering). There is increased itching.

Diagnostic Procedure

In systemic photoallergy caused by enteral/parenterally administered drugs, the photo patch test is often negative, because only a particular metabolite is the relevant photoallergen. Here, the diagnosis is performed by systemic photoprovocation, in which a control field with 10 J/cm2 UVA is irradiated, and the corresponding drug is then applied systemically. At the time of the highest plasma concentration, a further area of skin is irradiated with 10 J/cm2 UVA, and readings are performed after 24 and 48 h.


The characteristic perivascular lymphohistiocytic infiltrates lead to exoserosis and exocytosis with spongiosis and acanthosis, papillomatosis, and parahyperkeratosis. Light-damaged cells in the epithelium are rare.


The photoallergen is removed. Acute or chronic disease is treated as in dermatitis or eczema of allergic origin. Light protection is provided by dense clothing and sunscreens, which are also effective in the UVA range.

7.3 Chronic Actinic Dermatitis (Hawk and Magnus 1979)


Chronic persistent photosensitivity, which includes the terms persistent light reaction, photosensitive eczema, chronic photosensitive dermatitis, and actinic reticuloid, some of which are used synonymously.


Chronic actinic dermatitis is relatively rare. Older men are mainly affected, and occasionally also middle-aged and older women. The disease exacerbates during the sunny seasons.


Originally, the disease was called persistent light response. The epidemic-like occurrence, attributed to the use of halogenated salicylanilides in deodorizing soaps, was prevalent. Photoallergic dermatitis develops into chronic persistent photosensitivity. At this stage, electromagnetic radiation alone, without further allergen intake, is sufficient to maintain the dermatitis. At the same time, there is a change in the action spectrum. While photoallergic dermatitis is provoked by UVA, eczema is caused by UVB radiation during the persistent light reaction. In the further course, the action spectrum can expand to UVA and even visible light. The pathomechanism is unclear (Fig. 15). It is believed that UVB irradiation leads to the formation of an antigenic photo product, which may originate from the original endogenous carrier protein.
Fig. 15

Hypothetical pathogenesis of chronic actinic dermatitis and classification into its subtypes

In addition to the persistent light reaction, similar clinical pictures have meanwhile been described under the names of actinic reticuloid, photosensitive eczema, and chronic photosensitive dermatitis. Possibly this also includes the light engraved atopic eczema. It is characteristic for the course of the disease that a secondary photosensitization is added to a pre-existing chronic inflammatory dermatosis; in this case, electromagnetic radiation alone is sufficient to maintain the dermatitis.

Clinical Features

A chronic, mostly lichenified dermatitis (eczema) develops in the light-exposed skin areas, in which scattering phenomena also occur in skin that is covered by clothing, but insufficiently protected. The skin is inflammatory reddened, often livid red, diffusely upholstered, thickened, furrowed, and covered with scales. The agonizing itching leads to excoriations. In extreme cases, cushion-like inflammatory swellings occur, such as in cutaneous lymphomas, corresponding to a facies leontina. Preferred sites are the forehead, cheeks, auricles, earlobes, neck, and the back of the hand. Often, the retroauricular region and submental triangle remain free or are affected to a lesser extent, due to the low exposure to light. If the course is severe, the entire integument is affected. The action spectrum can be broad, and then extend from UVB to visible light.

The light sensitivity is high. Small amounts of light, which penetrate even through thin clothing, are sufficient to allow chronic skin inflammation to develop, even on the covered parts of the body.

Differential Diagnosis

In particular, a systemically triggered, photoallergic reaction must be distinguished with a continuous supply of the photosensitizer. An aerogenic contact dermatitis, which is frequently caused by plants from the group of compositae, in which the allergenic sesquiterpenlactones reach the skin through floating plant parts also needs to be distinguished. Other differential diagnoses include mycosis fungoides and chronic generalized atopic eczema.


The symptoms are mostly chronic, lichenified, spongiotic dermatitis with hyperkeratosis, parakeratosis, minor papillomatosis, and plump or psoriatic acanthosis (eczema morphology). The inflammatory, predominantly lymphohistiocytic infiltrate is very dense, often ligamentous, and pushes the older elastically altered connective tissue into the depths. Strong infiltrate epidermotropism resembles initial mycosis fungoides. In these cases, the term actinic reticuloid is used, in which suppressor T cells appear to predominate.


The disease is highly chronic. With longer duration, the light sensitivity increases. The patients are profoundly affected by the severe eczema, which is continuously reinforced by even the smallest doses of light.



The focus is on avoiding the triggering radiation. Since the action spectrum can be broad and then extends from the UVB to the UVA range and into the visible light, intensive light protection, in particular, must be provided. In extreme cases, artificial lighting at the workplace or at home may suffice for continuous eczema maintenance. Such patients are severely impaired. In general, shifting leisure activity to the evening and night hours, wearing light-protecting clothing, and covering, tinted preparations such as make-up or skin-colored lotions help. The calcineurin inhibitors tacrolimus or pimecrolimus are symptomatically effective.


In addition to immunosuppressive measures using systemically applied glucocorticoids, azathioprine, and cyclosporine A, PUVA treatment has established itself as the method of choice. The initiation of treatment may be difficult due to the possible UVA sensitivity. Initial doses below the eczema threshold dose must then be selected. A combination with systemic glucocorticoids or immunosuppressants (azathioprine) is helpful in this initial phase. In the further course, the PUVA treatment takes place as with psoriasis. Phototherapy alone, in the sense of acclimatizing to light, is not successful with these patients, as the dermatitis is continuously intensified by it.

7.4 Persistent Light Reaction (Wilkinson 1962)

The initial description was made in the context of an epidemic of photocontact allergies in workers exposed to halogenated salicylanilides in the workplace. Over time, these individuals developed persistent photosensitivity. The characteristic criteria of persistent light reaction are chronic eczematous skin changes in light-exposed skin (Fig. 16), preceding acute photocontact dermatitis, action spectrum in the UVB or additional UVA range, and the detection of a photoallergen in the photo patch test.
Fig. 16

Persistent light reaction

7.5 Actinic Reticuloid (Ive et al. 1969)

This term was used to describe a photodermatosis similar to the persistent light reaction in older men, which showed histological similarity to a T-cell lymphoma. The actinic reticuloid is defined by infiltrated papules and plaques in light-exposed skin, a T-cell lymphoma-like histological image, photosensitivity to UVB and UVA, often also to visible light, and a negative photo patch test.

7.6 Photosensitive Eczema (Ramsay and Kobza-Black 1973)

The authors described a group of older men who developed light sensitivity after existing eczema of a different genesis. Photo patch tests were negative, and the action spectrum was limited to the UVB range. Photosensitive eczema is therefore characterized by a preceding chronic dermatitis (eczema), which increasingly intensifies in light-exposed skin, an action spectrum limited to UVB, and a negative photo patch test. The epicutaneous test often identifies contact allergens.

7.7 Chronic Photosensitive Dermatitis (Frain-Bell et al. 1974)

As with photosensitive eczema, in the past only older men with chronic eczema were described in the prehistory. However, the photodiagnostic investigations showed a broad action spectrum and positive epicutaneous tests, as well as photo patch tests. This clinical picture is defined by a preceding chronic dermatitis (eczema), clinically and histologically chronic dermatitis in light-exposed skin, mostly broad action spectrum, which includes UVB, with or without UVA or visible light, as well as positive reactions to the patch and/or photo patch test, in some patients.

Since these different clinical pictures show many similarities and overlapping criteria, the use of the term chronic actinic dermatitis as an umbrella term has prevailed. As subtypes, the special forms can supplement the diagnosis of chronic actinic dermatitis. Some cases of photosensitive mycosis fungoides are clinically indistinguishable from chronic actinic dermatitis. Histopathological and molecular biological examinations must be used for differentiation.

8 Light Protection

Sensible handling of solar radiation, protective clothing, and application of light-filter substances are the essential pillars of suitable UV protection. The aim of these measures is to avoid the undesirable effects of solar radiation, which range from sunburn, premature skin aging (light aging), to photocarcinogenesis in healthy people. In patients with photodermatoses, the symptoms should be alleviated by these measures.

8.1 Avoidance of Excessive Solar Radiation

The best light protection is the avoidance of solar radiation, but for most people this is not desirable. Patients diagnosed with extreme light sensitivity, such as xeroderma pigmentosum, albinism, chronic actinic dermatitis, and others, must completely avoid sun exposure to prevent their deleterious effects. Glass panes provide protection against UVB radiation, but this is not enough for people with photodermatoses who react to UVA or visible light. Here, certain additional devices are necessary, such as crystal-clear UV-impermeable special films for house and car windows, combined with thermal insulation and protection in the area of visible light, if required.

It is important to avoid the sun between 11.00 and 15.00 in summer, as this is when UVB radiation is most energetic. In autumn and winter, UVB radiation is significantly less effective, so the risk is lower. UVA radiation is less dependent on time of day or season.

8.2 Clothing

In addition to avoiding the sun, suitable clothing is the most important factor in UV protection, and should not be neglected because of the widespread use of light protection filters. It is not easy to determine the protection factor of textiles in individual cases. Roughly estimated, textiles offer protection if, when held against a light source, they do not let any radiation through.

The ability to protect against UV radiation can be more accurately determined by determining the ultraviolet- protection factor (UPF), which is a measure of UVB protection corresponding to the sun protection factor for sun creams. In summer, it is recommended that clothing with a UPF of >30 be worn. The density of the fabric determines the height of the UPF. The UPF can also be increased by impregnation with UV-absorbing chemicals. Wet, close-fitting clothing offers less UV protection. Hats should have a wide brim; as several studies have shown, baseball caps provide insufficient protection.

8.3 Sunscreen Products

8.3.1 Basic Principles

Sunscreens are measured according to their sun protection factor (sun protection factor: SPF). Simply put, the LSF expresses the extension of the irradiation time until a UV-induced erythema occurs. For example, a person who develops an erythema in the midday sun within 15 min can extend this time from 15 min to 225 min by applying a sun cream with a SPF of 15. The sun protection factor is determined by measuring the minimum erythema dose (MED) in unprotected and protected skin. The quotient of the minutes of these two determinations gives the SPF.

In Europe (COLIPA), the USA, and Australia there are slightly diverging standards for determining the LSF. However, the differences are so small that sun protection against UVB is sufficiently well represented in all methods. The meaning of extremely high SPF (>60) has been controversially discussed; it has generally been agreed to divide sunscreens into four groups, for ease of understanding:
  • Low protection: SPF 6, 10

  • Medium protection: SPF 15, 20, 25

  • High protection: SPF 30, 50

  • Very high protection: SPF >50

The sun protection factors indicated on packages are limited to the specified values.

There are no generally accepted valid determination methods for UVA protection and protection against radical formation in the visible and infrared spectral range.

Both in vitro and in vivo methods are used worldwide. Taking into account the UVB and UVA protection factors, DIN 67502 was introduced as the standard procedure in Europe. Other protocols have been developed to measure immune protection (IPF) and to determine the factor of mutation protection. However, these in vitro tests have not been successful.

Due to the significant formation of radicals by visible light and infrared radiation, appropriate protection is also increasingly required here. There are as yet no filter systems that protect by absorption in analogy to the UV range. Only the physical light protection substances are effective in these areas, due to their very good scattering effect.

Another development concerns light stabilizers that not only have a high SPF, but also contain DNA repair enzymes. These repair enzymes are able to repair photo products within the DNA through the mechanism of photoreactivation. This also leads to the prevention of UVB-induced immunosuppressive effects. The development of actinic keratoses can also be reduced by these light stabilizers.

While the vast majority of dermatologists and photobiologists recommend the use of sunscreen products, there are also warnings. These postulate that people, assuming that they are protected, often stay in the sun for longer periods, and thus suffer considerable cumulative damage. Furthermore, vitamin D deficiency should also be borne in mind; this could lead to problematic osteoporosis. There are no evidence-based studies to support the thesis that sunscreen products themselves increase the risk of melanoma. It is still recommended that sunscreens be used.

8.3.2 Ingredients

Sunscreen products contain both chemical and physical filters (Table 9). Chemical filters absorb the high-energy UV radiation, and then emit it as low-energy, long-wave radiation.
Table 9

Ingredients of sunscreen products



Chemical UVB filters

Aminobenzoates (280–320 nm)

P-Aminobenzoic acid (PABA

Amyldimethyl PABA

Octyldimethyl PABA

Glyceryl PABA

2-Ethylhexyl PABA

Ethylhexyldimethyl PABA (padimat 0)

Cinnamates (290–320 nm)

2-Ethoxyethyl p-methoxycinnamate

Diethanolamine p-methoxycinnamate

Salicylates (290–320 nm)

Octyl salicylate

Triolamine salicylate

2-Ethylhexyl salicylate

Homomenthyl salicylate

Triethanolamine salicylate


Digalloyl trioleate



Phenylbenzimidazole sulfateägreat

Chemical UVA filters

Benzophenones (250–365 nm)





Eusolex 2020

Dibenzoylmethane (avobenzone parsol 1789)

Physical UVB and UVA blockers



Magnesium silicate

Magnesium oxide

Titanium dioxide

Iron oxide

Zinc oxide

DNA repair enzymes



Physical filters are tiny particles that block and reflect light. They are inert, and, unlike chemical filters, do not cause contact or photocontact allergies. Furthermore, these substances cause hardly any irritation around the eyes, which is especially important for athletes and children. These particles can also be added to chemical filters to achieve very high sun protection factors (>60).

Some factors should be considered when using light stabilizers:
  • SPF: For most people, sunscreens with a SPF of 15–30 that protect against UVB and UVA are sufficient. If very intensive exposure to the sun occurs, higher factors may be applied. Skiers, mountaineers, sailors, and other professionals should use products with a SPF of 30–50. This also applies to patients with photodermatoses.

  • Basis: Gels penetrate better and act more rapidly, but can have a drying effect. They burn more often around the eyes, even when they are applied carefully. Sweating causes gels to reach sensitive areas of the skin. Most people prefer lotions, as creams dry out less, but have an occlusive effect that can be disturbing, especially in summer. Lipsticks with light-protection filters are recommended.

  • Special features: The stability of light protection agents on the skin, as well as water resistance, are essential for their effectiveness. Water resistance is usually included in the determination of the LSF. As a rule of thumb, 50% of the LSF should still be maintained after swimming. The methods to check this are different, and a standard rule does not exist. The risk of contact or photocontact allergy to light stabilizers must be considered. Therefore, in the evaluation of photosensitive dermatoses, an epicutaneous test and a photoepicutaneous test should also be performed, with a light protection filter.

8.4 Artificial Tanning Agents

Dihydroxyacetone is the only widely used artificial tanning agent. It is used in concentrations between 2.5–10% (usually 5%), binds to the horny layer, and colors the outer horny layers brown, by means of oxidation. The paint is not washable, but fades over time through the natural process of desquamation. Initially, application within a test area is recommended, as individual differences in tinting may be esthetically disturbing in individual cases. The application should be as uniform as possible, on dry skin. If the effect is insufficient, the application can be repeated after a few hours.

Other tanning agents are water-soluble pigments, which are used as body make-up, and can also be applied over shower baths or baths.

Cave: Neither dihydroxyacetone nor other tanning agents provide protection against UV damage. Although harmless, they mislead patients into believing that they are protected, and therefore expose these people to more frequent and prolonged exposure to harmful radiation.

Betacaroten is sold in many countries as a tanning pill. In sufficient doses, it leads to a yellow-orange skin coloring, which is pronounced in the palmoplantar region. The substance offers no protection against UV radiation, but blocks some visible light, so that it is used therapeutically in erythropoietic protoporphyria. Overdose leads to hyperkarotenemia, and patients should be warned in this regard.

In the past, psoralens were used as tanning agents. Due to the unacceptable risk-benefit ratio, this method is obsolete today, and should only be prescribed in strict medical indications.

8.5 Tanning Studios

Tanning studios are commercially very successful in some Western countries. This trend has alarmed many dermatologists, as uncontrolled UV doses that are in the therapeutic range are applied. The increasing use of UVA emitters has given both operators and users a false signal of safety. Tanning studios advertise with the conveyance of well-being and protection against sunburn, while UVA tanning provides very little protection against UVB radiation. Although pure UVA does not cause sunburn, there are risks of phototoxic and photoallergic reactions, premature skin aging, and also risks of induction of non-melanocytic tumors and possibly melanomas. As the widespread use of these studios did not begin until 1980, better long-term data are needed in order to accurately estimate their carcinogenic risk. Patients with photodermatoses bear the risk of acute exacerbation of the disease after visiting tanning salons. So far, there are no internationally accepted regulations concerning the control of tanning studios.

While the laws in the USA vary from state to state, a law was passed in Germany in 2009 preventing minors from using sunbeds in tanning studios or similar facilities.


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2020

Authors and Affiliations

  1. 1.Center for Dermatology, Allergology and DermatosurgeryHELIOS University Clinic Wuppertal, University Witten-HerdeckeWuppertalGermany

Section editors and affiliations

  • Roland Kaufmann
    • 1
  1. 1.Head of the Department of Dermatology, Venerology and AllergologyFrankfurt University HospitalFrankfurt am MainGermany

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