Keywords

Figure 31.1 illustrates two patients with two of the disorders discussed in this chapter.

Fig. 31.1
figure 1

Upper panel reveals a young woman who presented with acute generalized photo-induced erythema, acral vasculopathy of the fingers and toes, oral erosions, and proteinuria, all features of systemic lupus erythematosus. Lower panel reveals an elderly woman with dermatomyositis. Note the brightly red erythema sparing sun-protected sites

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FormalPara Case

A patient presents with a few to several weeks’ history of confluent red smooth macules and patches. The onset may be acute or insidious and favors photo-exposed sites. This presentation is exclusive of the patients with total skin erythroderma, in whom the differential diagnosis is different (see Chap. 26 on exfoliative erythroderma).

FormalPara Clinical Differential Diagnosis

Depends on the onset and duration of the eruption. If acute and short-lived, then the patient is likely to have morbilliform drug eruption or exanthem (not addressed here); and if subacute to chronic, then

  • dermatomyositis, DM

  • photosensitive rash of systemic lupus erythematosus, SLE

  • photosensitive dermatitis/photo-drug eruption, and

  • subacute cutaneous lupus erythematosus, SCLE.

Clinical Clues

Lesions of SCLE are almost always discrete (annular polycyclic or psoriasiform, and rarely pityriasiform), unlike those of DM and the photosensitive rash of SLE , which are often diffuse. However, SCLE has been described as presenting in a diffuse manner as exfoliative erythroderma, even with bullae and TEN-like presentation. In my experience, this occurs in patients who are extremely photosensitive, following excessive sun exposure, resulting in the first episode of SCLE in an unsuspecting patient. Few patients with SCLE may have mild SLE.

The first episode of the photosensitive eruption in patients with undiagnosed SLE or DM may appear similar to and be confused with photosensitive drug eruption . Involvement of the eyelids with edema and purplish erythema is a characteristic of DM, while erythema, in the butterfly distribution, in the absence of eyelid involvement strongly favors SLE.

Photosensitive Drug Eruption

May be toxic (often seen in dermatology with doxycycline) or allergic (a long list of medications) with great overlap and imperfect distinction (hence, the more inclusive term “photosensitive drug eruption” is preferred). Phototoxic reaction mimics sunburn and is greatly photo-distributed. Photoallergic reaction mimics spongiotic dermatitis, usually subacute, both clinically and histologically. Some photosensitive eruptions are histologically and clinically lichenoid. These are more often papular.

How Helpful Is the Pathology?

Not helpful + 

Histological Findings

The combination of basal vacuolization, mild superficial and deep lymphocytic infiltrate, usually with increased dermal mucin, is characteristic of DM, SCLE and the acute photo-induced rash of SLE. If the photoeruption of SLE becomes chronic and persistent, it may acquire hyperkeratosis, dilated follicles, and a more prominent perifollicular infiltrate, reminiscent of discoid LE.

Spongiotic and lichenoid photo-drug eruptions mimic spongiotic dermatitis and lichenoid interface dermatitis. There are no specific features that indicate photosensitivity as a cause in either pattern of other causes. Some authors have proposed that the infiltrate in photosensitive dermatitis is likely to extend below the papillary dermis. This is however not reliable. Phototesting remains the gold standard for proving photosensitivity. Most patients with drug-induced photosensitivity are highly sensitive to Ultraviolet A (UVA). Not only is the minimal erythema dose markedly decreased but also UVA irradiation often reproduces the lesions.

How About Direct Immunofluorescence (DIF)?

DIF is also not helpful.

I have evaluated hundreds of direct immunofluorescence (DIF) biopsy specimens over the past 30 years from dermatologists asking to differentiate between DM and LE. These requests are often made after receiving a pathology report that indicates that the differential diagnosis is DM and LE. Some pathologists then recommend that the clinician obtain DIF in order to differentiate between the two disorders. Rarely has the test been helpful. In most cases, DIF is negative or mild and nonspecific even in cases that later prove to be LE. DIF is most likely to be positive in chronic discoid lesions and is frequently negative in acute and subacute lesions of lupus.

The literature quotes a figure of around 70 % positivity of DIF in SCLE and a similar figure in DM. While it is true that there are immune deposits at the dermo–epidermal junction in both DM and SCLE , the intensity is mild and the pattern is often not continuous along the dermo–epidermal junction; hence, the diagnosis of DM and LE cannot be confirmed nor excluded. A rare lesion of SCLE may demonstrate granular immunoglobulin deposits within basal and to a lesser degree suprabasal keratinocytes cytoplasm and nuclei. This pattern was initially reported in patients with mixed connective tissue disease with high titers of anti-RNP antibodies but later found in other patients with other connective tissue diseases along with other antinuclear antibodies.

In SLE patients with anti-double stranded DNA antibodies and generally renal disease DIF of lesional skin, and sometimes normal skin is frequently positive. There is an obvious deposition of immunoglobulins and complement in the form of a band (thus the historical term Lupus band test) of granular and/or of homogeneous material.

How About Blood Tests?

Serologic testing is very useful in the differentiation of the above disorders.

The antinuclear antibody test (ANA), whether by traditional immunofluorescence or by the more recent quantitative ANA-Direct test, is positive in 80–85 % in DM and SCLE and 100 % in SLE.

The pattern is speckled in DM and SCLE, and homogeneous and or peripheral/rim in SLE. Disease-specific antibodies in SCLE are SSA/Ro and SSB/La, while there are multiple autoantibodies in DM and SLE.

Conclusions

In the evaluation of red, smooth patches, a biopsy specimen is useful to differentiate between vacuolar interface dermatitis of DM, SLE, and SCLE on one side and photodermatitis, spongiotic, or lichenoid on the other. Serological testing is extremely helpful. Phototesting may be necessary occasionally.