Photosensitivity and Joint Dysfunction
Cutaneous photosensitivity may be associated with joint dysfunction in five separate ways. Photosensitivity may cause joint dysfunction through physical inhibition of flexion or extension following cutaneous damage, such as in congenital erythropoietic porphyria, erythropoietic protoporphyria, polymorphic light eruption and chronic actinic dermatitis. The disruption may be permanent as in congenital erythropoietic porphyria, but more commonly resolves, as in the other conditions. Secondly, photosensitivity may lead to a disease causing joint dysfunction, and this appears to be the case with both lupus erythe- matous and dermatomyositis in certain instances. In other words exposure to ultraviolet causes cutaneous changes, which may in certain circumstances lead to the diseases of lupus or dermatomyositis, which then causes the joint dysfunction. A further association is that diseases which demonstrate photosensitivity may by chance have joint dysfunction as part of that disease, but not clearly linked directly to the photosensitivity. Such disorders are psoriasis, acne rarely, and drug photosensitivity. Yet another way in which photosensitivity may be associated with joint dysfunction is that light may be used to treat the disease which causes the joint dysfunction, such as for example in the UVA phototherapy of lupus, albeit an uncommon mechanism of treating that disease. Other diseases in this category are psoriasis, rarely acne, polymorphic light eruption, chronic actinic dermatitis and solar urticaria, all conditions associated with joint dysfunction which may be treated with light therapy. Finally the joint dysfunction itself of lupus may occasionally respond to UVA phototherapy.