Photosensitivity diseases: a review of sun-exposed skin rashes

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Rhinophyma, a severe form of rosacea, in a 60-year-old man.
Rhinophyma, a severe form of rosacea, in a 60-year-old man.

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Several acquired and inherited dermatologic disorders manifest as rashes on sun-exposed skin following exposure to solar radiation. These conditions are referred to as photosensitivity diseases. In this article, we highlight some specific types of photosensitivity dermatosis: the porphyrias, lupus erythematosus, rosacea, drug-induced photosensitivity, polymorphous light eruption, and solar urticaria. 

Porphyrias

The porphyrias are a group of photosensitivity diseases resulting from an acquired or inherited defect in the enzymes required to produce adequate amounts of heme (found in hemoglobin, myoglobin, and cytochrome P-450 enzymes), with the subsequent accumulation of porphyrins. The accumulation and deposition of porphyrins in the skin leads to photosensitivity.1 

Two common types of porphyria with a significant photosensitivity component are porphyria cutanea tarda (PCT) and erythropoietic protoporphyria (EPP). These types of porphyria are described as nonacute because they manifest primarily as rashes on sun-exposed skin. In contrast to the nonacute porphyrias, the acute porphyrias have significant systemic and neurologic manifestations. 

PCT is the most common nonacute porphyria (prevalence of 1 in 10,000).2 PCT is due to a dysfunction of the enzyme uroporphyrinogen decarboxylase (UROD), which is either genetic or acquired. Acquired dysfunction of UROD has been associated with excess levels of alcohol, iron, or estrogens. Following exposure to light, pigmentation, vesicles, bullae, erosions, crusts, and scarring of the face, forehead, and forearms may develop. The diagnosis of PCT is aided by the finding of increased porphyrin levels in the plasma, feces, or urine, which may have a characteristic deep-red color. Patients who have PCT may be treated with repeated phlebotomy to reduce blood iron levels or by withdrawal of alcohol, iron, or estrogens.3 

EPP is the second most common nonacute porphyria (prevalence between 1 in 50,000 and 1 in 75,000).2 It is due to an inherited dysfunction of the enzyme ferrochelatase, which causes protoporphyrin to combine with iron to form heme. Symptoms usually first appear in early childhood.4 Exposure to light may immediately result in burning, stinging, swelling, and pruritus. With repeated exposure to light, scars, lichen planus, and nail changes may develop.5 EPP can be diagnosed by the finding of increased porphyrins in the plasma or stool. It can be treated with oral ß-carotene and avoidance of the sun.6 

Lupus erythematosus

Photosensitivity reactions are very common in patients with lupus. Lupus is an autoimmune disorder with numerous subtypes, defined by whether the disease manifestations are solely dermatologic or both dermatologic and systemic. Cutaneous lupus erythematosus (CLE) is limited to the skin, and systemic lupus erythematosus (SLE) involves both the skin and other organs. The cutaneous manifestations of lupus can include malar rash, discoid rash, and photosensitivity. The systemic manifestations of lupus can include nonerosive arthritis, serositis, renal disorders, neurologic disorders, hematologic disorders, immunologic disorders, and the presence of antinuclear antibody.7 Following exposure to sunlight, diffuse erythema or exacerbation of the dermatologic manifestations of the disease may develop in patients with lupus. In this article, we will focus on the photosensitivity that can arise in patients with lupus, specifically in those with either of two subtypes of CLE. 

Subacute cutaneous lupus erythematosus (SCLE) is a subtype of CLE that has a significant photosensitivity component. The rash associated with SCLE appears as plaques and/or papulosquamous lesions on sun-exposed areas, such as the shoulders, chest, and extremities.5 The diagnosis of SCLE can be aided by serologic findings such as elevated levels of antinuclear antibodies, anti-Ro antibodies, anti-La antibodies, and rheumatoid factor.8 The diagnosis can also be made through skin biopsy and subsequent direct immunofluorescence microscopy.9 Although some patients with SCLE may respond to nonsystemic treatment, such as topical tacrolimus and/or the avoidance of natural and artificial ultraviolet (UV) light, most patients require systemic therapy with an antimalarial agent, such as hydroxychloroquine.10 

Another subtype of CLE with skin manifestations of significant photosensitivity is discoid lupus erythematosus (DLE). DLE appears as circular (discoid), dark red, scaly plaques that are associated with features such as scarring and loss of skin pigment. The lesions develop on sun-exposed areas, such as the extremities, face, and upper chest.5 A malar rash and alopecia are also common in patients with DLE. The cutaneous signs of DLE are exacerbated by sun exposure. The diagnosis of DLE can be aided by skin biopsy and direct immunofluorescence. Although most patients with DLE respond to the avoidance of sunlight and the application of topical corticosteroids, some may require systemic therapy with an antimalarial agent, such as hydroxychloroquine.10

Rosacea

Rosacea is a dermatologic disease characterized by erythema, solar urticaria, or telangiectasia of the face. Rosacea manifests following repeated exposure to anything that causes facial blushing, such as sunlight, heat or cold, and spicy food. Alcohol consumption and some emotions may also cause facial blushing.11

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