1.3.4 Photodermatoses

Polymorphous light eruption (PMLE) is the most common photodermatosis (prevalence 10% to 20%), followed in decreasing order of frequency by photoaggravated dermatoses, drug-induced photosensitivity, chronic actinic dermatitis (CAD) and solar urticaria (SU). Some photodermatoses are extremely rare, such as hydroa vacciniforme.

The clinical picture varies for each of the different processes. Skin eruptions usually develop or exacerbate in areas exposed to UV radiation following sun or artificial light radiation. The severity of symptoms, as well as the duration of sun exposure, may vary over time and be related to the season and climate. Lesions may manifest as - papules (PMLE, actinic prurigo, CAD), - blisters (PMLE, porphyrias, photoallergy or photo-toxicity), - urticarial lesions (SU, erythropoietic protoporphyria), - lichenified plaques or eczematous lesions (CAD, photoallergic contact dermatitis). Occasionally, scarring may occur (porphyria cutanea tarda, discoid lupus erythematosus, actinic prurigo, hydroa vacciniforme).

The eruption may develop within minutes as in solar urticaria (SU) or few hours or days after light exposure as in PMLE. “Hardening” phenomenon, defined as improvement of the eruption during the summer with increasing sun exposure may be observed in PMLE and SU. In photoaggravated dermatoses the evidence of characteristic lesions of the underlying dermatoses may be helpful for the diagnosis.

The most relevant idiopathic immune-mediated and drug-induced photodermatoses include:

Polymorphic light eruption (PMLE): Clinically polymorphous itchy rash that affects exposed skin (occasionally sparing the face) developing minutes to hours after sun exposure and tends to resolve in hours, days or rarely weeks. Macular, papular, papulovesicular, urticarial, erythema multiforme- and plaque-like lesions. More prevalent in young women. Recurrent rash every spring/summer. Probable delayed-type hypersensitivity reaction against endogenous cutaneous photoantigen/s.

Actinic prurigo (AP) is a rare, sunlight induced, itchy papular or nodular eruption, often excoriated. More frequent in native North, Central and South Americans.

Hydroa vacciniforme (HV). Rare acquired photodermatosis, usually with onset in childhood, and characterized by vesicle, crust and scar formation that follows exposure to sunlight.

Possible association with latent Epstein–Barr virus (EBV) infection.

Chronic actinic dermatitis (CAD) is a rare, persistent chronic (UV- and rarely visible) light-induced dermatosis. CAD occurs most commonly in elderly men. Eczematous, itchy eruption, often lichenified. Tends to persist over many years.

Solar urticaria (SU) is a rare UVB, UVA and visible light chronic inducible urticaria. Wheals confined to sunlight-exposed skin appear a few minutes after visible radiation exposure. Sudden onset and the lesions resolve in one or two hours. 50% of cases of Idiopathic SU resolve spontaneously within 5 years.

Drug- and exogenous chemical-induced photosensitivity include:

Phototoxic and photoallergic skin reactions which can be triggered by a variety of topical and systemic medications (mainly due to UVA light).

Phototoxic (non-immunologically mediated) reactions are the result of direct tissue injury caused by a phototoxic agent and radiation. Clinically may resemble acute sunburn. It can occur in all individuals exposed to adequate doses of the agent and the activating wavelengths of radiation.

Topical medications inducing phototoxicity include NSAIDs, antibiotics, corticosteroids, dyes, coal tar, psoralens, benzocaine, etc.

Systemic drugs may include: Antimicrobials (tetracyclines, quinolones, voriconazole), diuretics (furosemide, hydrochlorothiazide), NSAIDs (ketoprofen, naproxen), antimalarials, psoralens, retinoids, UVB calcium channel blockers, etc.

Photoallergic reactions are type IV delayed hypersensitivity response that occur only in sensitized individuals, and requires only a minimal concentration of the photoallergen. Clinically develop within 24 h after exposure to the photosensitizer and sunlight and resembles eczematous dermatitis.

Systemic medications causing photoallergy include NSAIDs (piroxicam, celecoxib) sulphur- containing drugs (hydrochlorothiazide, sulfadiazine, sulphonamides, sulfonylureas), antimalarials, antimicrobials (chloroamphenicol), phenotiazines, chlorpromazine, etc.

Some disorders usually restricted to area(s) of light exposure (PMLE and SU), whereas others may spread to distant areas (photoallergic reactions) or may also affect non-exposed skin (CAD or actinic prurigo). The lesions are sharply-demarcated, symmetrically distributed in sun-exposed areas including forehead, cheeks, V-region of neck, nape of neck, dorsum of the hands, and extensor aspects of forearms.

Photosensitive cutaneous disorders can be classified into four main categories:

1. Idiopathic photodermatoses, immunologically mediated where light alone triggers the disease e.g. polymorphic light eruption, solar urticaria, hydroa vacciniforme.

2. Drug- and chemical-induced photosensibility, including a subgroup of phototoxic or photoallergic dermatoses triggered by exogenous topical or systemic substances, and a second subgroup of metabolic disorders in which there is an endogenous accumulation of phototoxic substances because of enzymatic defects, e.g. cutaneous porphyrias.

3. Genodermatoses characterized by a defective DNA repair, in which there is increased sensitivity to light e.g. xeroderma pigmentosum secondary to a defect in DNA-repair enzymes.

4. Photoaggravated dermatoses. Include a heterogeneous group of disorders.

TABLE 1- PHOTODERMATOSES. CLASSIFICATION

I-  IDIOPATHIC PHOTODERMATOSES (Immunologically mediated)

II-   DRUG- AND CHEMICAL-INDUCED PHOTOSENSITIVITY

III - DEFECTIVE DNA REPAIR DISORDERS

IV-           PHOTOAGGRAVATED DERMATOSES

Histological changes are often non-specific, however, skin biopsy is helpful in the diagnosis of polymorphous light eruption and chronic actinic dermatitis. Immunophenotypic markers studies and gene rearrangement analyses are useful in differentiating chronic actinic dermatitis from cutaneous T-cell lymphoma, which may show sometimes clinical similarities.

Variable in the different disorders. PMLE may persist indefinitely, but often gradually improves, and occasionally remits, SU often presents a fluctuating course, with periods of improvement and remission, whereas in phototoxic/photoallergic skin reactions a variable duration of photosensitivity is noted after stopping a photoactive drug (from hours to moths).

A proper systematic evaluation, including a detailed clinical history are often most helpful. Neonatal lupus and genodermatoses present in infancy, hydroa vacciniforme (HV) in children, PMLE in young adults, and phototoxicity and chronic actinic dermatitis (CAD) are more common in elderly. Questioning regarding possible exposure to various photosensitizers, such as topical and systemic medications, cosmetics, fragrances and plant extracts may suggest a diagnosis of phototoxicity or photoallergy. Any underlying illness, personal or family history of autoimmune or connective tissue diseases, could add towards making the correct diagnosis.

Photodiagnostic procedures are essential. Monochromatic UVA and UVB sensitivity testing, photo patch testing, provocative challenge with UV and/ or visible light. Assessment of cutaneous response consists either in the development of lesions and the minimal erythema dose (MED). Photopatch testing is performed when photoallergic contact dermatitis is suspected. In some cases, patch testing should be recommended to rule out associated allergic contact dermatitis.

Photoprotection is the mainstay of treatment in all photodermatoses. To recommend to use protective clothing and sunscreen covering the action spectrum of the dermatoses (UVA in phototoxicity and visible light in solar urticaria). SPF 30 provides same protection as avoiding sunlight.

Recommend discontinuation of topical or systemic drugs. Topical steroids and antihistamines can be used for symptomatic relief. Any underlying primary illness (e.g. collagen vascular diseases) should be treated. Preventive UV phototherapy and⁄or psoralen plus UVA (PUVA) may be useful in some cases.

High-potency topical corticosteroids or even short courses of oral corticosteroids are helpful in symptomatic patients. Other possible systemic treatments for chronic, refractory cases: antimalarials or immunosuppressive drugs such azathioprine, ciclosporin, mycophenolate, JAK inhibitors or thalidomide.