They are popularly known as “sun allergy” but, in reality, it is not the correct clinical term. They are photodermatoses. It is a group of skin diseases whose primary symptom or phenomenon is pathological photosensitivity. Photosensitivity is an abnormal reaction to ultraviolet radiation. The most common are acquired idiopathic photodermatoses, with spontaneous onset or unknown cause, such as polymorphic solar eruption.
Causes of sun allergy
Abnormal reaction to ultraviolet radiation
Photodermatoses, popularly known as sun allergies, are caused by an abnormal reaction to ultraviolet radiation and can be:
– Acquired idiopathic photodermatoses such as: polymorphous solar eruption, actinic prurigo, Bazin’s hydroa vacciniforme, solar urticaria or chronic actinic dermatitis (photosensitive eczema, actinic reticuloid, persistent reactivity to light and photosensitivity dermatitis). Chronic actinic dermatitis is believed to have an immunological basis.
– Disorders due to defective DNA repair such as: xeroderma pigmentosum, Bloom syndrome or Cockayne syndrome.
– Photosensitization by chemical substances (exogenous or endogenous): porphyrias, photoallergies and photosensitivity. They can be associated with the same drugs as: antiarrhythmics, contraceptives, anticonvulsants, antihypertensives, oral antidiabetics, antiparasitics, psoralens, cytostatics, antidepressants, diuretics, antipsychotics, antihistamines, non-steroidal anti-inflammatory drugs and/or antimicrobials.
– Dermatoses exacerbated by ultraviolet radiation such as: acne vulgaris, carcinoid syndrome, dermatomyositis, disseminated superficial actinic prorokeratosis, lichen planus, lupus erythematosus, pellagra, rosacea, flat warts, pityriasis alba or Haley-Haley disease.
– Solar urticaria. It is a type 1 hypersensitivity reaction mediated by immunoglobulin E (Ig E). It is very rare and usually appears, especially in women, between the ages of 20 and 40. It begins with itching and erythema and within a few minutes, wheals appear on sun-exposed areas such as the face and hands that disappear in one or two hours, and do not leave a scar, as sun exposure is avoided.
Symptoms of sun allergy
They depend on the type of photodermatosis
The signs and symptoms of so-called sun allergy depend on the type of photodermatosis. The most common are photodermatoses that arise spontaneously or have an unknown cause (idiopathic):
– Polymorphic solar eruption: It affects women more than light-skinned men (phototype I/IV). It can appear from childhood until, generally, the age of 30. They usually have a family history. It typically occurs in spring and fades as the sunny and hot months progress. Lesions appear on the skin, generally on the chest, (papules, papulovesicles, plaques, vesiculoblisters or eczema), although it is true that you may only feel itching. Lesions that occur between 30 minutes from the beginning of sun exposure until several hours later. These lesions disappear between one and seven days later without leaving a scar.
– Actinic prurigo: It usually appears between the ages of five and ten, especially in girls, and disappears at puberty. Patients usually have a family history. Actinic prurigo presents with a rash that can remain year-round but worsens in summer. It presents as open (excoriated) papules and nodules, very itchy and, normally, with eczema, lichenification and scabs.
– Chronic actinic dermatitis encompasses photosensitive eczema, actinic reticuloid, persistent reactivity to light and photosensitivity dermatitis: It has an insidious onset. It is diagnosed more frequently in older men and begins with itching on the face, neck and back of the hands. The itch evolves into eczematous lesions, papules and infiltrated patches that may appear in exposed areas but later appear in areas normally covered by clothing.
– Bazin’s Hydroa vacciniforme: It is very rare and usually appears in children up to ten years of age with clear phototype I/II. It usually coexists with atopic dermatitis. Erythema appears, usually on the face, after prolonged sun exposure that evolves into vesiculoblisters and leaves scars when healing. There may be fever and general malaise.
– Photosensitization by chemical substances (exogenous or endogenous): porphyrias, photoallergies and photosensitivity: Lesions appear compatible with an exaggerated sunburn, with no proportion between the intensity of sun exposure and that of the lesions. In the area exposed to the sun there may be erythema, edema, vesicles, blisters, hyperpigmentation, burning, stinging and pruritus. If the cause is photoallergic, eczematous plaques with peeling and vesiculoblisters of insidious onset and itching appear.
Diagnosis of sun allergy
Physical examination, medical history and tests
The doctor will need to know the family history, the usual or recent treatments that the patient is following or has followed, how the disease began, what its evolution is and the previous history of photoexposure, as well as what the skin response to sunscreens is.
Additionally, a complete blood test, blood biochemistry, detection of antinuclear antibodies (ANA), antibodies (anti DNA, anti Ro and anti LA), porphyrins in urine and blood, and immunofluorescence biopsy will be requested.
It is key to make a differential diagnosis to be able to conclude on the type of photodermatosis that affects the patient and to be able to apply the appropriate treatment.
Sun allergy treatment and medication
Suitable for the type of photodermatosis
Treatment must be appropriate to the type of photodermatosis:
– Polymorphous solar rash: Avoid sun exposure and use broad-spectrum sunscreens. In severe cases, prophylactic photochemotherapy in low doses can be considered. In patients in whom the rash is not controlled with these measures, topical treatment with corticosteroids or a short course of oral corticosteroids will be prescribed.
– Actinic prurigo: It is recommended to cover the skin, avoid sun exposure and use broad-spectrum photoprotectors. Thalidomide is also prescribed in intermittent cycles at low doses.
– Chronic actinic dermatitis: Full screen photoprotectors with low allergenic potential are prescribed.
– Solar urticaria. The treatment is based on avoiding sun exposure and using broad-spectrum photoprotectors. To help the skin get used to (hardening phenomenon), a treatment of oral antihistamines and beta-carotenes can be prescribed a month before.
– Bazin’s Hydroa vacciniforme: It is very rare and usually appears in children up to the age of ten. It usually coexists with atopic dermatitis. Sun exposure must be avoided and prophylactic photochemotherapy is usually considered in some cases.
– Photosensitization due to chemical substances (exogenous or endogenous): Eliminating the substance that causes the symptoms is the main treatment, although possible chronicification must be monitored. Sunscreens do not protect against photosensitization by chemical substances. For example, if you are following a treatment with antibiotics, you should avoid sun exposure during the treatment and up to three days after finishing it.
Sun allergy prevention
Sun protection and avoiding sun exposure
Use sunscreens that protect the skin from sun exposure, appropriate to the type of skin and state of health; and avoiding sun exposure, even with clothing that covers arms and legs as well as hats, are the main prevention measures against possible photodermatosis.
It is also important to remember that if you are following a treatment with antibiotics, you should not sunbathe or be exposed to light alone during the treatment and up to three days after finishing it.
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