Introduction:
Solar urticaria is a rare chronic disorder. The affected patients' skin is highly sensitive to sun exposure and other ultraviolet light (photosensitivity), resulting in repeated episodes of raised, erythematous (red), and itchy rashes on the sun-exposed areas of the skin known as urticaria. The condition is usually considered to be benign. However, it is extremely incapacitating, affecting the patient's day-to-day activities and seriously altering their quality of life.
What Is the Etiology of Solar Urticaria?
The leading cause of solar urticaria is direct exposure to sunlight. The radiation that comes between ultraviolet B and visible light. That is, from wavelength 300 nanometers to 500 nanometers are responsible for urticaria rashes. Ultraviolet sources other than sunlight also cause urticaria in these patients.
The exact mechanism of the rash development is not known. However, some studies suggest that an immune reaction plays a vital role. The body's immune system identifies the photodamaged cells as foreign bodies and initiates a histamine reaction (allergic reaction). The allergic reaction will lead to itching, inflammation, redness, and many other symptoms.
What Is the Epidemiology of Solar Urticaria?
Solar urticaria is rare and can occur in people of any race. Only 650 cases of solar urticaria have been reported worldwide, although it is more. Commonly observed in people with white skin. It accounts for approximately 7 % of all photodermatoses and 0.5 % of all urticaria cases. The condition is usually present in a person's mid-thirties. However, some rare cases in neonates and early childhood have also been reported. It has a female predominance. In about 16 % of the patients, solar urticaria was associated with other skin conditions like chronic allergies.
What Is the Pathophysiology of Solar Urticaria?
The pathophysiology of solar urticaria has yet to be entirely understood. However, it is considered an immediate hypersensitivity reaction mediated by IgE antibodies occurring post-sun exposure. When photosensitive skin is irradiated by sunlight, an endogenous substance in the epidermis or the serum called chromophore is activated. Activation of the chromophore will convert it into an immunologically active photo allergen. The photo allergen will initiate an immune reaction that results in mast cell degranulation, caution urticaria lesions.
Radiations in the shorter wavelength region, like the ultraviolet B rays, usually cause immunological allergic reactions. Radiations in the more extended wavelength regions can inhibit the immunologic reactions caused by the ultraviolet rays. The phenomenon is known as the double spectrum of action.
In some rare cases, solar urticaria is caused by an exogenous substance like some medications. Some examples of medications that may cause solar urticaria are
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Atorvastatin.
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Chlorpromazine.
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Tetracycline.
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Oral contraceptives.
What Is the Microscopic Appearance of Skin Affected by Solar Urticaria?
Microscopically the dermis and epidermis have a similar appearance to the classical urticaria. The skin shows mild to moderate edema with neutrophilic and eosinophilic infiltration around the blood vessels. The infiltration may be superficial to deep. Deep infiltrate may be perivascular or interstitial. The lesions usually have a purpuric appearance. However, leukocytoclastic vasculitis is absent.
What Are the Symptoms of Solar Urticaria?
The sun-exposed areas of skin will develop redness and edematous blemishes within a few minutes of exposure. Direct sun exposure or skin covered with white and light clothes will develop solar urticaria. Usually, covered skin has a more severe reaction to sun exposure. The commonly presented symptoms are
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Erythematous rashes.
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Intense itching.
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Burning sensation.
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Periorbital angioedema.
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Dermographism (eruption of the lesion on applying pressure or trauma).
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Nausea.
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Syncope.
Though systemic symptoms are not uncommon in solar urticaria, anaphylactic shock is a rare observation. The skin lesions and rashes disappear within a few hours in 75 % of cases. The intensity and duration of the exposed light determine the duration and severity of urticarial lesions.
How Is Solar Urticaria Diagnosed?
A case of solar urticaria is suspected based on the patient's recall of a previous incident. The clinical presentation of solar urticaria is similar to a typical urticarial lesion and some other photodermatoses. The differential diagnosis includes
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Lupus erythematosus.
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Polymorphous light eruption.
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Photo-contact dermatitis.
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Drug-induced photosensitivity.
The diagnosis is confirmed by photo-testing the patient's skin with visible light, ultraviolet A, and ultraviolet B. Often, natural light is also used for testing purposes. The photo-testing will help determine the radiation wavelength that triggers the immunological reaction. It will help identify the condition and aid in management by taking precautions. The histopathological examination will help rule out photodermatoses, and photo patch testing is done to rule out photo contact dermatitis and drug-induced photosensitivity.
What Are the Treatment Strategies for Solar Urticaria?
A definitive treatment guidance is not available for solar urticaria. However, the first and foremost advice given to the patients is to avoid sun exposure and use dark clothing and broad-spectrum sunscreens. Various treatment strategies have been implemented with a variable success rate. Second-generation antihistamine receptor antagonist agents, a strategy used for treating classical urticaria, are the most accepted first-line treatment for solar urticaria. The drugs include:
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Loratadine.
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Fexofenadine.
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Cetrizine.
The drugs are efficient in inducing relief in most cases. However, the dose required is two to four times greater than that for classical urticaria. Studies suggest that chronically sun-exposed areas are more tolerant to further damage, so treatment with photochemotherapy and phototherapy can induce tolerance.
Cases of refractory solar urticaria are treated with
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Intravenous immunoglobulins.
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Plasma exchange.
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Cyclosporine.
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Afamelanotide.
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Omalizumab.
Often, combination therapy is employed to achieve better results and maintain clinical outcomes.
What Are the Complications of Solar Urticaria?
Solar urticaria is considered to be caused by hypersensitivity type-1 reactions. In case of severe attacks, it may lead to episodes of syncope, bronchospasm, and life-threatening anaphylaxis.
Conclusion:
In conclusion, solar urticaria can be triggered by chronic exposure to sun rays. Though rare and benign, it has a chronic course. The condition usually appears in the mid-thirties and has a debilitating effect on the quality of life. Histamines and other inflammatory compounds are released in reaction to UV light due to an aberrant immune response, which is the cause of the condition. Although there is no known cure for solar urticaria, there are several ways to treat the symptoms, including antihistamines, clothing that protects from sun rays, and immunosuppressive medications. Phototherapy may also be utilized in extreme situations. People with solar urticaria are capable of leading regular lives with the right care and avoidance of triggers. Proper precautions will help prevent the development of urticarial lesions and aid the patients in leading better lives.
