Solar urticaria is one of the uncommon forms of physical urticaria. In the case of solar urticaria, the appearance of wheals and itchiness occurs after exposure to light, particularly sunlight. In common language light urticaria is (wrongly) sometimes called “sun allergy”.
Solar urticaria can occur at any age. Possible women are more frequently affected than men, however, little is yet known about the ratio among the sexes and the duration of solar urticaria. Solar urticaria often persists for many years, sometimes decades. Many solar urticaria patients suffer from further forms of urticaria in addition, such as, for instance symptomatic dermographism (urticaria factitia).
Solar urticaria typically begins suddenly in spring or summer. A few seconds or minutes after exposure to light (UVA, UVB or visible light), itchy wheals form on the skin in the areas exposed to the light. In rare cases wheal formation can also first appear several hours after solar exposure. All parts of the skin protected from the light generally remain unaffected. Lightweight clothing does not completely ward off UVA rays and visible light, this means that solar urticaria can also appear on “covered” areas of the body. Once the sun exposure is discontinued, the symptoms often disappear again in one or two hours. Exposure to lower intensity rays may only cause redness or small wheals, therefor it is not always easy to differentiate solar urticaria from other skin reactions triggered by light. If the entire body is exposed to the triggering rays, then serious symptoms like respiratory distress, dizziness or an anaphylactic shock may occur.
Light tests have shown that patients frequently on react to a certain part of the light range, i.e. only to radiation within a specific wavelength. These wavelengths are called action spectra. Some patients with solar urticaria are intolerant to visible light, other only react to, for us invisible, UVA radiation (340–400 nm wavelength) or to UVB radiation (280-320 nm) (Ill. 14a). Occasionally patients react both to visible light and also to UV radiation.
It is interesting to note that some patients, whose urticaria is only triggered by the light of the sun, can tolerate artificial light without any problems. For some 70 % of the patients with solar urticaria there is a specific wavelength range, beside the action spectrum, that actually suppresses the formation of wheals. Usually the wavelengths of the inhibition spectrum are longer than those of the action spectrum: If, for instance, the action spectrum is in the UVA range of wavelengths around 320-400 nm, the inhibition spectrum would potential be in visible light (400-780 nm). In most patients with solar urticaria the inhibition of urticaria, however, only occurs if the inhibition spectrum directly follows exposure to the action spectrum.
Heat urticaria is a very rare form of physical urticaria, wheals and itchiness occur on patches of skin that are exposed to heat (e.g. warm water or warm air). Therefore heat urticaria, if it occurs in summer, can be confused with solar urticaria. In the case of solar urticaria, as a rule only such areas of the skin are involved which have been exposed to the sun, whereas in the case of heat urticaria the wheals occur more on skin under clothing (heat generation).
To be on the safe side heat and light tests should confirm the diagnoses. Sometime skin changes may also be due to the so-called polymorphous light dermatitis which resembles those of solar urticaria. However, skin changes of polymorphous light dermatitis last much longer (days!). The same applies to photoallergic and phototoxic contact eczema and more rare diseases like lupus erythematosus and porphyria.
Contrary to the case of spontaneous chronic urticaria, little is known about the possible connection between solar urticaria and infections, food additives, allergies or other causes. Therefore solar urticaria is usually defined as “idiopathic” (= cause not clarified) and as a rule, a search for the cause is not recommended.
A light test should be carried out to diagnose solar urticaria. This involves exposing the skin (or parts of the skin) to light of various wavelengths to establish the triggering wavelength range. The test is made with a so-called “photo patch” on patches of the skin not usually exposed to the sun, e.g. on the back or buttocks (Ill. 14b). The light test may also be positive, for other skin diseases triggered by light; i.e. may trigger skin symptoms. However, it is typical for solar urticaria that the wheals appear within a few minutes after radiation on the test areas. These wheals are volatile and almost always subside after 1-2 hours.
Because it is not exactly known yet how and why solar urticaria occurs, there is currently no known approved effective therapy, which combats the cause of the disease. An attempt is therefore being made to either prevent the urticarial outbreak by protection against light or to alleviate the symptoms.
One relatively simple way to provide protection against exposure to the sun is to use sunscreens with a high light protection factor which filter out a wide range of light, as well to wear appropriate clothing and hats and to primarily stay in the shade. Unfortunately these measures are only effective for patients who react to ultraviolet light, but are of little benefit for urticaria triggered by visible light.
Another option of symptomatic treatment is the administration of antihistamines which block the effects of histamine. Apart from antihistamines all the drugs can be used for treatment which are currently also recommended for spontaneous chronic urticaria.
An alternative therapy that may be successful, but hardly feasible in reality, is a light desensitisation treatment. For this therapy initially only parts of the body, later the whole body, are exposed to light of the triggering wavelengths (phototherapy). The intensive exposure leads to the desensitisation of the skin to light and wheals do not occur so easily. A disadvantage is that the success of this treatment diminishes again when interrupted and continuous light treatments on the other hand, are associated with potential, negative long-term effects.