Solar urticaria is process that takes place with very little frequency and it is characterized by the development of hives and erythema a few minutes after the skin has been exposed to sunlight. There can be found six types of solar urticaria in function of the wavelength of light which triggers the reaction and which is not possible to passively transfer the phenomena with serum.
- The types I and IV are observed with wavelengths of 2800 to 3200 angstrom and with wavelengths of 4000 to 5000 angstrom respectively. Both types can be passively transferred with serum.
- Types II, III and V are induced by wavelengths of 3200 to 4000 angstrom, of 400 to 500 nanometers and of 2800 to 5000 angstrom respectively and these types cannot be passively transferred with serum.
- The type VI is a metabolic disorder (erythropoietic protoporphyria), on which the protoporphyrin IX acts as a photosensitizer at a wavelength of 4000 angstrom.
Although the pathogenesis of this condition is not known with assurance, it has been proven in some cases the release of mediators (histamine, ECF-A and NCF) done by mast cells on these patients after the light provocation or sunlight exposure. Besides, in the types I and IV the test of transfer suggest that the irradiation of skin produces a photoproduct which is an antigen and reacts with any serum factor giving place to the release of mediators and urticaria. On types II, III and V the mechanism of the condition is unknown.
Clinical manifestations for solar urticaria
The presence of this condition is more frequent in adults although it can also emerge at any age. Urticaria hives emerge one to three minutes after the skin has been exposed to sunlight. Typically appears in first instance the pruritus and it is followed the erythema and hives distributed on the areas that have been exposed to sunlight. The lesions are evanescent disappearing generally after one to three hours.
When big areas of the body are exposed, there can emerge angioedema and systemic symptoms such as bronchospasm, hypotension and headache. On the contrary when the exposition is not very intense and in cutaneous areas chronically affected like hands or face (areas of skin which are usually less reactive) only an erythema can be developed. Since the radiation of ultraviolet rays and visible light can pass through thin dresses, sometimes lesions can also emerge on covered zones. Fluorescent light is also capable of producing the appearance of urticaria hives. Once the condition appears it tends to last for many years (ten years in average).
Diagnosis for solar urticaria
It can be confirmed with a photo-test which consists on reproducing the lesions a few minutes after the exposure of skin to a natural source of light (sunlight), or artificial light (quartz lamp or a fluorescent tube). The use of filters can separate the radioactivity on its different types. The response abnormally delayed is useful to establish a difference for patients with a polymorphous solar flare.