Excerpt from Urticaria, Solar

Synonyms, Key Words, and Related Terms: solar urticaria, sun hives, allergy, allergic reaction, anaphylaxis, anaphylactoid reaction, angioedema, photodermatosis, pruritus, solar irradiation, minimum urticarial dose, MUD, polymorphous light eruption, PMLE, erythropoietic protoporphyria, lupus erythematosus, photocontact dermatitis, miliaria rubra, psoralen–UV-A, PUVA, phototherapy, UV-A, broadband UV-B, narrowband UV-B, photochemotherapy, methoxsalen

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Background: Solar urticaria is a rare photodermatosis characterized by pruritus, stinging, erythema, and wheal formation after a brief period of exposure to natural sunlight or an artificial light source emitting the appropriate wavelength.

Initially described by Merklen in 1904, the reaction is localized to exposed areas of the skin, although it can occur through thin clothing. Solar urticaria disappears within several minutes to a few hours, without pigmentary change if further sun exposure is avoided. This disorder can be quite disabling and difficult to manage. It often has a sudden, dramatic onset, and little information is available regarding its duration and eventual outcome.

Pathophysiology: Solar urticaria is possibly caused by an antigen-antibody reaction. Solar irradiation may induce an antigen in the serum or plasma of affected individuals. Intradermal injection of serum from a solar urticaria patient passively, but not consistently, transfers the condition to a healthy individual.

The following types of solar urticaria have been proposed:

Type I: This type is an immunoglobulin E (IgE)–mediated hypersensitivity to specific photoallergens generated only in solar urticaria patients.
Type II: This type is an IgE-mediated hypersensitivity to nonspecific photoallergens found in both solar urticaria patients and healthy individuals.

Passive transfer test findings are positive in patients with type II solar urticaria, but they may be positive or negative in those with type I.

The wide action spectrum (290-800 nm) implicated for this condition may be related to the specific photoallergen and its molecular weight. Diversity in the reported action spectra may be due to differences in photoallergens. In addition, spectra believed to be responsible for either inhibition or augmentation of the reaction have been detected. Complex interactions occur between the various wavelengths and the photoallergen.

The result of these interactions is mast cell degranulation with subsequent histamine release. Mediators other than histamines may also be involved. Inhibition of solar urticaria with light suppresses the wheal-flare response following intradermal injection of photoactivated autologous serum but does not suppress the wheal and flare associated with compound 48/80.

Frequency:

In the US: Solar urticaria comprises only 4% of US patients with photosensitive disorders.

Internationally: Solar urticaria comprises 5.3% of the cases of photosensitive dermatoses worldwide.

Mortality/Morbidity: The mortality rate has not been determined. In some cases, skin eruption is accompanied by symptoms such as headache, nausea, vomiting, bronchospasm, and syncope.

Race: The condition occurs in all races.

Sex: A slight female predilection is noted.

Age: Solar urticaria has a wide range of onset (10-70 y). The mean age of onset is 35 years, but it has been reported to occur in infancy.