Phototesting
Solar urticaria has a wide action spectrum. Perform phototesting using broadband ultraviolet B (UV-B; 290-320 nm), UV-A (320-400 nm), and visible light sources (400-800 nm). Most patients with solar urticaria have provocative wavelengths in the UV-A and visible ranges, especially green or blue.
Duration of exposure under visible light should be less than an hour. Typically, the light emitted by a slide projector is used. Measures must be taken to avoid excessive heat output from the light source in order to eliminate the possibility of cholinergic or heat-induced urticaria instead of actual solar urticaria. A water filter may be placed in front of the light source to absorb excess heat.
Phototesting with narrowband UV-B (311-313 nm) is recommended if therapy with this light source is being considered. Occasionally, these tests do not produce the expected reaction.
Phototesting with other light sources (eg, solar simulators, lasers) may be necessary in difficult-to-establish cases. Provocation with natural sunlight may be performed if ambient conditions allow.
Many patients with solar urticaria have an inhibition spectrum. If their skin is first exposed to wavelengths known to induce solar urticaria and is then immediately afterward, or possibly concurrently, exposed to radiation within the inhibition spectrum, the urticarial reaction is either eliminated or diminished.
Laboratory Studies
There are no specific laboratory tests for diagnosis. In order to rule out other conditions, perform serologic tests for antinuclear antibody, Ro, and La antibodies to exclude connective-tissue disease (eg, lupus erythematosus).
Perform testing to exclude metabolic causes (eg, porphyrias). Evaluate the plasma porphyrin level; if abnormal results are found, follow with a quantitative 24-hour urinary and fecal porphyrin measurement, as well as erythrocyte porphyrin determination.
Histologic Findings
Histologic changes are typically found in the dermis in the form of vasodilatation, increased permeability of the vascular endothelium, and edema. Eosinophil infiltration and deposition of eosinophil granule proteins in the dermis are prominent during the early stages of the lesion. Neutrophils are found in increased numbers around the upper dermal vessels. Dermal mast cells may increase in number. After 24 hours, the dermal infiltrate is predominantly composed of mononuclear leukocytes.
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Solar urticaria. Courtesy of DermNet New Zealand (http://www.dermnetnz.org/assets/Uploads/reactions/solar-urticaria.jpg).
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Solar urticaria. Courtesy of DermNet New Zealand (http://www.dermnetnz.org/assets/Uploads/reactions/solar-urticaria1.jpg).
