Testing with precise light ranges has shown that patients are frequently only sensitive to a portion of the light spectrum, i.e. react only to radiation within a specific wavelength range (Ill. 7). These wavelength ranges are called the action spectrum. Most patients with solar urticaria (approx. 60%) are intolerant to visible light, roughly 30% react only to UVA radiation (300-400 nm wavelength, invisible to us). Less common is intolerance to UVB radiation (280–320 nm). It occasionally happens that patients react both to visible light and UV radiation. Interestingly, there are also patients whose urticaria is only triggered by the light of the sun, but for whom artificial light represents no problem.
For some 70% of patients with solar urticaria, there is a specific wavelength range beside the action spectrum that actually suppresses urticaria formation. This is called the "inhibition spectrum." The wavelengths of the inhibition spectrum are generally longer than those of the action spectrum: if, for example, the action spectrum lies in the UVA range with wavelengths of 320–400 nm, the inhibition spectrum would potentially lie in the visible light (400–780 nm). For most patients with solar urticaria, however, the inhibition of the urticaria only occurs if the exposure to the inhibition spectrum follows chronologically directly after the exposure to the action spectrum.
With heat urticaria, a very uncommon form of physical urticaria, wheals and itchiness occur at those spots on the skin that have been exposed to heat (e.g. warm water or warm air). This means that heat urticaria first occurring during the summer is occasionally misdiagnosed as solar urticaria. With solar urticaria, it's almost always the case that only those parts of the skin exposed to light are affected, whereas heat urticaria involves wheals on covered parts of the skin (heat generation). In case of doubt, heat and light testing should be performed to ascertain a clear diagnosis.
Sometimes the skin changes observed with polymorphous light eruption resemble those of solar urticaria. There are a few key differences, however. Those from polymorphous light eruption tend to last much longer (days!) and have an eczema-like, knotty character. The same is true of photoallergic and phototoxic contact eczema, and rare diseases like lupus erythematodes and porphyria.