Solar urticaria (SU) is a rare condition in which exposure to ultraviolet or UV radiation, or sometimes even visible light, induces a case of urticaria or hives that can appear in both covered and uncovered areas of the skin. It is classified as a type of physical urticaria. The classification of disease types is somewhat controversial. One classification system distinguished various types of SU based on the wavelength of the radiation that causes the breakout; another classification system is based on the type of allergen that initiates a breakout.The agent in the human body responsible for the reaction to radiation, known as the photoallergen, has not yet been identified. The disease itself can be difficult  to diagnose  properly because it is so similar to other dermatological disorders, such as polymorphic light eruption or PMLE. The most helpful test is a diagnostic phototest, a specialized test which confirms the presence of an abnormal sunburn reaction. Once recognized, treatment of the disease commonly involves the administration of antihistamines, and desensitization treatments such as phototherapy. In more extreme cases, the use of immunosuppressive drugs and even plasmapheresis may be considered.The initial discovery of the disease is credited to P. Merklen in 1904, but it did not have a name until the suggestion of "solar urticaria" was given by Duke in 1923. However, their research contributed to the study of this uncommon disease. More than  one hundred cases have been reported in the past century.
Solar urticaria may occur in all races but studies monitoring 135 African Americans and 110 Caucasians with photodermatoses found that 2.2% of the African Americans had SU and 8% of the Caucasians had the disease showing that Caucasians have a better chance of getting the disease. The age ranges anywhere from 5–70 years old, but the average age is 35 and cases have been reported with children that are still in infancy. Solar urticaria accounts for less than one percent of the many documented urticaria cases. To put that into a better perspective, since its first documented case in Japan in 1916, over one hundred other instances of the disease have been reported. History
Solar urticaria was first identified by P. Merklen in 1904. Just a year later, in 1905, Ward became the first to induce urticaria through exposure to the sun in a controlled environment. The first documented case came in Japan in 1916. The name "solar urticaria" was proposed in 1923. In 1928 urticaria was induced for the first time. This was carried out by phototesting with increasing amounts of radiation of varying wavelengths. In 1942 the disease was passively transferred to normal volunteers using serum from patients with solar urticaria. References


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