Criteria:
(1) onset of pruritus and urticaria following exposure to sunlight or indoor lighting
(2) urticaria develop primarily in light exposed areas
(3) reactions can be elicited by phototesting
While often localized to limited portions of the body, sunbathing can result in a systemic reactions with anaphylaxis.
Classification:
(1) by chronology of development
(2) by light spectrum inciting reaction
(3) primary or secondary to other causes
Chronology of development:
(1) immediate (within minutes)
(2) delayed 18-72 hours after exposure
Secondary causes include erythropoietic protoporphyria (light causes photoactivation of the complement system). This can be tested for measuring erythrocyte protoporphyrin, fecal protoporphyrin and fecal coproporphyrin.
Differential diagnosis:
(1) drug-induced photoallergic reaction
(2) polymorphous light eruption
(3) heat urticaria
