Background As little has been published on the course of idiopathic solar urticaria (SU) patients cannot receive comprehensive prognostic advice.
Objective To determine the prognosis and photobiological characteristics of idiopathic SU.
Design Historical cohort study, with inception cohort followed up from time of diagnosis. Follow-up for a median of 4 years (range, 3 months to 26 years) after diagnosis.
Setting Tertiary referral center for the investigation of photodermatoses in Scotland.
Patients The study included 87 patients, 61 (70%) of whom were female, with phototest-confirmed idiopathic SU between 1975 and 2000. Sixty patients (69%) were followed up clinically, and 25 patients (29%) were phototested on 2 or more occasions.
Interventions Investigations at time of diagnosis included monochromator phototesting. Further monochromator phototesting was performed in those patients in whom it was clinically indicated (select subgroup), and all patients who could be traced received a follow-up questionnaire.
Main Outcome Measures Characteristics of SU, responsible wave bands, and prognosis for clinical resolution.
Results The prevalence of idiopathic SU in Tayside, Scotland, is estimated to be 3.1 per 100 000. Action spectra were typically broad, with 63% reacting to more than 1 wave band, and the most common provoking wavelengths were the longer UV-A and the shorter visible ones. The majority of subjects were affected perennially (68%), by radiation transmitted through glass (83%) and thin clothing (76%). Coexistent polymorphic light eruption occurred in 20 patients (23%), and another photodermatosis occurred in 6 patients, 3 of whom had chronic actinic dermatitis. In those with SU alone, the mean age at onset was 41 years. The probability of clinical resolution at 5 and 10 years after diagnosis was 0.12 (95% confidence interval, 0.06-0.24) and 0.26 (95% confidence interval, 0.15-0.43), respectively.
Conclusion Idiopathic SU is a chronic disease. The majority of this cohort was still affected after 5 and 10 years.
