Solar Urticaria

A rare photodermatosis characterized by an abrupt onset of transient erythema, wheals, and pruritus appearing within minutes of exposure to light.

Epidemiology

Exact, prevalence and incidence of solar urticaria are not known. It is reported as representing only 4-8% of all photodermatosis presentations and estimated to account for only 0.4% of all urticarias. To date, more than 650 cases have been described worldwide. Females are predominantly affected.

Clinical description

Wheal formation and pruritus are characteristic, but other manifestations have been reported such as headaches, general malaise, increased heart rate, low blood pressure and syncope. Symptoms almost always clear within 24 hours. The disease can initially appear in all age groups, although it most commonly presents during the third or fourth decade of life. It is a chronic disease requiring long-term treatment. However, some patients (up to 15%) experience spontaneous remissions.

Etiology

The etiology and pathophysiology are not clearly defined; however, the disease may be caused by an antigen-antibody reaction. There are two possible mechanisms, antibodies are either targeted against an abnormal photoallergen which is generated only in patients with SU, or they are targeted against a normal photoallergen generated both in patients and in normal individuals. The immune response involves a photoallergic reaction resulting in histamine release by mast cells.

Diagnostic methods

Phototesting is an important tool for confirming the diagnosis, to assess the responsible wavelengths, the minimal whealing (urticaria) dose (MUD) and the response to therapy.

Differential diagnosis

The major differential diagnoses to be considered include polymorphic light eruption (which generally appears only 2-3 days after sun exposure and lasts more than 24 hours), lupus erythematosus tumidus (which typically occurs more than a week after sun exposure) and erythropoietic protoporphyria, (an inherited disease that may have similar clinical manifestations).

Management and treatment

Treatment is difficult and referral to a specialist is recommended. Treatment recommendations are based on retrospective studies and small case series. They include broad‐spectrum sunscreens, antihistamines, leucotriene receptor antagonist (LRA), UV phototherapy (hardening), azathioprine, ciclosporin, intravenous immunoglobulins and omalizumab. The European consensus is first-line treatment with non-sedative anti-histamines. If the disease is inadequately controlled, treatment is stepwise with increased dose, followed by omalizumab and then ciclosporin.

Prognosis

Therapeutic intervention can significantly improve the quality of life but response is variable. Spontaneous resolution of symptoms may occur in 15% of patients within 10 years. Some patients have to avoid outdoor activities completely, severely impacting their quality of life.