Urticaria: solar urticaria

Solar urticaria is a rare type of inducible urticaria caused by UV radiation or visible light.

This chapter is set out as follows: 

• An allergic type I hypersensitivity reaction mediated by histamine and other agents 

• It can affect any age

• Symptoms - a sore, itchy rash that arises on sun-exposed areas within minutes of UV exposure and resolves within 30 minutes of UV avoidance. Triggered by UVA, UVB and visible light

• The differential diagnoses include:
  • Lupus erythematosus
  • Erythrohepatic protoporphyria

Distribution

• Mainly exposed sites, although the face and dorsum of hands may be spared. Non-exposed sites covered by thin clothing may be affected

• It may be necessary to rule out lupus erythematosus by checking blood tests for ANA and ENA (particularly anti-Ro and La autoantibodies). ENA levels can be positive even if the ANA screen is negative - most labs will not check ENA (if ANA negative) unless it is specifically asked for on a request form stating that lupus is possible

Step 1: general measures

• Provide a patient information leaflet on UV protection, including through windows - for more information on sunscreen, including the Dundee formulation, refer to the related chapter Photodermatoses: an overview 

Step 2: antihistamines

• Non-sedating antihistamines will be required, often at high doses

Step 3: referral

• Patients are often best referred to a dermatologist for:
  • Further investigations - it can be useful to photo-test patients to assess which wavelengths trigger their symptoms and how easily the urticaria is provoked
  • Treatment - consideration of graduated phototherapy in early spring to help improve tolerance, or immunosuppressive therapy in severe cases