What is the recommended approach for testing and diagnosing solar urticaria?

Diagnostic Approach for Solar Urticaria

The recommended approach for testing and diagnosing solar urticaria includes phototesting with defined radiation sources (UVB, UVA, visible light) to determine the action spectrum and minimal urticaria dose (MUD), which is essential for confirming the diagnosis and guiding treatment decisions. 1

Clinical Presentation and Initial Assessment

• Solar urticaria presents with erythema, wheals, and/or pruritus occurring within minutes after sun exposure, with lesions typically disappearing within 24 hours 2
• The most commonly affected areas are the V of the neck and arms, similar to polymorphous light eruption, but with much faster onset and resolution 2
• A thorough history should focus on timing of symptom onset after sun exposure, duration of lesions, and any history of atopy (present in approximately 48% of patients) 2, 3

Diagnostic Testing Protocol

Phototesting (Primary Diagnostic Tool)

• Phototesting with multiple light sources is the gold standard for diagnosis of solar urticaria 1
• Testing should include:
  • UVB (including narrowband UVB 311nm)
  • UVA (including UVA-1)
  • Visible light (green and red light) 1
• The diagnosis is confirmed when wheals develop within 30 minutes of exposure to specific wavelengths 2
• Determination of the minimal urticaria dose (MUD) is essential for both diagnosis and treatment planning 4

Important Diagnostic Considerations

• A negative phototest result from a single light source does not exclude solar urticaria - testing with multiple light sources is necessary 2
• If phototesting with artificial light sources yields negative results, a provocative test with natural sunlight should be performed 2
• Common action spectra include:
  • UVA (found in 70% of patients)
  • UVA-1 (found in 89% of patients)
  • Visible light (found in 37-67% of patients)
  • UVB (found in 44% of patients) 1, 3

Differential Diagnosis

• The diagnosis of urticaria is primarily clinical, and investigations should be guided by the history 5
• Key differential diagnoses include:
  • Polymorphous light eruption (distinguishable by longer duration of lesions)
  • Other physical urticarias (which may coexist with solar urticaria)
  • Chronic spontaneous urticaria with sun as a trigger factor 5, 2

Testing Algorithm

1. Initial evaluation: Document clinical history focusing on timing of lesion appearance and resolution after sun exposure 2
2. Phototesting protocol:
   • Test with multiple light sources (UVB, UVA, visible light) 1
   • Determine action spectrum and minimal urticaria dose (MUD) 4
   • If initial tests are negative, proceed with additional light sources or natural sunlight testing 2
3. Additional testing: Consider screening for atopy or other associated conditions (present in approximately 25% of cases) 3

Common Pitfalls and Caveats

• False-negative phototesting can occur with single light sources - always test with multiple wavelengths 2
• Solar urticaria may be associated with other forms of urticaria (particularly dermographic urticaria in 21% of cases) 3
• The condition typically affects younger patients (peak age 20-30 years) and may spontaneously resolve within 4-6 years in many patients 3
• Testing should be performed by centers experienced in photodermatology to ensure accurate diagnosis 4, 1

Treatment Planning Based on Diagnostic Results

• Treatment should be tailored according to disease severity and specific action spectrum identified during testing 4
• First-line therapy includes H1-antihistamines, which can be combined with leukotriene receptor antagonists based on severity 4
• For severe cases, rush hardening with UVA-1 phototherapy based on the identified action spectrum may be effective 1
• In refractory cases, omalizumab may be considered as a last resort therapy 1, 6