How should I evaluate and manage an otherwise healthy adult with intermittent heat‑induced urticaria?

Evaluation and Management of Intermittent Heat-Induced Urticaria

The most likely diagnosis is cholinergic urticaria, which is triggered by stimuli that provoke sweating rather than heat itself; confirm with detailed history, provocation testing, and treat with standard-dose second-generation H1 antihistamines as first-line therapy. 1, 2

Distinguish Between Heat Urticaria and Cholinergic Urticaria

The key diagnostic distinction is critical because these conditions are often confused:

• Cholinergic urticaria is triggered by stimuli that induce sweating (exercise, emotional stress, hot showers, spicy foods) when core body temperature rises, producing small 1-3mm itchy wheals that appear within minutes and resolve within 1 hour. 1, 3
• True heat urticaria is rare, occurs predominantly in women aged 20-45 years, and produces well-demarcated wheals only at the site of direct heat contact (typically requiring temperatures ≥44°C), not from generalized warming or sweating. 4
• The British Association of Dermatologists emphasizes that cholinergic urticaria is induced by the stimulus for sweating rather than overheating per se, even though raised core temperature is the usual reason for sweating. 1

Essential History Elements

Document these specific details to guide diagnosis:

• Wheal characteristics: Individual lesions lasting 2-24 hours suggest ordinary urticaria; lesions persisting beyond 24 hours raise concern for urticarial vasculitis and require skin biopsy. 2
• Specific triggers: Exercise, hot baths/showers, emotional stress, spicy foods (cholinergic); direct heat application to skin (heat urticaria); overheating plus physical exertion (exercise-induced anaphylaxis). 1, 4, 5
• Wheal size and distribution: Small 1-3mm punctate wheals favor cholinergic urticaria; larger wheals at contact sites suggest heat urticaria. 3, 4
• Associated systemic symptoms: Weakness, wheezing, headache, flushing, nausea, diarrhea, or syncope occur in approximately 50% of heat urticaria cases and warrant more aggressive management. 4
• Timing: Cholinergic urticaria appears within minutes of trigger and resolves within 1 hour; heat urticaria appears soon after heat exposure but may have delayed familial forms. 2, 4

Diagnostic Testing

• No routine laboratory work is needed for typical intermittent physical urticaria; investigations should be guided by history alone. 2, 6
• Provocation testing is essential to confirm the diagnosis and establish trigger thresholds:
  • For suspected cholinergic urticaria: exercise challenge or hot water immersion to induce sweating. 3, 5
  • For suspected heat urticaria: apply localized heat stimulus (warm water bottle, heated probe) to determine critical temperature threshold (mean ~44°C). 4, 5
• Consider screening tests only if history suggests chronic urticaria with poor antihistamine response: complete blood count with differential, ESR or CRP, thyroid autoantibodies, and thyroid function tests. 2

First-Line Treatment

Second-generation H1 antihistamines at standard licensed doses are the cornerstone of therapy for both cholinergic and heat urticaria. 2, 3

• Approximately 60% of heat urticaria patients respond to standard-dose nonsedating antihistamines, though full symptom relief occurs in only a minority. 4
• About 40% of pediatric urticaria patients achieve adequate control with antihistamine monotherapy. 2

Adjunctive Measures

• Avoid aggravating factors: Overheating, emotional stress, alcohol, aspirin, and NSAIDs (which trigger mast-cell degranulation). 2
• Short-course oral corticosteroids may shorten severe episodes but should be limited to brief courses; long-term use is inappropriate. 2
• Heat desensitization programs can be effective for heat urticaria when implemented systematically. 4
• Physical desensitization may benefit some patients with cholinergic urticaria. 3, 5

Escalation for Refractory Cases

If standard-dose antihistamines fail:

• Omalizumab (anti-IgE therapy) has proven effective in recent case reports for both heat urticaria and refractory chronic inducible urticaria. 3, 4
• Consider adding H2 antihistamines, hydroxyzine, or doxepin, though evidence for benefit is limited. 3
• Immunomodulatory agents (cyclosporine, tacrolimus) are reserved for severe chronic urticaria resistant to all other treatments. 3

Critical Pitfalls to Avoid

• Do not perform extensive laboratory work-ups for typical intermittent physical urticaria; they add no clinical value. 2, 6
• Do not confuse cholinergic urticaria with exercise-induced anaphylaxis: the latter involves systemic symptoms (hypotension, respiratory compromise) and requires epinephrine, not just antihistamines. 4
• Do not miss urticarial vasculitis: if wheals persist beyond 24 hours or leave bruising/hyperpigmentation, obtain a lesional skin biopsy. 2
• Recognize systemic symptoms: weakness, dyspnea, syncope, or gastrointestinal symptoms accompanying heat-induced wheals warrant emergency evaluation and consideration of anaphylaxis protocols. 4