What causes red rashes that appear with sweating or increased heart rate and then subside?

Cholinergic Urticaria

The red rashes that appear with sweating or increased heart rate and then subside are most consistent with cholinergic urticaria, a physical urticaria triggered by stimuli that induce sweating rather than heat itself. 1

Clinical Presentation

Cholinergic urticaria is characterized by:

• Small, transient weals (hives) that develop rapidly when core body temperature rises or sweating is triggered 1
• Lesions typically appear within minutes of the provoking stimulus (exercise, hot showers, emotional stress, spicy foods) 1
• Individual weals resolve within 30-60 minutes, distinguishing this from ordinary urticaria where lesions persist 2-24 hours 1
• The rash is often pruritic and may be accompanied by a prickling or burning sensation 1

Mechanism and Triggers

• Cholinergic urticaria is primarily induced by the stimulus for sweating rather than overheating per se, though elevated core temperature is the usual reason for sweating 1
• Common triggers include physical exercise, hot baths or showers, emotional stress, and consumption of hot or spicy foods 1
• The condition represents a reproducible physical urticaria where the same physical stimulus consistently induces the reaction 1

Diagnostic Approach

When evaluating this presentation:

• The rapid onset with activity/heat and quick resolution (under 1 hour) is pathognomonic for cholinergic urticaria 1
• No routine laboratory testing is required for typical presentations of physical urticaria 1
• If symptoms are atypical or persist beyond expected timeframes, consider measuring inflammatory markers (ESR, CRP) to exclude urticarial vasculitis, where lesions persist for days 1
• Challenge testing can confirm the diagnosis by reproducing symptoms through exercise, hot water immersion, or other heat-inducing activities 1

Important Distinctions

This presentation should be differentiated from:

• Ordinary urticaria: spontaneous weals lasting 2-24 hours without specific physical triggers 1
• Exercise-induced anaphylaxis: may progress to systemic symptoms including respiratory compromise and hypotension, not just localized skin changes 1
• Other physical urticarias: cold urticaria, delayed pressure urticaria (takes 2-6 hours to develop, persists up to 48 hours), or symptomatic dermographism 1

Management Strategy

First-line treatment consists of nonsedating H1 antihistamines taken regularly, not just as needed 1:

• Cetirizine, desloratadine, fexofenadine, levocetirizine, or loratadine taken once daily 1
• If standard doses are ineffective, updosing antihistamines up to 4 times the standard dose may be beneficial 1
• Cetirizine may cause sedation at higher doses 1

Lifestyle modifications are critical since defining the stimulus provides opportunity to minimize or prevent urticaria 1:

• Avoid rapid temperature changes and overheating 1
• Gradual warm-up before exercise 1
• Avoid hot showers/baths when possible 1
• Minimize stress and avoid known aggravating factors including alcohol 1

Clinical Pitfalls

• Do not confuse this with anaphylaxis unless systemic symptoms develop (respiratory distress, hypotension, gastrointestinal symptoms) 1, 2
• The absence of fever distinguishes this from infectious exanthems 3, 4
• Aspirin and NSAIDs should be avoided as they may worsen urticaria 1
• Some physical urticarias may be especially persistent, so counsel patients that while prognosis is generally excellent, resolution may take time 1