What are the likely causes and recommended treatment for an intermittent pruritic rash confined to the legs of a frequent exerciser?

Itchy Rash on Legs in a Frequent Gym-Goer

Start with a second-generation non-sedating H1 antihistamine such as cetirizine 10 mg daily or loratadine 10 mg daily for exercise-induced urticaria, which is the most likely diagnosis in this patient. 1, 2

Most Likely Diagnosis: Exercise-Induced Urticaria

The intermittent nature of the rash that "comes and goes" in a frequent exerciser strongly suggests exercise-induced urticaria, which presents with isolated hives (10-15 mm wheals), generalized pruritus, flushing, and warmth triggered specifically by exercise. 2 This condition does not progress to systemic symptoms like vascular collapse or laryngeal edema, distinguishing it from the more serious exercise-induced anaphylaxis. 2, 3

Initial Treatment Algorithm

First-line therapy:

• Begin with cetirizine 10 mg daily or loratadine 10 mg daily 1, 2
• Offer at least two different second-generation antihistamines to trial, as individual responses vary significantly between agents 1, 2
• Other options include fexofenadine, desloratadine, or levocetirizine 2

If inadequate control after 2-4 weeks:

• Escalate to 4 times the standard dose (e.g., cetirizine 40 mg daily or loratadine 40 mg daily) 1, 2

If high-dose antihistamines fail:

• Add omalizumab 300 mg subcutaneously every 4 weeks, allowing up to 6 months for response 1, 2
• If insufficient response, increase to 600 mg every 2 weeks as maximum dose 2

Third-line option:

• Cyclosporine up to 5 mg/kg body weight added to antihistamines, effective in 65-70% of severe cases, but requires monitoring of blood pressure and renal function every 6 weeks 1, 2

Critical Screening Questions

Rule out food-dependent exercise-induced anaphylaxis (FDEIA):

• Ask specifically about temporal relationship between food ingestion and exercise within 4-6 hours 1, 2, 3
• Common culprits include wheat and soy 2
• If FDEIA suspected, advise avoiding exercise for 4-6 hours after consuming culprit foods 1, 2

Screen for medication triggers:

• Ask about NSAID or aspirin use before exercise, as 13% of exercise-induced anaphylaxis cases involve NSAID ingestion 1, 2, 3
• If identified, eliminate these medications 1

Assess for warning signs of exercise-induced anaphylaxis:

• Progression beyond isolated hives to angioedema, gastrointestinal symptoms, laryngeal edema, or vascular collapse requires immediate epinephrine and represents a medical emergency 1, 2, 3
• Approximately one-third experience transient loss of consciousness and two-thirds develop upper respiratory obstruction 2, 3

Alternative Diagnoses to Consider

Cholinergic urticaria:

• Presents with characteristic punctate wheals (1-3 mm, not 10-15 mm) triggered by core body temperature increase from exercise OR passive warming 1, 2, 3
• Responds particularly well to hydroxyzine 1, 2

Fungal infections (tinea corporis/cruris):

• Given gym environment exposure, consider tinea if rash has defined borders or scaling 4
• Terbinafine 1% cream applied daily for 1 week achieves 94% mycological cure rate 4
• Prevention includes thorough drying after showers, daily sock changes, and not sharing towels 4

Cutaneous larva migrans:

• If recent travel to tropical areas, presents as intensely itchy serpiginous rash migrating 1-2 cm per day 4
• Treat with ivermectin 200 μg/kg single dose or albendazole 400 mg daily for 3 days 4

Adjunctive Measures

Minimize aggravating factors:

• Identify and reduce overheating, environmental heat exposure, stress, and alcohol consumption 2

Symptomatic relief:

• Apply cooling antipruritic lotions such as calamine or 1% menthol in aqueous cream 2
• Topical hydrocortisone 1% cream can provide anti-itch relief 5

Common Pitfalls

The key pitfall is missing exercise-induced anaphylaxis, which requires the patient to carry epinephrine at all times and exercise with a companion trained in EpiPen use. 4 However, if the patient has only isolated hives without systemic progression, this is exercise-induced urticaria and does not require epinephrine. 2, 3

Another pitfall is assuming all exercise-related rashes are urticaria when fungal infections like tinea are extremely common in gym environments and require antifungal rather than antihistamine therapy. 4, 6, 7