What is the optimal treatment plan for an 18‑year‑old man with tinea corporis due to Trichophyton tonsurans who has persistent lesions after two months of oral terbinafine 250 mg daily and topical terbinafine cream?

Treatment Plan for Terbinafine-Resistant Tinea Corporis

Switch to itraconazole 200 mg daily for 4 weeks as second-line therapy for this Trichophyton tonsurans infection that has failed to clear after two months of terbinafine. 1

Rationale for Treatment Change

Your patient has demonstrated clinical improvement but persistent lesions after 8 weeks of oral terbinafine—well beyond the standard 2-4 week duration recommended for tinea corporis caused by Trichophyton species. 1 This represents treatment failure, not simply incomplete response. The British Association of Dermatologists guidelines explicitly state that when there has been initial clinical improvement but ongoing positive mycology (or in this case, persistent lesions), you should proceed to second-line therapy rather than simply extending the current regimen. 1

The key algorithmic decision point: Has there been initial clinical improvement? Yes → Proceed to second-line therapy. No initial improvement → Also proceed to second-line therapy. 1

Recommended Second-Line Regimen

• Itraconazole 200 mg orally once daily for 4 weeks is the guideline-recommended second-line agent for Trichophyton infections that fail terbinafine. 1
• Itraconazole has excellent activity against both Trichophyton and Microsporum species, making it ideal when terbinafine fails. 1
• Critical administration detail: Itraconazole must be taken with food and acidic beverages (orange juice, cola) to optimize absorption. 1

Pre-Treatment Requirements

Before prescribing itraconazole, you must:

• Obtain baseline liver function tests (ALT, AST) and complete blood count. 1, 2
• Review all concurrent medications for critical drug interactions—itraconazole is contraindicated with terfenadine, astemizole, sertindole, midazolam, and cisapride due to enhanced toxicity. 2
• Check for heart failure history—itraconazole is absolutely contraindicated in patients with heart failure due to negative inotropic effects. 1, 2
• If the patient takes statins, warfarin, digoxin, or ciclosporin, be aware that itraconazole markedly increases their serum concentrations and requires dose adjustments or close monitoring. 2

Essential Adjunctive Measures

Addressing concomitant tinea corporis is critical—a recent study found that concomitant tinea corporis significantly increases treatment failure risk (odds ratio 3.9) in Trichophyton tonsurans infections. 3 Your patient has persistent lesions on legs and arm, suggesting this is a factor.

Topical Therapy

• Continue or restart topical antifungal therapy alongside oral itraconazole—combination therapy provides antimicrobial synergy and suppresses resistant mutants. 2
• Apply topical terbinafine, clotrimazole, or miconazole cream to all affected areas twice daily. 2, 4

Environmental Decontamination

• Decontaminate or replace contaminated clothing and bedding to eliminate fungal reservoirs. 2
• Wash all clothing, towels, and bedding in hot water. 2
• Consider discarding items that cannot be adequately decontaminated. 2

Critical Diagnostic Consideration: Terbinafine Resistance

While uncommon, terbinafine resistance in Trichophyton rubrum has been documented with point mutations in the squalene epoxidase (SQLE) gene. 5 Although your patient has T. tonsurans (not T. rubrum), the prolonged terbinafine exposure raises this possibility.

• If itraconazole also fails, consider requesting fungal culture with terbinafine susceptibility testing and SQLE gene sequencing. 5
• Documented terbinafine-resistant cases have responded to long-term itraconazole therapy. 5

Monitoring During Itraconazole Therapy

• Monitor hepatic function tests if treatment extends beyond 4 weeks or if the patient has pre-existing liver abnormalities. 1, 2
• Common adverse effects include headache and gastrointestinal upset. 1
• Re-evaluate at 4 weeks with clinical examination and consider repeat KOH/culture to confirm mycological cure—clinical improvement without mycological clearance predicts recurrence. 6

If Itraconazole Fails

Should itraconazole fail after 4 weeks:

• Third-line option: Fluconazole 150-450 mg once weekly for at least 6 months, though this is significantly less effective. 1, 2
• Alternative agents for refractory cases: Voriconazole has been used successfully in exceptional circumstances, though it should be reserved for truly refractory infections. 1, 5

Common Pitfall to Avoid

Do not simply extend terbinafine duration. The British Association of Dermatologists guidelines specifically state that "prolonging treatment does not improve efficacy" for terbinafine, and at higher doses, terbinafine is more effective—but you've already used the standard dose for twice the recommended duration. 1 Continuing terbinafine will not achieve cure and delays appropriate second-line therapy.