What are the concerns with a 17-year-old male with post-infectious glomerulonephritis and impaired renal function taking cotrimazole and terbinafine for a possible fungal infection?

Terbinafine Should Be Discontinued Immediately in This Patient

In a 17-year-old male with post-infectious glomerulonephritis and impaired renal function, terbinafine is absolutely contraindicated and must be stopped immediately. 1, 2

Critical Contraindication: Renal Impairment

Terbinafine is explicitly contraindicated in patients with creatinine clearance ≤50 mL/min. 1, 2 The FDA drug label clearly states that "in patients with renal impairment (creatinine clearance less than or equal to 50 mL/min) the use of terbinafine tablets has not been adequately studied." 2

Why This Matters for Post-Infectious Glomerulonephritis

• Post-infectious glomerulonephritis commonly causes acute kidney injury with reduced glomerular filtration rate during the acute phase 3
• Terbinafine is primarily cleared by the kidneys, with approximately 70% of the administered dose eliminated in urine 2
• In patients with renal impairment, terbinafine clearance is decreased by approximately 50% compared to normal volunteers, leading to drug accumulation 2
• The British Association of Dermatologists explicitly lists renal impairment as a contraindication rather than recommending dose adjustment 1

Additional Serious Concern: Rhabdomyolysis Risk

Terbinafine has been associated with severe rhabdomyolysis that can cause acute kidney injury requiring dialysis. 4

• A case report documented a previously healthy 22-year-old male who developed severe rhabdomyolysis (CK >100,000 U/L) and anuric acute kidney injury after only 9 days of terbinafine 250 mg daily 4
• In a patient with already compromised renal function from glomerulonephritis, this risk is substantially amplified 4
• Early recognition is critical, as delayed treatment can lead to electrolyte abnormalities, worsening acute kidney injury, and disseminated intravascular coagulation 4

Immediate Management Steps

1. Discontinue Terbinafine Immediately

• Stop all terbinafine administration now 1, 4
• Document the contraindication in the medical record 1

2. Assess Current Renal Function

• Obtain immediate serum creatinine and calculate creatinine clearance 1, 2
• Check for signs of rhabdomyolysis: measure creatine kinase (CK), assess for muscle pain, weakness, or dark urine 4
• Monitor electrolytes, particularly potassium 4

3. Monitor for Complications

• If CK is elevated or patient has muscle symptoms, initiate aggressive hydration and consider bicarbonate therapy 4
• Serial monitoring of renal function is essential given the dual insult of glomerulonephritis and potential drug toxicity 4

Alternative Antifungal Options

For patients with renal impairment requiring antifungal therapy, topical treatments are strongly preferred. 1

First-Line Alternatives (Topical Therapy)

• Amorolfine 5% lacquer applied once or twice weekly for 6-12 months 5, 1
• Ciclopirox 8% lacquer applied once daily for up to 48 weeks 5, 1
• These topical agents have minimal systemic absorption and are safe in renal impairment 1

Second-Line Systemic Options (Only if Absolutely Necessary)

• Itraconazole may be considered if hepatic function is normal, though it also requires dose adjustment when creatinine clearance <30 mL/min 1
• Fluconazole requires 50% dose reduction when GFR <45 mL/min and is contraindicated in severe renal impairment 5, 1
• Griseofulvin is an alternative but also requires caution in renal impairment 5

Critical Pitfall to Avoid

Never assume that a medication is safe simply because it has been prescribed for 4 days without obvious immediate complications. 4 The case report of terbinafine-induced rhabdomyolysis occurred after 9 days of therapy, demonstrating that serious toxicity can develop after the initial treatment period 4. In patients with pre-existing renal disease, the risk of accumulation and toxicity is substantially higher 2, 6.

Regarding Cotrimoxazole (Co-trimoxazole)

While the question mentions "cotrimazole," this likely refers to co-trimoxazole (trimethoprim-sulfamethoxazole). This agent also requires dose adjustment in renal impairment and carries nephrotoxic potential 6, 7. If this medication is also being used, renal function should be closely monitored and dosing adjusted appropriately based on creatinine clearance 6.