A 68-year-old man with type 2 diabetes and chronic kidney disease (eGFR 35 mL/min/1.73 m²) presents with widespread crusted plaques, hyperkeratosis, and nail dystrophy. Microscopy confirms *Sarcoptes scabiei*. He has failed two courses of topical permethrin (5%) applied 1 week apart. His wife is unaffected. What is the most appropriate next step in management?

Explanation

## Crusted Scabies: Recognition and Management ### Clinical Recognition: This Is Crusted Scabies **Key Point:** The combination of widespread crusted plaques, hyperkeratosis, nail dystrophy, and failure of standard topical therapy indicates **crusted scabies** (Norwegian scabies), not uncomplicated scabies. **High-Yield:** Crusted scabies occurs in immunocompromised patients (advanced age, diabetes, CKD, immunosuppression). The mite burden is 100–1000× higher than in uncomplicated scabies, making topical agents alone insufficient. **Clinical Pearl:** The fact that the wife is unaffected suggests immunocompromise in the index patient rather than high environmental transmission—crusted scabies is less contagious but far more difficult to treat. ### Why Topical Permethrin Alone Has Failed | Factor | Impact | |--------|--------| | **Mite burden** | 100–1000× higher in crusted scabies; topical penetration inadequate | | **Hyperkeratosis** | Thick scale barrier reduces drug penetration | | **Systemic factors** | Diabetes + CKD impair immune response and healing | | **Standard dosing** | Designed for uncomplicated scabies; insufficient for crusted variant | **Mnemonic:** **CRUSTED scabies = Compromised immunity, Resistant to topical monotherapy, Urgent systemic treatment, Severe mite burden, Topical keratolytics adjunctive, Extensive body involvement, Difficult diagnosis often delayed** ### Correct Management Approach 1. **Oral ivermectin 200 µg/kg** (first-line for crusted scabies) - Achieves systemic bioavailability to penetrate hyperkeratotic plaques - Repeat dose at 1–2 weeks to kill newly hatched mites - Safe in CKD (minimal renal excretion; hepatic metabolism) 2. **Adjunctive topical keratolytics** (salicylic acid 5–10%, urea 10–40%) - Debulk hyperkeratotic plaques - Enhance penetration of systemic agents - Applied before or concurrently with ivermectin 3. **Isolation precautions** (if hospitalized) - Crusted scabies has high transmission risk despite lower contagiousness in uncomplicated disease - Contact precautions recommended in healthcare settings ### Why Other Options Are Incorrect **Option 0 (Repeat topical permethrin):** A third topical application without systemic therapy will fail again. The mite burden and hyperkeratotic barrier exceed topical penetration capacity. This represents failure to escalate care appropriately. **Option 2 (Inpatient hospitalization alone):** While isolation may be appropriate, hospitalization without systemic ivermectin therapy is ineffective. The decision to hospitalize should be concurrent with ivermectin initiation, not instead of it. **Option 3 (Benzyl benzoate 10%):** Benzyl benzoate is an alternative topical agent but is not superior to permethrin in crusted scabies and does not overcome the fundamental problem—topical agents cannot penetrate thick scale in high mite-burden disease. It is also not preferred in CKD due to potential systemic absorption and toxicity. [cite:Irvine 26e Ch 7; Harrison 21e Ch 211]