A 62-year-old man with advanced dementia and poor hygiene presents with widespread crusted plaques over his entire body, including the face, palms, and soles. He has minimal pruritus despite severe infestation. Microscopy of skin scrapings reveals hundreds of mites. His caregivers report he has not responded to two courses of permethrin applied 1 week apart. What is the most likely diagnosis and the most appropriate next treatment?

Explanation

## Diagnosis: Crusted (Norwegian) Scabies **Key Point:** Crusted scabies is a severe, highly contagious variant characterized by widespread hyperkeratotic crusted plaques, minimal pruritus (paradoxically), and massive mite burden (thousands to millions). It occurs in immunocompromised, elderly, or institutionalized patients. ## Clinical Features Distinguishing Crusted Scabies | Feature | Typical Scabies | Crusted Scabies | |---------|-----------------|------------------| | **Mite burden** | 10–15 mites | Thousands to millions | | **Pruritus** | Intense, nocturnal | Minimal or absent | | **Distribution** | Finger webs, wrists, axillae | Widespread: face, scalp, palms, soles | | **Morphology** | Burrows, papules | Thick hyperkeratotic crusts | | **Transmission** | Skin-to-skin contact | Highly contagious (fomites, respiratory) | | **Risk groups** | General population | Immunocompromised, elderly, institutionalized | | **Response to topical agents** | Excellent | Poor (crusts block penetration) | **High-Yield:** The combination of massive mite burden, crusted plaques, minimal itch, and failure to respond to topical permethrin is pathognomonic for crusted scabies. ## Why Topical Agents Fail in Crusted Scabies 1. **Thick crusts** act as a barrier, preventing adequate penetration of topical scabicides 2. **Massive mite burden** requires systemic therapy for adequate distribution 3. **Immunocompromise** (dementia with poor self-care, advanced age) impairs local immune response ## Treatment of Crusted Scabies **Key Point:** Oral ivermectin is the treatment of choice for crusted scabies. ### Ivermectin Dosing - **Dose:** 200 µg/kg (typically 12 mg for adults) - **Schedule:** Two doses, 1–2 weeks apart (some sources recommend a third dose at week 3) - **Mechanism:** Systemic distribution allows penetration through thick crusts and reaches high concentrations in skin ### Adjunctive Measures 1. **Keratolytic agents:** Salicylic acid 5–10% or urea 10% ointment to soften and remove crusts before ivermectin 2. **Topical scabicides:** May be applied after crust removal for enhanced efficacy 3. **Isolation precautions:** Crusted scabies is highly contagious; contact isolation recommended in institutional settings 4. **Environmental decontamination:** Thorough cleaning of living spaces and frequent laundering of fomites **Clinical Pearl:** In this case, the patient's dementia and poor hygiene are risk factors for crusted scabies. The failure of permethrin despite correct application and the massive mite burden on microscopy confirm the diagnosis. ## Mnemonic: CRUSTED Scabies Features **C** — Crusts (thick, hyperkeratotic) **R** — Resistant to topical therapy **U** — Unresponsive to standard permethrin **S** — Systemic ivermectin needed **T** — Thousands of mites **E** — Elderly/Immunocompromised **D** — Dementia, debilitation **Warning:** Do not continue topical permethrin in crusted scabies; it wastes time and allows continued transmission. Switch to oral ivermectin immediately. [cite:Park 26e Ch 8; Fitzpatrick's Dermatology 10e Ch 19]