A 68-year-old man with poorly controlled diabetes mellitus type 2 and chronic corticosteroid use (for COPD) presents with severe, widespread pruritus and generalized erythema with numerous crusted lesions over his entire body, including the scalp, face, and intertriginous areas. He reports symptom onset 2 weeks ago. Skin scraping and microscopy reveal numerous mite burrows and mites at all stages of development. His wife, who is immunocompetent, has no symptoms. What is the most likely diagnosis and the most appropriate management?

Explanation

## Clinical Diagnosis: Crusted Scabies (Norwegian Scabies) ### Key Diagnostic Features **High-Yield:** Crusted scabies is a severe, highly contagious variant of scabies characterized by: | Feature | Typical Scabies | Crusted Scabies | |---------|-----------------|------------------| | **Mite burden** | 10–15 mites | Thousands to millions | | **Clinical presentation** | Localized burrows, mild erythema | Widespread crusted plaques, erythroderma | | **Distribution** | Wrists, interdigital spaces, genitalia | Entire body including scalp, face, ears | | **Pruritus severity** | Moderate to severe | Often minimal (paradoxically) | | **Contagiousness** | Moderate | Extremely high | | **Risk factors** | None specific | Immunosuppression, chronic corticosteroids, diabetes, malignancy, HIV/AIDS | | **Microscopy findings** | Few mites | Abundant mites at all life stages | **Key Point:** This patient has TWO major risk factors for crusted scabies: chronic corticosteroid use and poorly controlled diabetes (both impair cell-mediated immunity). The widespread distribution, crusted morphology, and abundant mites on microscopy confirm the diagnosis. **Clinical Pearl:** The absence of symptoms in the immunocompetent wife is typical — crusted scabies is a manifestation of the host's impaired immune response, not increased mite virulence. The wife may still be a carrier and should receive prophylactic treatment. ## Management of Crusted Scabies ### Why Standard Permethrin Is Insufficient 1. **Mite burden:** Millions of mites overwhelm topical therapy alone. 2. **Crusted barrier:** Thick crusts impede penetration of topical agents. 3. **Systemic therapy required:** Ivermectin achieves high concentrations in skin and can reach mites within crusts. ### Recommended Treatment Protocol ```mermaid flowchart TD A["Crusted Scabies Diagnosis"]:::outcome --> B["Ivermectin 200 µg/kg PO"]:::action B --> C{"Dosing Schedule"}:::decision C -->|"Days 1, 2, 8, 9, 15"| D["5 doses total"]:::action D --> E["+ Permethrin 5% topical"]:::action E --> F["Apply to entire body"]:::action F --> G["Repeat topical at days 8 & 15"]:::action G --> H["Treat all household contacts"]:::action H --> I["Even asymptomatic contacts"]:::action I --> J["Follow-up at 2-4 weeks"]:::outcome ``` **High-Yield:** Ivermectin dosing for crusted scabies: - **Dose:** 200 µg/kg orally - **Schedule:** Days 1, 2, 8, 9, and 15 (5 doses total) - **Rationale:** Multiple doses are needed because ivermectin does not reliably kill eggs; repeated dosing targets newly hatched mites over 2–3 weeks. **Key Point:** Topical permethrin 5% cream is applied concurrently to: - Penetrate crusts and kill mites in the stratum corneum. - Reduce mite burden and accelerate clinical improvement. - Applied on days 1, 8, and 15 (coinciding with ivermectin doses). ### Ancillary Measures 1. **Debridement:** Gentle removal of crusts with keratolytic agents (salicylic acid 5–10%) before topical permethrin application improves penetration. 2. **Treat all contacts:** All household members and healthcare workers must receive ivermectin prophylaxis (single dose 200 µg/kg) even if asymptomatic, due to extreme contagiousness. 3. **Environmental control:** Wash all bedding, clothing, and towels in hot water; disinfect fomites. 4. **Immunosuppression review:** Optimize diabetes control; consider reducing or tapering corticosteroids if clinically feasible. 5. **Infection control:** Isolate patient from immunocompromised individuals during treatment. **Mnemonic:** **CRUSTED SCABIES = C**orticosteroids/**C**omorbidities, **R**educed immunity, **U**ncontrolled diabetes, **S**evere/widespread, **T**housands of mites, **E**rythroderma, **D**ebridement + ivermectin, **S**ystemic therapy, **C**ontacts treated, **A**ncillary measures, **B**arrier function impaired, **I**mmunity compromised, **E**xtended dosing, **S**evere contagiousness. [cite:Irvine's Dermatology 10e Ch 24; Harrison 21e Ch 397]