Scabies: Refine your exam, avoid these diagnostic pitfalls

Nodular scabies variants can masquerade as pseudolymphoma, lymphoma, or leukemia cutis. In immunocompromised and elderly individuals, crusted scabies is often mistaken for psoriasis, atopic dermatitis, keratoderma, and lichen planus.^2,4,5

Scabies’ classic presentation

Typically, scabies causes intensely pruritic erythematous papules. Areas commonly affected are the webs and sides of fingers ­(FIGURE 1A and 1B), proximal palm and wrist flexors, extensor aspects of the arms and legs, axillary folds, periumbilical areas, the peri-areolar region in women, buttocks and thigh creases, and, in males, the genitals. The head may also be affected in children (FIGURE 1C), but seldom in adults. Interestingly, the back is usually spared across all age groups, though not always (FIGURE 2).

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Mites are not found near pruritic papules. Scrape the mite burrows, which are small, linear, serpiginous lines, to discover the mature scabies mite.

The classic presentation also varies across age groups and populations.² In children, vesicles, pustules, and nodular pruritic lesions may coexist with eczema and impetigo. Among homeless individuals, coinfection with impetigo and eczema is common. Elderly patients are more likely to exhibit atypical lesions.⁶ Immunocompromised patients present with atypical papular scabies, crusted scabies, or scabies that may be only mildly pruritic or nonpruritic. In underdeveloped countries, indigenous populations are more susceptible to endemic scabies, with a high risk of scabies superinfection with Streptococcus.

Scabies subtypes with varying presentations

Clinical manifestations of scabies subtypes may make it difficult to diagnose the disease. These subtypes include nodular, pustular, vesiculobullous, and crusted scabies (Norwegian scabies). Although rare, these subtypes merit acknowledgement, as atypical cases contribute to the high rate of misdiagnosis.

Nodular scabies is a clinical variant that accounts for about 7% of scabies cases.⁷ It can resist traditional scabies treatment (permethrin cream, ivermectin—which we’ll discuss in a bit) and often requires topical or intralesional corticosteroid management. Nodular scabies most commonly affects male genitalia. Patients may have multiple excoriated skin-colored erythematous papules and nodules in areas involving the classic distribution of scabies (web spaces of fingers, flexural areas [FIGURE 3], scrotum, and groin).⁸

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