The role of dermatoscopy in establishing the diagnosis of scabies: a case of crusted scabies concurrence with psoriasis vulgaris

Introduction

Scabies is a parasitic world-wide dermatosis caused by an obligate human parasite Sarcoptes scabiei var. hominis. The female mite burrows in the horny layer of host’s skin, laying the eggs that hatch into larvae, nymphs, and eventually adult forms in two weeks’ time [1], while the life cycle of the mite lasts for 4–6 weeks [2]. Classic scabies symptoms that involve intense pruritus, worsening at night, small erythematous papules, vesicles and nodules (usually 0.5 cm in diameter) in typical areas (periumbilical, interdigital spaces, wrists and extensor aspects of the limbs axillary folds, genitalia and breasts, waist, buttocks), usually starts 3–4 weeks post primary infestation or 1–3 days post re-infestation [2]. Scabies affects all socio-economic groups, nevertheless it occurs most commonly in low-income and middle-income countries, which is explained by overcrowding and limited access to an effective treatment [3]. The infestation occurs by skin-to-skin contact, including sexual transmission. Although adult mite is capable of surviving outside the human host for 24–36 hours, it is not frequently acquired from infested fomites, except for crusted scabies cases [2, 4]. Crusted scabies, historically known as “Norwegian scabies”, is a severe and extremely contagious form of scabies with massive skin infestation – patients are commonly infected with over 1 million mites [5], which leads to crusting of the skin, thickening of the stratum corneum and subungual hyperkeratosis. The hyperkeratotic lesions may contain up to 4700 mites per gram of exfoliated epidermis [6]. Due to immune system malfunction associated with immunosuppression, malnourishment, neurological disease or in bedridden elderly, those mentioned are at the highest risk of crusted scabies [7]. It is estimated by the World Health Organization (WHO) that there are over 200 million people suffering from scabies globally at any time, yet there are no data for crusted scabies to be found [8]. As a result of the occurrence of only mild itching or its absence, along with a clinical presentation not corresponding to the classic scabies, characterized by hyperkeratosis, often with the presence of warty scales over bony prominences, the establishment of a definitive diagnosis is frequently delayed, leading to potential outbreaks that mainly affect institutions such as age care facilities, hospitals, childcare centers, kindergartens, schools, sheltered workshops, prisons and orphanages [9].

Objective

To emphasize the role of dermoscopy as the fastest and non-invasive examination in establishing the definitive diagnosis of scabies.

Case report

A 77-year-old nursing home male resident with a history of psoriasis for over 30 years was admitted to the Department of Dermatology due to erythroderma. A significant exacerbation of the lesion occurred approximately 4 weeks prior to hospitalization. Physical examination revealed generalized, poorly defined erythematous-papular lesions and patches, covered by scales (fig. 1). The palmoplantar regions were covered by yellow and grey plaques and crusts with linear fissures. Onycholysis, subungual hyperkeratosis and pitting – typical features of nail psoriasis were present (figs. 2, 3). The patient denied pruritus. The dermoscopic examination revealed “delta wing” sign and numerous aggregates of burrows (fig. 4). Laboratory tests presented eosinophilia, elevated C-reactive protein levels and serum IgE concentration. Treatment involved ointments consisting of sulfur 10% ointment, permethrin 5% cream and crotamiton cream. As post-treatment, topical mild steroids and emollients have been applied. After a couple of days of hospitalization erythematous patches with prominent scale regressed. Further oral therapy with ivermectin 200 µg/kg has been scheduled on days 1, 2 and 8 [4].

Discussion

In the absence of proper treatment in immunocompromised hosts, the condition typically spreads progressively and may ultimately affect the entire body. Differential diagnostics is crucial and encompasses other dermatoses characterized by hyperkeratosis like psoriasis, seborrheic dermatitis, palmoplantar keratoderma, Darier disease [10]. Drug-related eruptions capable of mimicking multiple dermatoses and atopic dermatitis should also be considered as one of the causes of extensive erythematous-scaly lesions. The diagnosis that is difficult to be suspected on the characteristics of itch, clinical findings and suggestive history must be supported by more specific methods such as microscopic examination of epidermal scrapings that identifies mites, eggs or fecal pellets (“scybala”), nevertheless the negative microscopic result does not exclude scabies [4]. According to 2020 Consensus Criteria for the Diagnosis of Scabies, handheld dermatoscopy is an examination allowing confirmation of the diagnosis of scabies if the mites are identified [11]. Dermoscopy performed by a trained specialist has a 95% confidence interval for positive likelihood ratio of 4.1 to 10.3 and a negative likelihood ratio of 95% confidence interval 0.06 to 0.2 for detecting scabies [12], sensitivity of 98.3% and specificity of 88.5% [13]. Recent rising interest in ultraviolet dermoscopy resulted in introducing a new diagnostic method in the field. UVA light is confirmed to detect Sarcoptes scabiei as a bright reflex of white or green point-shaped area [14]. The UV dermoscopic image consists of the body of a female mite (a novel sign of an oval-shaped reflex) and the borders of the tunnel, reflecting in linear blue luminescence [14, 15].

Conclusions

In the presented case, diagnostic challenges arose from the coexistence of psoriasis with scabies infection, culminating in erythroderma, resulted from inadequate or absent therapeutic interventions. Polarized dermatoscopy as a widely used, fast and non-invasive examination can identify typical scabies characteristics, such as skin burrows, mites presented as the “delta” sign at the end of the burrow [16] and eggs. The UV dermoscopy might provide even better image than polarized dermatoscopy, especially with hyperkeratotic lesions that make it difficult to clearly identify the tunnel content in the polarized mode. Both methods significantly improve the process of diagnosing scabies.

Funding

No external funding.

Ethical approval

Not applicable.

Conflict of interest

The authors declare no conflict of interest.

References