Diagnosis of scabies by dermoscopy

Review articles¹ and dermatology textbook presentations of scabies² often contain no images of its dermoscopic features and, in fact, may omit mention of dermoscopy as an aid in diagnosis. Scabies often occurs in populations where performing traditional scabies preparations may be challenging. Dermoscopy’s utility in these settings is that it is non-invasive, painless, and highly diagnostic. Confirmation of this diagnosis is important because treatment is recommended for many individuals (many of whom may be asymptomatic); treatment failure is not uncommon (for example, as a result of incorrect use of medication, poor compliance, or re-infestation) and can lead to doubt about diagnosis if not firmly established initially; and post-scabetic itch is common and can lead to doubt about diagnosis. Additionally, because scabies may mimic a host of other dermatologic conditions, it is easily misdiagnosed if not confirmed. Patients (or parents) can be shown mites through the dermatoscope, via dermoscopic photographs of the mite on the camera’s LCD screen, or shown the ex vivo microscopic findings to assure “buy in” to the diagnosis to foster compliance with treatment.

Depicted are images of the dermoscopic features of scabies (figs 1–3). Figure 1 depicts a feature that I have not found previously reported, specifically “mini-triangle signs” in maturing scabies’ eggs.

Figure 1. Classic dermoscopic image of triangle or “delta wing jet” sign of dense scabies head parts (long red arrow), relatively translucent scabies body (long black arrow), scabies eggs (short red arrows), and classic S shaped burrow.

The “mother” mite is visible to the extreme right hand side of the burrow (long arrows). Also, upon careful inspection, translucent ovoid eggs, many showing their own “mini triangle signs”, are evident (several short red arrows). This feature is not commonly described. Heine Delta 20X dermatoscope with Nikon Coolpix 4500 camera.

Figure 2. Microphotograph of mineral oil scabies preparation at site corresponding to fig 1, showing mite (below heavy black arrows), eggs (orange arrows), and scybala (yellow arrows).

In preparing an ex vivo scabies preparation, mineral oil preserves scabies’ scybala, whereas potassium hydroxide does not. The technique for obtaining a scabies preparation is that of a very superficial shave biopsy (without anaesthesia), with the scalpel blade virtually parallel to the skin.

Figure 3. Microphotograph of scabies scybala (yellow arrows) as the only diagnostic microscopic finding confirming presence of scabies.

Because the scybala may be the only diagnostic finding, mineral oil is the preferred agent for preparing slides. Both potassium hydroxide and mineral oil preparations will demonstrate eggs and mites.

Dermoscopy employs magnification and light in such a way as to render the skin surface translucent, allowing visualisation of patterns that are often not visible with traditional clinical inspection. Originally employed to improve the detection of melanoma, it has since been used in a plethora of dermatologic conditions. Digital dermoscopic photographs can be viewed and magnified immediately on camera or computer screens.

Argenziano et al first reported the dermoscopic “triangle sign”, which represents the “head” portion of the mite, and “the delta wing jet with contrail” sign, corresponding to the head of the mite and the trailing burrow.³ Confirmation of scabies by dermoscopy is less time consuming than traditional methods of identifying mites. At the minimum, dermoscopy enables the mites to be located, which allows traditional mineral oil scabies preparations to be targeted precisely.