ABSTRACT
Onychomycosis means fungal infection of the nail unit. Dermatophytes are responsible for the majority of cases, others are due to molds and yeasts. Distal and lateral subungual onychomycosis due to Trichophyton rubrum is the most common type. Fungi enter the nail unit through the hyponychium and infect the area beneath the nail plate, moving proximally and showing the following dermoscopic patterns: jagged proximal margin of the onycholytic area, sharp structures (spikes), and white-yellow longitudinal striae directed to the proximal fold (which corresponds to the outbreak of fungal invasion). The “ruin appearance” is instead the dermatoscopic pattern of subungual hyperkeratosis that develops owing to the build-up of dermal debris from fungal invasion ( Figures 10.1–10.4). Dermatophytoma is an aggregation of hyphae and scales in the subungual region that can be better appreciated by dermoscopy as yellow-orange patch, connected to the distal edge of the nail plate by a thin narrow channel ( Figure 10.5 ). Fungi can also produce dark pigment, rarely in bands and more at random (fungal melanonychia). Together with dermatophytoma it is a sign of treatment resisitance.
