Onychomycosis, II: Diagnostic approach

Four main portals for fungi can be identified on the nail, each resulting in different clinical patterns of infection: 1 - Via the distal subungual area and the lateral nail groove leading to distal lateral subungual onychomycosis. The fungus invades the horny layer of the hyponychium and/or the nail bed, then the undersurface of the nail plate which becomes opaque. Endonyx onychomycosis is a variant of this type. 2 - Via the dorsal surface of the nail prate, producing superficial onychomycosis. Superficial white onychomycosis is normally confined to the toenails. Superficial black onychomycosis, the counterpart of the latter, is very rare. 3 - Via the undersurface of the proximal nail fold which appears normal in proximal subungual onychomycosis. In patients with AIDS the term 'acute proximal nail dystrophy' might be appropriate for this type of infection. Proximal leuconychia associated with paronychia is produced by non dermatophyte-moulds. 4 - Secondary total dystrophic onychomycosis represents the most advanced form of all the types described above. In contrast to this form, primary total dystrophic onychomycosis is observed only in patients suffering from chronic mucocutaneous candidosis or in other immunodeficiency states. The diagnosis of onychomycosis always requires laboratory confirmation. Mycological diagnosis is based on detection of fungal elements in direct microscopy preparations and identification of the responsible fungus by culture. In repeated false negative mycological results, histopathological examination of nail keratin may be helpful.