[Etiopathogenesis, clinical picture and diagnosis of onychomycoses]

ETIOLOGY OF ONYCHOMYCOSES: Onychomycoses can be caused by dermatophytes, molds and yeasts. However, dermatophytes appear to be the chief organisms capable of a primary attack on the nail. By far the most frequent dermatophytes isolated from nails are Trichophyton rubrum, T. mentagrophytes var. interdigitale and Epidermophyton floccosum. Molds virtually only invade toenails, but their role as a primary pathogen is still debated. Yeasts have been isolated from diseased nails at highly different rates. Nails may be infected by two different dermatophytes, two dermatophytes and a yeast, a dermatophyte, a yeast and a mold, etc. PATHOGENESIS OF ONYCHOMYCOSES: The mode of infection is still under debate. In many cases palmar and/or plantar tinea, exists but can often remain asymptomatic for years. After spreading to the nail, the fungus invades the hyponychium or lateral nail sulcus to finally reach the nail bed where it moves proximally to the matrix. Proximal subungual onychomycosis probably starts with a fungal skin infection, whereas white superficial onychomycosis seems to be a culture of T. mentagrophytes on a softened nail surface. Total dystrophic onychomycosis may result from both distal and proximal subungual onychomycosis or from C. albicans in chronic mucocutaneous candidiasis. Candida infections occur most often due to previous Candida paronychia, but it appears that a number of cases of so called idiopathic onycholysis are also caused by C. albicans with damage to the hyponychium being the portal-of entry. CLINICAL PICTURE OF ONYCHOMYCOSES: Onychomycoses can be divided into four different types. Distal subungual onychomycosis is the most common. The most frequent presenting clinical features are thickening and opaci-fication of the nail plate along the distal and lateral borders. The discoloration ranges from white to brown. The edge of the affected nail is usually uneven and often one or more streaks of dystrophic discoloured nail extend towards the distal border. Proximal subungual onychomycosis is uncommon. A white spot appears beneath the proximal nail fold and may extend distally to involve the deeper layers of the whole nail. Superficial white onychomycosis is also uncommon. The surface is the initial site of invasion. The causative organisms produce small superficial white and powdery patches over the nail. The surface becomes rough and the texture softer than normal. Total dystrophic onychomycosis represents the most advanced from all the previous three types, especially the distal subungual onychomycosis. The nail matrix has become permanently scarred by chronic infection. The nail is thick, elevated, denser and opaque. Candidomycotic onychomycosis shows erythematous and swollen proximal and lateral nail folds. Consequently, the nail plate becomes detached from the eponychium. Mycotic onycholysis is characterized by detachment of the nail plate from the bed, distal nail erosions, and grayish-yellow paste-like material under the nail. DIAGNOSIS OF ONYCHOMYCOSES: The diagnosis of onychomycoses cannot be made on the basis of clinical observation alone. Direct microscopy plays an important role in diagnosing nail fungal infections. However, fungal cultures are the only definitive test that can be used to identify the genus and the species of the infectious organism. Histological examination is a routine technique useful for defining the nature and localization of fungi in the nail plate. Immunohistochemistry applied to onychomycosis is an experimental approach bringing prominent information about identification of fungi. In vivo confocal microscopy represents a technique of the future.