Onychomycosis: classification and diagnosis

Journal of Drugs in Dermatology, Jan-Feb, 2004 by Aditya K. Gupta, Jennifer E. Ryder, Richard C. Summerbell

Abstract

Onychomycosis is a common infection of the nail predominantly caused by anthropophilic dermatophytes, and to a lesser extent by yeasts (Candida species) and non-dermatophyte molds. The treatment of onychomycosis is dependent on several variables, including the type of onychomycosis and the causative organism. Various techniques have been used to accurately diagnose onychomycosis, with microscopy and culture being used most frequently. Histological examination of the distal nail plate can aid in confirming the presence of invasive nail disease, but histological examination should not be limited to the nail plate as it may also be helpful in diagnosing subungual onychomycosis. Nucleic acid-based identification techniques may also be valuable when diagnosing onychomycosis; however, multiple steps may be necessary to determine the causative species. Confocal microscopy may also be a fast and reliable method of diagnosing onychomycosis, though it has very limited ability to distinguish between dermatophyte and mold infections. Prior to treatment an accurate diagnosis can provide guidance about the choice of antifungal agent, especially since the causative organism may vary in its response to the antifungal therapies available.

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Introduction

Onychomycosis is a common infection of the nail, and it may affect approximately 6.5% to 8.7% of the North American population (1-3). It affects adults, particularly the elderly (>60 years old) (4,5), males (1), diabetics (6,7), immunocompromised individuals (e.g., HIV-positive patients) (8), smokers (9), and patients with psoriasis (10), as well as those with peripheral vascular (arterial) disease (9), previous tinea pedis, history of trauma to the nail (2), or a family history of onychomycosis.

Onychomycosis is caused predominantly by anthropophilic dermatophytes and less commonly by yeasts (Candida species) and non-dermatophyte molds. Organisms that cause onychomycosis can invade both the nail bed and the nail plate. The decision to treat onychomycosis, and the choice of agent to use, is dependent on several variables including the type of onychomycosis and the causative organism.

Anatomy

The nail plate, commonly referred to as the 'nail', is a durable keratinized structure that is constantly growing (11). The nail plate is held in place by the lateral and proximal nail folds. The latter is continuous with the cuticle, which is the layer of epidermis that extends from the proximal nail fold and adheres to the dorsal aspect of the nail plate. The nail matrix comprises the proximal one-quarter to one-third of the nail bed. The nail plate rests upon a vascularized nail bed that extends from the lunula (half moon) to the hyponychium (11). The hyponychium lies between the free edge of the nail plate and the distal groove (11). It is a reservoir for keratinous material and acts as a barrier to bacterial and fungal entry into the nail bed. The onychodermal band is part of the distal margin of the nail bed that has a contrasting hue in comparison with the rest of the nail bed (11).

Clinical Presentations

In 1998, Baran et al. (12) reported a new classification of onychomycosis, approximately 25 years after the first classification published by Zaias (13). Baran et al. (12) suggested onychomycosis should be categorized into five clinical types: distal and lateral (DLSO), superficial, proximal subungual (PSO), endonyx, and total dystrophic (TDO) (Figures 1 through 4). The basis of the new classification was the pattern of nail plate involvement by mode and site invasion, rather than fungal etiology: therefore, Candida onychomycosis is not a separate category in the new classification (12,13).

Zaias (13) initially described Candida onychomycosis in individuals with 'chronic cutaneous candidiasis syndrome', also called 'Candida granuloma', Candida onychomycosis is seen most often in patients with chronic mucocutaneous candidiasis (CMC). In non-immunocompromised subjects. Candida onychomycosis may be seen in association with paronychia (thickening of the subungual region) and onycholysis (detachment of the nail plate from the nail bed). This involves the fingernails to a much greater extent than toenails, and may be related to frequent exposure of the hands to water and/or chemicals, coupled with trauma.

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Distal lateral subungual onychomycosis (DLSO) is the most common presentation. The organism typically disrupts the onychodermal band enabling the fungus to reach the underside of the nail (ventral aspect) via the hyponychium, the nail bed, or the lateral nail fold (14). Although the fungal invasion originates in the nail bed, it can secondarily involve the inner (ventral) nail plate (15). Invasion of the nail plate may result in a yellowish-grey appearance. In some instances, the nail discoloration spreads in a transverse direction; these transverse networks correspond with "tunnels" that the pathogen has created within the keratin (16). DLSO may extend to the matrix and can result in paronychia, onycholysis, and subungual hyperkeratosis (12,14,16).

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