Onychomycosis is a fungal infection of the nail unit, more common in toenails than in fingernails,1 and caused by a variety of fungi including dermatophytes, nondermatophyte molds, and Candida.2,3 There are 4 to 5 subtypes related to the method of fungal invasion of the nail unit, the most common being distal lateral subungual onychomycosis (DLSO) (Figure 1).4,5 Here the fungus enters the distal lateral part of the nail bed, the region of the hyponychium, often as an extension of tinea pedis. Hyperkeratosis occurs under the nail plate, resulting in detachment of the nail plate from the nail bed (onycholysis), with subungual thickening (Figure 1).
Onychomycosis may occur without tinea pedis in only a couple of scenarios: if a pedicure has inoculated the fungus directly into the nail, or if the patient has used infected clippers. Older patients will often tell you that they donâ€™t have tinea pedis, until you ask them to take their socks off. They assume incorrectly that foot dryness is normal.
We also need to look for dermatophytomas, yellow-orange streaks and patches that can extend the full length of the nail (Figure 2). Dermatophytomas are often seen in patients who have been treated with oral antifungals. The nails clear up initially, but patients have a recurrence due to a cluster of fungi growing into a fungal abscess, which presents as a streak or patch under the nail. The presence of dermatophytomas is an important consideration when considering nail debridement.
Proximal subungual onychomycosis, a relatively uncommon subtype, occurs when the fungus invades under the cuticle or nail plate, and advances from the proximal to distal part of the nail.5 It occurs in individuals who are immunocompromised (it was originally reported as an early clinical marker for HIV infection),6 in transplant patients, and in others with primary or secondary immunodeficiency. It is difficult to treat until the immunosuppression has resolved. It may also arise secondary to trauma.5
White superficial onychomycosis is less common than DLSO, representing about 10% of onychomycosis cases.7 The superficial nail plate or dorsal aspect is usually involved initially. It is most commonly caused by Trichophyton mentagrophytes and several nondermatophyte molds (such as Fusarium, Aspergillus, and Acremonium spp.). The nail feels rough but may become soft and crumbly where the fungus has attacked,8 and it can be scraped off with a scalpel. Eventually the nail becomes totally dystrophic.9
Candida onychomycosis is rare and only affects immunosuppressed patients. It typically invades the entire nail plate and is associated with paronychia. Candida can be a primary invader of the nail in a rare condition called chronic muccocutaneous onychomycosis, in which the patient has an immune deficiency that allows candida to invade the nails and mucous membranes.