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Onychomycosis Defined

Click to see the slide showOnychomycosis is an infection of the nail by opportunistic fungi that include dermatophytes, nondermatophyte moulds and yeasts (mainly Candida species). The toenails are affected in 80% of all cases of onychomycosis; dermatophyte infection, mostly due to Trichophyton rubrum, is the cause in over 90% of cases (See the slide show).

Onychomycosis is classified clinically as distal and lateral subungual onychomycosis (DLSO), superficial white onychomycosis (SWO), proximal subungual onychomycosis (PSO), candidal onychomycosis and total dystrophic onychomycosis.

Distal and lateral subungual onychomycosis

DLSO accounts for the majority of cases and is almost always due to dermatophyte infection. It affects the hyponychium, often at the lateral edges initially, and spreads proximally along the nail bed resulting in subungual hyperkeratosis and onycholysis although the nail plate is not initially affected (learn more on nail anatomy). DLSO may confined to one side of the nail or spread sideways to involve the whole of the nail bed, and progresses relentlessly until it reaches the posterior nail fold.

Eventually the nail plate becomes friable and may break up, often due to trauma, although nail destruction may be related to invasion of the plate by dermatophytes that have keratolytic properties (more on fungal infections). Examination of the surrounding skin will nearly always reveal evidence of tinea pedis. Toenail infection is an almost inevitable precursor of fingernail dermatophytosis, which has a similar clinical appearance although nail thickening is not as common.

Superficial white onychomycosis

SWO is also nearly always due to a dermatophyte infection, most commonly T. mentagrophytes. It is much less common than DLSO and affects the surface of the nail plate rather than the nail bed. Discoloration is white rather than cream and the surface of the nail
plate is noticeably flaky. Onycholysis is not a common feature of SWO and intercurrent foot infection is not as frequent as in DLSO.

Proximal subungual onychomycosis

PSO, without evidence of paronychia, is an uncommon variety of dermatophyte infection often related to intercurrent disease. Immunosuppressed patients, notably those who are human immunodeficiency virus-positive, may present with this variety of dermatophyte infection; conditions such as peripheral vascular disease and diabetes also may present in this way. Evidence of intercurrent disease should therefore be considered in a patient with PSO.

Candidal onychomycosis

Infection of the nail apparatus with Candida yeasts may present in one of four ways: (i) chronic paronychia with secondary nail dystrophy; (ii) distal nail infection; (iii) chronic mucocutaneous candidiasis; and (iv) secondary candidiasis.
Chronic paronychia of the fingernails generally only occurs in patients with wet occupations. Swelling of the posterior nail fold occurs secondary to chronic immersion in water or possibly due to allergic reactions to some foods, and the cuticle becomes detached from the nail plate thus losing its water-tight properties.

Microorganisms, both yeasts and bacteria, enter the subcuticular space causing further swelling of the posterior nail fold and further cuticular detachment, i.e. a vicious circle. Infection and inflammation in the area of the nail matrix eventually lead to a proximal nail dystrophy.

Distal nail infection with Candida yeasts is uncommon and virtually all patients have Raynaud’s phenomenon or some other form of vascular insufficiency. It is unclear whether the underlying vascular problem gives rise to onycholysis as the initial event or whether
yeast infection causes the onycholysis. Although candidal onychomycosis cannot be clinically differentiated from DLSO with certainty, the absence of toenail involvement and typically a lesser degree of subungual hyperkeratosis are helpful diagnostic features.

Chronic mucocutaneous candidiasis has multifactorial aetiology leading to diminished cell-mediated immunity. Clinical signs vary with the severity of immunosuppression, but in more severe cases gross thickening of the nails occurs, amounting to a Candida
granuloma. The mucous membranes are almost always involved in such cases.
Secondary candidal onychomycosis occurs in other diseases of the nail apparatus, most notably psoriasis.

Total dystrophic onychomycosis

Any of the above varieties of onychomycosis may eventually progress to total nail dystrophy where the nail plate is almost completely destroyed.

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