Onychomycosis

Fungal infections of the nail organ are the most common nail disorders. Even though they are usually easily diagnosable they may be indistinguishable from nail psoriasis and the conditions may in fact occur together.

Superficial white onychomycosis is easy to diagnose: a tangential biopsy of the nail plate is taken with a no. 15 scalpel and sent to the laboratory. Formalin fixation is not necessary. The thin nail slice is processed and cut as usual and stained with periodic acid-Schiff reagent (PAS) or another stain for fungi. Under the microscope, chains of small, regularly sized fungal spores are seen on the nail plate surface and in its splits, giving evidence of a Trichophyton mentagrophytes infection. Larger spores and short, thick-walled hyphae of irregular calibre are characteristic of a mould infection. The nail plate does not exhibit any further pathological alterations and the subungual structures remain normal.

Endonyx onychomycosis can be diagnosed by examining nail clippings. Usually due to T. soudanense, it shows a dense infiltration of the nail substance with relatively thin hyphae.

To diagnose distal and distal lateral subungual onychomycosis, either nail clippings with adherent subungual hyperkeratosis or a nail biopsy are necessary. Clipped material shows variable amounts of irregular masses of hyphae and often also thick-walled arthrospores. In addition, the subungual keratotic material may contain small, dried neutrophilic abscesses and serum globules that are also PAS positive and may be mistaken for fungal elements if very small. Nail biopsies reveal important pathological alterations of the nail bed and matrix with subepithelial lymphocytic infiltrates, spongiosis, lymphocytic exocytosis and intraepithelial neutrophils, which often form Munro's micro-abscesses in the keratin just beneath the nail plate. If there are only few fungi the wrong diagnosis of psoriasis unguium may then be made. The nail plate's undersurface may be invaded by hyphae, which usually are seen in a longitudinal and parallel arrangement. This is proof that the affected nail is still growing normally.

For the diagnosis of proximal subungual onychomycosis, a disc of nail plate may be punched out of the nail plate; this is best done after soaking the digit in water for 10 minutes, to soften the nail plate. The punch is carefully advanced through the entire thickness of the nail plate until the reactive subungual keratosis is reached. The tissue sample is embedded, cut, and stained for fungi. In onychomycoses hyphae are seen to penetrate the entire thickness of the nail plate. Nail biopsies that include the proximal nail fold, nail plate, matrix and nail bed show hyphae in the stratum corneum of the underside of the proximal nail fold as well as fungi in different levels of the nail plate. Inflammatory changes are not pronounced as long as the fungi have not reached the nail bed epithelium.

Total dystrophic onychomycosis shows variably severe changes. An intact nail plate is no longer seen, being replaced by irregular keratotic debris containing large amounts of fungal elements, both spores and hyphae. The latter are arranged in a haphazard fashion. The keratin contains also neutrophils and neutrophilic abscesses. The matrix and nail bed often exhibit papillomatosis and pronounced spongiosis. There is a considerable oedema in the papillary dermis and a variably dense lymphocytic infiltrate.

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