Differential Diagnosis of Mycotic Nail Diseases

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Though not very frequent, most inflammatory nail diseases are characterized by their protracted and recalcitrant course. Fungal infections are the most frequent cause of nail disorders. In most cases, a dermatophyte infects the hyponychium and slowly extends proximally under the nail plate (distal subungual onychomycosis). The nail bed epithelium produces a reactive hyperkeratosis which is very rich in fungal elements and causes distal onycholysis. The proximal subungual onychomycosis starts at the eponychium, spreading along the underside of the proximal nail fold to the matrix and further distally. Dermatophytes are the most common causative organisms, though (secondary) infections with moulds are not rare in toenails. Nail discoloration, subungual hyperkeratosis with onycholysis and even nail destruction are important clinical signs. Superficial onychomycosis usually causes a whitish colour of the nail plate. The demonstration of fungal elements in KOH preparations and in culture depends on careful collection of material. Chronic paronychia is often due to Candida albicans. Firm swelling of the proximal nail fold (PNF) with loss of the cuticle and a putrid secretion from under the PNF are the cardinal symptoms, and cross ridging of the nail plate is a secondary phenomenon. Psoriasis is the skin disease that most often produces nail changes. Fine pitting, yellow discoloration or onycholysis due to circumscribed subungual psoriatic papules, thickening and gross nail destruction are common. Extensive very fine pitting may occur in alopecia areata; involvement of all nails is called trachyonychia (20-nail dystrophy). Some coarse pits and irregular deformation of the nail plate’s surface occur in eczema of the nail region. Lichen planus unguium may occur in isolation or together with typical skin lesions: usually, irregular longitudinal ridging, pits and/or nail dystrophy are seen. Painless separation of the nail plate from the nail bed is called idiopathic onycholysis. Nail dystrophy, discoloration, distal onycholysis, slow growth and thickening occurring on toenails are often observed in old age, in the presence of impaired blood supply, and after chronic repeated trauma.