Congenital disease

A number of usually rare congenital conditions can affect the nails. In the nail–patella syndrome, the nails (and patellae) are absent or rudimentary. The nails in pachyonychia congenita are thickened and discoloured from birth. Nail dystrophy is a feature of dystrophic epidermolysis bullosa (p. 90). Racket nails, characterized by a broad short thumbnail, is the commonest congenital nail defect. It is dominantly inherited and more common in women.

Trauma

Trauma, especially from sport, commonly causes nail abnormalities. Subungual haematomas usually occur when a fingernail has been trapped or a toenail stood on or stubbed, but the possibility of a subungual malignant melanoma must always be considered. Splinter haemorrhages are induced by trauma, although they also occur with infective endocarditis. Ill-fitting shoes contribute to ingrowing toenails, and chronic trauma predisposes to onychogryphosis – in which the big toenails become thickened and grow like a horn. Trauma may also induce onycholysis (separation of the nail from the nail bed). Constant picking of the thumbnail will produce a habit-tic dystrophy with transverse ridges and grooves. Brittle nails are a common complaint, usually due to repeated exposure to detergents and water, although iron deficiency, hypothyroidism and digital ischaemia are other causes.

Dermatoses

The nails are commonly involved in skin disease (Figs 1 and 2), and are routinely assessed in a dermatological examination. Details are given in Table 1, and a differential diagnosis of the changes is shown in Table 2. Treatment is aimed at the associated dermatosis; care of the hands (p. 38) is especially important.

Fig. 1 Psoriasis of the nails.

Pitting, onycholysis and brownish discoloration are apparent.

Fig. 2 Alopecia areata.

Thimble pitting of the nail is seen.

Table 1 Nail involvement in common dermatoses

Table 2 Differential diagnosis of nail changes in dermatoses and systemic disease