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The diagnosis and treatment of nail diseases require an in-depth knowledge of the anatomy, physiology, and pathology of the nail unit (see Chap. 8). It is an integral part of the tip of the digit, and its anatomy, growth characteristics, and functions vary between the different fingers and toes, which have to be considered when faced with a diseased nail. There are different ways to classify nail disorders and a compromise between a scientifically based etiologic classification and a more clinico-morphologically based one appears to be a practical approach to nail disorders. This chapter discusses specific nail changes either resulting from particular nail disorders or from dermatologic or systemic diseases, the peculiarities, including etiopathogenetic aspects, of which are not repeated here if they are not specific for the nail lesions.
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NAIL SIGNS AND NAIL-SPECIFIC CONDITIONS
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ANONYCHIA, HYPONYCHIA, AND DOUBLE NAIL
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Complete or almost complete lack of the nail is called anonychia, severe hypoplasia, or hyponychia. The condition is usually inborn, may be a genetic trait or the result of drug or toxin-induced lack of nail formation during embryogenesis. Several different types are known, ranging from a round tip of the digit without any visible change of the skin to an area that may correspond to the nail field, or a hyperkeratosis. When there is no terminal phalanx and no nail growth this is called Cooks syndrome or atelephalangia with anonychia. Hyponychia may be on all or several digits and is more common. A particular form with half-side index fingernail hypoplasia and a Y-shaped radiologic alteration of the distal phalanx is characteristic for Iso-Kikuchi syndrome.
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Micronychia may be a sign of phenytoin and alcohol fetopathy and is a constant feature of congenital onychodysplasia of Iso-Kikuchi (COIF [congenital onychodysplasia of index finger] syndrome).
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Racket nail is a short wide nail, mostly of the thumb, which develops from the age of 12 years on and is the result of a premature ossification of the epiphysis of the distal phalanx. The bone cannot grow longitudinally but continues to get broader because of apposition on the sides. This condition is autosomal dominant with variable expression and penetrance.
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Very short nails (brachyonychia) may develop in patients under chronic hemodialysis who develop a tertiary hyperparathyroidism with resorption of the bone of the terminal phalanx.
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A rudimentary double nail of the fifth toe is a relatively frequent finding in subjects of all races. The nail may be slightly wider and have a slight longitudinal indentation or be discernable as a complete accessory nail (Fig. 91-1).1
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The nail may show a variety of color changes that may be caused by ...