Congenital and hereditary nail dystrophies—an embryological approach to classification

Summary In many congenital and hereditary ectodermal syndromes, nail changes can often provide the earliest definitive and consistent signs by which the condition may be identified. In the dermatological, paediatric and genetic literature the nail signs are usually described on purely morphological grounds with little attention being paid to the embryological defect or defects underlying such changes. Consequently, any secondary changes produced by trauma to the nails may be confused with, and included in, the primary‘diagnostic’features. We propose a pathophysiological approach to the nail changes in congenital and inherited conditions: these will be classified as defects in the nail matrix, defects in the nail field or bed, and combined disorders originating from ectodermal and mesodermal tissues (Table 1). This is a simplistic classification and many disorders will result from a combination of these defects. Each of the defects will be illustrated by genodermatoses in which the knowledge of normal nail embryology and intra‐uterine development aids description and diagnosis.