[Sub-total hereditary leukonychia, histopathological and electron microscopy study of "milky" nails]

Leuconychia is the most common of ungueal discoloration or dyschromia. The hereditary form, sub-total or total, is very rare. We report the observation of a family in which thirteen members, distributed over four generations, presented a partial or subtotal leukonychia of all nails. On the basis of this observation and histopathological analysis of the propositus nail, we discuss the different hypotheses proposed to account for white nails.

A 45 year-old man without previous medical history, showed partial or sub-total leukonychia of his twenty nails. Clinical examination revealed soft nails with slow growth. Discoloration or dyschromia was associated with koilonychia on the fingers without other cutaneous or visceral abnormalities. A longitudinal biopsy of the whole ungueal apparatus of the large toe was performed. Histopathological analysis showed parakeratosis and an abnormal granular layer thickened on the proximal and ventral womb. These abnormalities were responsible for heterogeneous HES coloration, lamellar and dissociated aspect of the nail plates. Electron microscopy revealed dissociated keratin bundles and the existence of intracytoplasmic clear vacuoles probably of lipid origin. This aspect was observed on the proximal part of the dorsal tablet and disappeared in the distal portion of the nail.

According to Newton's theorem, a surface appears white when it reflects all the radiation of visible light. This mechanism can be proposed to explain leukonychia. On histological level, parakeratosis and dissociation of the keratin bundles may play a role in the modification of the solar light reflection by ungueal plates. In our case, parakeratosis and disorganization of keratin bundles were present in the white part of the nail, but also in the distal, pinkish crescent (distal dorsal plate). Electron microscopy analysis showed clear vacuoles located in the white part of the nail, whereas they were not seen in the distal part. These observations confirm the probable participation of parakeratosis and keratin abnormalities in this pathology but also suggest an important role of lipid vacuoles. Therefore, some white nails may in fact be "milky" nails. Finally, disorganization of the keratin bundles observed, was also reported in other genodermatosis e.g.: epidermolysis bullosa simplex. In this latter pathology, some hard keratin genes mutations (K5 and K14) have been demonstrated. According to these results, a genetic study is on going in this family in order to search for a mutation in one of the hard keratin genes.