Pachyonychia

 

Congenita

 

 PC is a group of autosomal dominant disorders characterized by severe nail thickening due to massive nail hyperkeratosis . Type 1 PC is associated with mutations of K6a and K16 genes, whereas type 2 PC is caused by mutations of K6b and K17. In PC both fingernails and toenails are thickened, yellow-gray, and transversely overcurved with marked subungual hyperkeratosis most notable at the distal portion of the nails . Nail trimming is impossible, because the nails are extremely hard. The abnormalities usually develop during childhood, but a late-onset variety of PC has been described. Focal palmo-plantar keratoderma is associated with the nail changes in 60 percent of patients or may result from mutations without detectable nail changes. Up to 75 percent of patients have oral leukokeratosis; hyperhidrosis of the palms and soles, follicular keratoses, and acral bullae (especially during warmer months) are common. Patients with type 2 PC also tend to have cystic lesions (steatocystoma multiplex, vellus hair cysts), which reflects the expression of K6b and K17 in adnexal structures. The cystic lesions begin during childhood and progressively appear. Coarse or twisted hair, early primary tooth loss, and natal teeth are other occasional features of type 2 PC, reflecting expression of the affected genes in the hair follicle apparatus and teeth.