Pachyonychia congenita (PC) is characterized by thickened and friable finger and toe nails often apparent at birth or soon after. There are painful plaques of callus-like hyperkeratosis (keratoderma) on palms and soles with underlying blisters, hyperhidrosis and some individuals may have spiny follicular hyperkeratosis elsewhere on the body. Patients who also have natal teeth, alopecia and epidermoid cysts were previously described as having PC type 2. Patients with oral leukoplakia (a finding absent in PC type 2) were previously described as having PC type 1. However, recent genotype/phenotype correlation reports favor the more general nomenclature of pachyonychia congenita (PC) due to high phenotypic overlap between the subtypes. Heterozygous mutations in five keratin genes have been associated with PC: KRT16, KRT6A, KRT17, KRT6B, and KRT6C. Patients with KRT6C mutations typically have callus-like palmoplantar keratoderma with little or no nail dystrophy.
In steatocystoma multiplex, multiple round or oval sebaceous (epidermoid) cysts develop, widely distributed over the back, anterior trunk, arms, scrotum, and thighs. Thickened nail plates, focal palmoplantar keratoderma and natal teeth are variable features.