This rare autosomal dominant condition is characterized by thickened nails (which are very difficult to cut), with yellow discoloration, follicular keratosis in 59 %, plantar keratosis in 72 %, and leukokeratosis in 57 % of cases. The nail abnormalities may be seen soon after birth, and paronychial inflammation occurs. Subcutaneous hamartomatous cysts, bullae, ichthyosis, and hyperkeratosis may develop. Blistering may occur over pressure areas, and there may be deafness, hair and dental abnormalities, hyperhidrosis, and corneal opacification. Oral leukoplakia is seen. Malignant changes may occur in the oral lesions, in cysts, or at sites of chronic plantar ulcerations, so that areas of chronic ulceration or bullous formation should be observed for the possible development of malignancy (Su et al. 1990). Steatocystoma multiplex is considered to be the same condition. Germline mutations in the K17 gene may underlie certain types of this condition (Smith et al. 1997); others may be due to missense mutations in keratin 17 (Smith et al. 1997; Feinstein et al. 1988; Hodes and Norins 1977; Stieglitz and Centerwall 1983).
Pachyonychia congenita (PC) types 1 and 2 are described. These are similar but type 1 gets oral leukokeratoses, whereas type 2 has premature dentition and multiple sebaceous cysts. Mutations in the keratin 17 (K17) gene can cause PC type 2 or steatocystoma multiplex PC types 1 and 2 with hypertrophic nail dystrophy, focal keratoderma, and sebaceous cysts (Corello et al. 1998). PC type 1 is due to K6a and K16 mutations (Swenssen 1999).