A unique pattern of dyskeratosis characterizes epidermolytic hyperkeratosis and epidermolytic palmoplantar keratoderma.

Hereditary skin diseases that are characterized ultrastructurally by intracellular clumpings of keratin tonofilaments, such as Darier disease and Ichthyosis Hystrix of Curth-Macklin, display epidermal dyskeratosis also histologically. Epidermolytic hyperkeratosis (EHK) and epidermolytic palmoplantar keratoderma (Voerner type) (EPPK) are 2 types of autosomal dominant keratodermas, which are also characterized ultrastructurally by intracellular clumpings of tonofilaments but usually without a clear description of histological dyskeratosis. The main aim of the present study was to characterize the histologic signs of keratin aggregation and clumping in the involved epidermis of EHK and EPPK. Two cases of EHK caused by KRT1 mutations and 4 cases of EPPK caused by KRT9 mutations were studied. The biopsies were obtained mostly from the involved skin of the palm. All 6 biopsies were studied histologically, and 4 biopsies (2 EHKs and 2 EPPKs) were also studied ultrastructurally. All 6 cases displayed the characteristic histological epidermolytic changes. In addition, intracytoplasmic and perinuclear eosinophilic homogenizations and round to oval eosinophilic inclusions were identified with varying frequencies in the involved epidermis of all 6 cases. These findings, which were more prominent in the EHK cases, corresponded most likely to the intracytoplasmic aggregates of tonofilaments and to the large round to oval dense clumps of tonofilaments, which were observed ultrastructurally. In conclusion, varying degrees of dyskeratosis are frequently present in EHK and EPPK and should be considered to be a histological characteristic of these disorders.