THE EPIDERMOID FORMATION AND ITS AFFINITY TO CONGENITAL CHOLESTEATOMA

The epidermoid formation was reinvestigated in stained step sections o f foetal and neonatal temporal bones to assess its relationship to con genital cholesteatoma. Epidermoid formations were found in 88 of 211 t emporal bones (42%) at from 12 weeks gestation to 6 months post-partum . The site was usually near the roof of the anterolateral epitympanum at an average of 389 mu m anterior to the anterior edge of the tympani c membrane. Seven epidermoid formations however, were sited on the med ial anterosuperior tympanic membrane surface. The mean surface diamete r of epidermoid formations was 242 mu m. Some were greater than 1500 m u m, which approaches the size of small congenital cholesteatomas. The y are often in proximity to a developmental epithelial fold which coul d close off to produce a cyst. These features support the concept that the epidermoid formation is a precursor of the small anterosuperior c ongenital cholesteatoma.