The epidermoid formation and its affinity to congenital cholesteatoma

The epidermoid formation was reinvestigated in stained step sections of foetal and neonatal temporal bones to assess its relationship to congenital cholesteatoma. Epidermoid formations were found in 88 of 211 temporal 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 microns anterior to the anterior edge of the tympanic membrane. Seven epidermoid formations however, were sited on the medial anterosuperior tympanic membrane surface. The mean surface diameter of epidermoid formations was 242 microns. Some were greater than 1500 microns, which approaches the size of small congenital cholesteatomas. They are often in proximity to a developmental epithelial fold which could close off to produce a cyst. These features support the concept that the epidermoid formation is a precursor of the small anterosuperior congenital cholesteatoma.