Congenital cholesteatomas are identical to epidermoid cysts, differing only in name and location.
They are intraosseous inclusions of ectoderm, and are therefore comprised of keratin debris and cholesterol. Characteristically, they are located at the petrous apex. In contrast middle ear cholesteatomas tend to be acquired, secondary to tympanic membrane perforation.
Unsurprisingly congenital cholesteatomas have imaging characteristics similar to those of epidermoid cysts
MRI shows sharply demarcated margins with smooth bony erosions.
- T1: low signal
- high signal (usually slightly brighter than CSF)
- usually will not attenuate on FLAIR imaging, however partial or heterogeneous attenuation may occur. This is in contrast to cholesterol granulomas which never attenuate
- T1 C+ (Gd): no enhancement of the mass itself; thin linear enhancement confined to the margins may be seen
A white epidermoid variant is also recognised in bone, in which case it becomes indistinguishable from a cholesterol granuloma, as both are bright on both T1 and T2 due to the high cholesterol component.