Cholesteatomas are collections of stratified squamous epithelium and associated keratin in an abnormal location, or in other words, “skin in the wrong place.” Cholesteatomas may be congenital or acquired. Congenital lesions are most common in the suprasellar cistern and cerebellopontine angle. Congenial cholesteatomas may occur in the middle ear although approximately 98% of these lesions are acquired.

Acquired cholesteatomas are unique to the middle ear cavity. The skin lining the tympanic membrane(TM) is felt to gain access to the middle ear cavity via defects in the membrane (either areas of perforation or retraction pockets). The pars flaccida portion of the TM lacks a fibrous layer and is especially susceptible to retraction. Extension of squamous epithelium via this route results in cholesteatoma formation in Prussak’s space. This space is bordered medially by the head of the malleus and body of the incus, laterally by the attic wall, and superiorly by the lateral malleolar ligament. Cholesteatomas in Prussak’s space tend to erode the adjacent scutum and displace the malleus head and incus body medially. Lesions in this location also tend to spread in the path of least resistance posteriorly into the posterolateral attic. Pars tensa chlesteatomas tend to involve the recesses of the posterior tympanum (sinus tympani and facial recess). Aggressive cholesteatomas can extend beyond the middle ear and into the petrous apex.

Cholesteatomas manifest on CT as abnormal soft tissue densities within the middle ear cavity. Mass effect and osseous erosion help differentiate these lesions from chronic granulation tissue which may be associated with chronic otitis media. Other differential considerations for abnormal soft tissue densities within the middle ear include paragangliomas, aberrant internal carotid artery, and dehiscent internal jugular vein. These lesions tend occur in characteristic locations which are atypical for cholesteatomas. On MR, cholesteatomas do not enhance after the administration of gadolinium unlike these lesions which will either enhance or demonstrate a flow void.