Head and Neck Imaging

Cholesteatoma

tumour-like mass of exfoliated keratin within a sac of stratified squamous epithelium, most often occurring in the middle ear. Cholesteatoma is usually an acquired disease ('secondary cholesteatoma'), but can be congenital ('primary cholesteatoma').

Acquired cholesteatoma is believed to result from ingrowth of squamous epithelium through marginal tympanic membrane perforations, from retraction pockets or from ingrowth into the middle ear of the basal layer of the tympanic membrane. Acquired cholesteatoma is usually related to chronic otitis media. Secondary cholesteatoma usually arises from the upper part of the tympanic membrane (pars flaccida or Shrapnells membrane) and will develop primarily within Prussaks space.

High resolution CT is an excellent technique for showing the location and extent of the lesion prior to surgery. On CT-images cholesteatoma usually presents as a more or less rounded soft tissue mass, often centred within the epitympanic recess (Fig.1); lesions are commonly associated with erosion of the lateral epitympanic wall (more specifically the scutum) and the ossicular chain (Fig.2). Depending on size and the tumour's position, middle ear and inner ear structures may be vulnerable to pressure- and enzyme-related erosion (Fig.3). Associated findings are thickening of the tympanic membrane and inflammatory polyps in the medial part of the external auditory canal. Differentiation from noncholesteatomatous chronic inflammation, granulation tissue or cholesterol granuloma, possibly coexisting with cholesteatoma, is difficult. MRI may provide additional information, as the signal characteristics and/or enhancement pattern of these entities are different from those of cholesteatoma (see also granulation tissue ear).

Acquired cholesteatoma of the external auditory canal is rare (Fig.4). It is thought to be caused by circumscribed periostitis, with embedding of the cholesteatoma in the bony wall (see also keratosis obturans). There is an association with acquired postinflammatory atresia of the external ear canal. Post-traumatic cholesteatoma is a separate entity, being a late complication of a longitudinal fracture temporal bone; entrapment of epithelium may occur before callus formation occurs and may give rise to the formation of a cholesteatoma.

Congenital (primary) cholesteatoma is an ectoblastic derived tumour, originating from epithelial rests. They have been found everywhere in the temporal bone. In these cases, the tympanic membrane always appears intact. In some locations, such as the petrous apex, the lesion may already be large at first presentation because of the paucity of symptoms produced (Fig.5).

Rarely, a cholesteatoma may occur in a paranasal sinus. On CT it resembles a mucocele.

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Fig.4

Axial (a) and coronal (b) CT image of left temporal bone. Soft tissue mass eroding through the posterocranial wall of the external auditory canal into the mastoid (arrows). External auditory canal cholesteatoma.
Cholesteatoma, Fig.1
Cholesteatoma, Fig.2 (a)
Cholesteatoma, Fig.2 (b)
Cholesteatoma, Fig.3
Cholesteatoma, Fig.4 (a)
Cholesteatoma, Fig.4 (b)
Cholesteatoma, Fig.5 (a)
Cholesteatoma, Fig.5 (b)