Cholesteatoma is histologically-equivalent to an epidermoid cyst and is composed of desquamated keratinizing stratified squamous epithelium forming a mass. They usually present with conductive hearing loss.
The mass is lined by epithelium (facing inwards) which continues to grow, thereby shedding additional cells into the mass. Their cholesterol content (which is not always present) is responsible for their name, although 'keratoma' is probably a more apt term.
Cholesteatomas of the temporal bone and middle ear can be divided into:
- congenital cholesteatoma: accounting for only 2%
- acquired cholesteatoma: 98%
- external ear canal cholesteatoma
- mural cholesteatoma
- petrous apex cholesteatoma
Conventional non-contrast MR imaging with diffusion-weighted imaging is recommended in all patients with a suspicion of cholesteatoma. An MRI should be performed especially in patients with previous surgery for cholesteatoma since recurrence or residual tumour can be detected with great accuracy. If negative, it can obviate "second look" surgery. It is important to prepare the patient for the examination (clear the external auditory canal or the postoperative cavity) to avoid a false positive diagnosis.
The standard examination is a T2-weighted series in the coronal and axial plane, followed by a non-echo planar DWI series (b-values 0, 1000). On the DWI images with b-value 1000 s/mm2, a cholesteatoma becomes apparent as a hyperintense area. The signal intensity should be higher than visible on the DWI images with b-value 0 s/mm2. On the ADC map, low signal should be visible in the same area, confirming the presence of diffusion restriction.
A CT scan should be added in those cases where a cholesteatoma is detected with MRI. CT is required for preoperative planning (reconstruction of ossicles if needed) and to exclude perforation of the bony tegmen.
Pars flaccida cholesteatoma originates in Prussak's space and usually extends posteriorly.
Pars tensa cholesteatoma originates in the posterior mesotympanum and tends to extend posteromedially.
- cholesterol granuloma
In contrast with cholesteatoma, these show high signal on the ADC map. With these findings, recurrent cholesteatoma can be detected with 100% specificity. Cholesteatomas down to a size of 2 mm can be detected with this technique on a 1.5 T MRI machine.
Further differential diagnosis is to be made with:
- cerumen: which shows similar image characteristics to cholesteatoma but is located in the external ear canal
- abscess formation in the middle ear: can also show similar imaging findings but has a completely different clinical appearance
Important CT features to comment on when reporting a cholesteatoma:
- erosions of the
- dehiscence of the
- the integrity of the
- the presence of cholesteatoma in the sinus tympani (which is the most hidden recess of the middle ear): to avoid residual disease
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