Acquired cholesteatoma

Acquired cholesteatomas are far more common than congenital cholesteatomas and are almost always closely related to the tympanic membrane and pneumatized portion of the temporal bone from which most are thought to arise ⁹.

Cholesteatomas occur far more commonly in the middle ear than in the external auditory canal. This article relates to middle ear acquired cholesteatomas. See the external auditory canal cholesteatoma article for details on that entity.

Epidemiology

Acquired cholesteatomas make up 98% of all middle ear cholesteatomas ⁹.

Clinical presentation

The vast majority of acquired cholesteatomas develop due to chronic otitis media and are usually associated with perforation of the tympanic membrane. Clinical presentation usually consists of conductive hearing loss, often with purulent discharge from the ear ⁶. 

Patients may also present due to one of many complications, which include: 

• labyrinthine fistula (perilymphatic fistula)

• cochlear fistula: less common

• labyrinthitis

• facial nerve dysfunction, including the rare inflammatory neuroma of the facial nerve

• extension through the inner ear into the internal acoustic meatus leading to deafness

• extension into the middle cranial fossa with possible meningitis, cerebral abscess, etc.

• extension into the petrous apex (rare) with similar complications to petrous apicitis

Pathology

Cholesteatomas are composed of densely packed desquamated keratinizing squamous cells, arising from a peripheral shell of inward-facing epithelium. As cells mature, they continue to be shed into the mass, resulting in slow growth ^1-3. 

Etiology

There are four hypotheses that relate to the formation of cholesteatomas; all may be true ^1,6:

• invagination/negative pressure

  • probably the most common cause

  • results from Eustachian tube dysfunction and tympanic membrane retraction, with debris and keratin eventually obstructing the neck of the retraction

• invasion/migration

  • in the setting of a previous perforation

  • keratinized cells 'invade' the middle ear through the perforation

• basal cell hyperplasia and papillary ingrowth: invasive hyperplasia of the basal cell layer of the tympanic membrane as a result of infection

• metaplasia: as a result of chronic irritation from middle ear infection

Location

There are a number of subtypes of acquired cholesteatoma, classified by location ¹¹:

• middle ear cholesteatoma cholesteatoma (most common)

  • pars flaccida type

  • pars tensa type

• external auditory canal cholesteatoma (rare)

Radiographic features

CT

CT is the modality of choice for diagnostic assessment of cholesteatomas, due to its ability to demonstrate the bony anatomy of the temporal bone in exquisite detail. Cholesteatomas appear as regions of soft tissue density, exerting mass effect and resulting in bony erosion, with the latter the hallmark of cholesteatoma ¹¹. 

Findings depend on the part of the tympanic membrane the middle ear cholesteatoma arises from: 

• pars flaccida type (more common)

  • superior extension: most common, it expands into Prussak space, eventually eroding the scutum, displacing the ossicles medially, and eroding the aditus ad antrum posteriorly (sometimes forming a common cavity involving the epitypmanum and aditus ad antrum) ¹¹

  • inferior extension: less common, but more frequently seen in children ^ref

• pars tensa type

  • posterosuperior extension (more common of this type): extends medial to the incus and displaces the ossicles laterally ¹¹

  • anterior and inferior extension ^ref

MRI

Although MRI is unable to adequately delineate bony anatomy, it can potentially distinguish non-specific opacification from cholesteatomas. It is particularly useful in the postoperative setting when CT may be indeterminate, since granulation tissue, scarring and recurrent cholesteatoma may all appear similar ². 

Signal characteristics

• T1: low signal

• T2: high signal

• T1 C+ (Gd): no enhancement

• DWI: diffusion restriction

Diffusion-weighted imaging is particularly useful when distinguishing a cholesteatoma from other middle ear masses. It is the only entity that demonstrates high signal intensity on DWI. However, the sequence is prone to artefact and care must be taken how the sequence is performed and interpreted ². Non-echo planar DWI is superior for the diagnosis of cholesteatoma and is therefore preferred if it is available ⁸. DWI (especially non-EPI DWI) is particularly useful in cases of suspected post-surgical recurrence.

The mechanism responsible for a high signal on DWI remains somewhat uncertain but is thought to represent either T2 shine-through alone or in combination with true restricted diffusion ^2-4.

Treatment and prognosis

Surgical excision is curative. However, recurrence is not uncommon because the lesion is often difficult to remove completely. 

Differential diagnosis

The differential is of a middle ear mass with bony erosion ¹¹:

• facial nerve schwannoma

• tympanic paraganglioma

• other tumors, e.g. rhabdomyosarcoma, squamous cell carcinoma, metastasis, giant cell tumor

• Langerhans cell histiocytosis

• chronic otitis media: characterized by tympanic membrane retruction

Cholesteatoma is difficult to differentiate on CT from other middle ear masses without bony erosion with MRI helpful in differentiating cholesteatoma from other entities ¹¹:

• cholesterol granuloma: high T1 signal, no enhancement, no restriction diffusion

• granulation tissue / fibrosis / scar tissue

• post-inflammatory ossicular fixation

• mucosal edema

Post-operative

Following resection of a cholesteatoma, the differential for a soft-tissue middle ear mass includes the entities above, but is usually restricted to three entities ^2,10:

• recurrent cholesteatoma

  • low T1 signal

  • no enhancement

  • increased signal on DWI

• granulation tissue

  • intermediate T1 signal

  • enhancement

  • low signal on DWI

• scarring

  • low T1 and T2 signal

  • low signal on DWI

Practical points

• careful review of areas not visible on otoscopy should be performed including the sinus tympani, facial recess, and anterior epitympanic recess ¹¹