Acquired cholesteatoma

Acquired cholesteatomas make up 98% of all middle ear cholesteatomas and are almost always closely related to the tympanic membrane, from which most are thought to arise. 

Clinical presentation

The vast majority of acquired cholesteatomas develop as a result of chronic middle ear infection 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 : episodic vertigo typical; lateral semicircular canal is most frequently involved
• cochlear fistula : less common
• perilymphatic fistula : can be spontaneous or as a result of surgery to treat existing labyrinthine or cochlear fistulas
• labyrinthitis
• facial nerve dysfunction including the rare inflammatory neuroma of the facial nerve.
• extension through inner ear into 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 complication as petrous apicitis

Pathology

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

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

1. 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
2. invasion / migration
   • in the setting of a previous perforation
   • keratinized cells 'invade' the middle ear through the perforation
3. basal cell hyperplasia / papillary ingrowth
   • invasive hyperplasia of the basal cell layer of the tympanic membrane as a result of infection
4. metaplasia
   • as a result of chronic irritation from middle ear infection

Radiographic features

CT

CT is the modality of choice for diagnostic assessment of cholesteatomas, due to it's ability to elegantly demonstrate the bony anatomy of the temporal bone. Cholesteatomas appear as regions of soft tissue attenuation, exerting mass-effect and resulting in bony erosion. 

Findings depend on the part of tympanic membrane the cholesteatoma arises. 

• pars flaccida (82%) 
  • superior extension : most common; it expands into Prussak's space, eventually eroding the scutum and displacing the ossicles medially
  • inferior extension : less common, but more frequently seen in children
• pars tensa
  • posterosuperior (78%) : extends medial to the incus and displaces the ossicles laterally
  • anterior and inferior (22%)

MRI

Although MRI is not able to adequately delineate bony anatomy, it has the ability to potentially distinguish non-specific opacification from cholesteatomas. It is particularly useful in the post-operative setting, when CT may be indeterminate since granulation tissue, scarring and recurrent cholesteatoma may all appear similar ². 

Cholesteatomas demonstrate:

• T1 : low signal
• T2 : high signal
• T1 C+ (Gd) : no enhancement
• DWI : increased signal 

DWI is particularly useful when distinguishing a cholesteatoma from other middle ear masses. It is the only entity that demonstrates restriction. The sequence is however prone to artifact and care must be taken in how the sequence is performed, and interpreted ². 

The mechanism responsible for 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 mass if often difficult to remove completely. 

Differential diagnosis

The differential of a middle ear mass includes:

• cholesterol granuloma
  • high T1 signal
  • no enhancement
  • no restriction diffusion
• mucoid impaction
• glomus tympanicum
• facial nerve schwannoma

In the post operative setting the differential for a soft-tissue attenuating middle ear mass includes the entities above, but is usually restricted to three entities ²:

• recurrent cholesteatoma
  • increased signal on DWI
  • no enhancement post gad
  • high T1 signal
• granulation tissue
  • intermediate T1 signal
  • enhancement post gad
  • low signal on DWI
• scarring
  • low T1 and T2 signal
  • low signal on DWI