External auditory canal cholesteatoma

External areturnsauditory canal cholesteatomas are uncommon locations (EACCs) are rare, the external auditory canal being an unusual site for cholesteatomascholesteatoma, which are usually in the middle ear or petrous apex.  When they occur lateral to the tympanic membrane, they are referred to as external auditory canal cholesteatomas.  

Epidemiology

The external acoustic canal is a rare location for a cholesteatoma with an estimated incidence around 1.2 per 1,000 new otological patients. The overall incidence rate in one large study was 0.30 per year per 100,000 inhabitants ¹. For comparison, the annual incidence of middle ear cholesteatoma is around 9.2 per 100,000.

Pathology

A distinction from keratosis obturanshas been introduced only in the last few decades. Both mainly consist of epithelial debris in the external acoustic canal. The most useful findings confirming an external auditory canal cholesteatoma are focal osteonecrosis with or without sequestration and lack of epithelial covering of the bony surface ². The distinction between keratosis obliterans and cholesteatomas is not merely one of pedantry, but impacts on treatment strategy ³.

Classification

External auditory canal cholesteatoma can be divided into:

• primary (idiopathic/spontaneous)
  • rare
  • no antecedent cause identified
• secondary
  • more common
  • postoperative, postinflammatory, posttraumatic, radiation-induced

A special type of secondary cholesteatoma of the external auditory canal which is worth mentioning separately are those associated with congenital atresia of the external auditory canal.

Radiographic features

Plain filmradiograph

The Schüller, Stenvers, and Mayer views of the petrous bone have been largely replaced by CT.

CT

High-resolution bone window temporal bone CT is the method of choice to localise the lesioncholesteatoma and to demonstrate bony erosions. They appear as soft tissue attenuating lesions. 

MRI

CannotMRI cannot show the early involvement of compact bone. Its role in EACC is unknownnot defined.

Treatment and prognosis

Small lesions can be treated endoscopically under local anaesthesia, whereas larger lesions need surgery to remove the cholesteatoma and affected bone areas. Grafting of defects may be necessary. Prognosis depends on the extent at the time of diagnosis.

History and etymology

External auditory canal cholesteatomas were first reported by Toynbee in 1850.

Differential diagnosis

General imaging differential considerations include

• keratosis obturans
• necrotising otitis externa