Internal auditory canal exostosis

Internal auditory canal (IAC) exostoses are bony growths that can narrow the IAC lumen, sometimes causing neurological symptoms due to nerve compression.

Epidemiology

Unlike their counterpart in the external auditory canal, IAC exostoses are uncommon and can be difficult to detect ¹.

Clinical presentation

IAC exostoses are usually found incidentally and do not cause any symptoms. When they do, however, they can be:

• sensorineural hearing loss

• non-pulsatile tinnitus

• vestibular dysfunction: ataxia/loss of equilibrium/vertigo

Pathology

Exostoses are grossly broad-based and microscopically demonstrate parallel concentric layers of subperiosteal bone, abundant osteocytes, and no fibrovascular channels ².

Radiographic features

To improve detection, we must look carefully at thin-cut CT or axial T2 sequences with thin slices (e.g. 3D CISS and FIESTA).

CT

Shows narrowing of the IAC due to a wide-based bony growth protruding into its lumen. It’s usually bilateral although asymmetrical. There is no radiographic evidence of bone marrow.

IAC exostoses can be difficult to detect if there is prominent beam hardening and streak artifact from the temporal bone.

MRI

• T2: hypointense wide-based bony growth.

• T1: hypointense, same intensity as cortical bone.

• T1 C+ (Gd): no contrast enhancement.

Treatment and prognosis

Patients with mild symptoms can be managed conservatively. Surgery is reserved for severe cases with neurologic deficits.

Differential diagnosis

• vestibular schwannoma

  • soft tissue mass

  • intense contrast enhancement

• Paget's disease

  • generalized bone involvement

• fibrous dysplasia

  • diffuse involvement of the entire temporal bone