Divertikel Meatus acusticus internus

Role of MRI
as first-line modality in the detection of previously undiagnosed otosclerosis: a single tertiary institute experience. A 64-year-old male patient with heavy bilateral SNHL (case 7). Axial CISS image (a) shows curvilinear T2 hyperintensities (arrow) surrounding the left cochlea. Subsequent axial HRCT image (b) confirms extensive very hypodense plaques (arrows) in the left pericochlear region, likely long-standing disease. Axial CISS image (c) at a lower level also shows a small right IAC diverticulum (dashed arrow), confirmed on subsequent axial HRCT image (d) as an ‘ear-like’ outpouching anterior to the right IAC (dashed arrow). Also note the large plaque in the right FA region (short arrow)
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An internal auditory canal (IAC) diverticulum or notch, also known as cupping of the internal auditory canal, is a small focal outpouching arising from the anterolateral wall of the internal acoustic canal (IAC). This is a benign normal variant and distinct from, although may occur in conjunction with, cavitary otosclerosis.

Epidemiology

IAC diverticula are identified in approximately 5% of petrous temporal bone CT scans . Approximately 60% of patients have bilateral diverticula . A minority of patients demonstrate concurrent otosclerosis .

Clinical presentation

The clinical significance of IAC diverticulum is not well established in the medical literature. An association with hearing loss, especially sensorineural hearing loss, has been dispelled. When an IAC diverticulum is present, careful evaluation for evidence of concurrent otosclerosis should be saught as this could indicate an alternative cause of hearing loss. However, the presence of an IAC diverticulum in isolation should have no relation to the history of hearing loss.

Microscopic appearance

Histologic specimens demonstrate smooth osseous margins along the walls of the diverticulum with intact structure of the surrounding bone .

Radiographic features

CT

Internal auditory canal diverticula are best seen on thin slice petrous temporal bone CTs. They consist of a small CSF isodense outpouching from the anterolateral wall of the IAC near the dense otic capsule. They vary in size from a small focal notch to a larger diverticulum .​ This is distinct from cavitary otosclerosis which typically demonstrates internal density slightly greater than that of the adjacent CSF and ill defined margins, in addition to evidence of fenestral and/or retrofenestral otosclerosis.

MR

Internal auditory canal diverticula imaged by MR are best seen on thin slice T2 weighted images through the petrous temporal bone. They appear as a well demarcated outpouching along the anterior inferior wall of the IAC with CSF signal within the defect .

Differential diagnosis

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