Role of Multi-detector CT and MRI in Diagnosis of Congenital Inner Ear Anomalies

Introduction:

Inner ear malformation (IEM) with associated sensoryneural hearing loss (SNHL) is a major cause of childhood disability.

The embryology of the inner ear must be known as many of the inner ear malformations present as a result of the arrest during the various stages of embryology.

Congenital malformations of the inner ear may be considered in two broad categories:

1. malformations with pathologic changes that involve only the membranous labyrinth. 2. malformations that involve both the osseous and the membranous labyrinth.

Congenital sensorineural hearing loss arises as a result of abnormalities in the inner ear, the vestibulocochlear nerve, or the processing centres of the brain. The abnormality may have a genetic cause or be a sequel of infection or injury at birth; in some cases, no cause is identified.

One of the paths in the investigation of congenital sensorineural hearing loss (CSNHL) is to try to characterize its aetiology through the inner ear evaluation using high resolution computer tomography (CT) scans. With minor malformation, it is not always possible for a simple visual inspection to recognize if the structure in the inner ear is normal or not.

Both CT and MR can be used to look at inner ear malformations but often both techniques are complementary. CT is preferred when associated middle or external ear malformations must be excluded. Magnetic resonance is preferred when subtle changes in the membranous labyrinth or abnormalities of the nerves in the internal auditory canal must be visualised Moreover, some changes inside the membranous labyrinth can only be seen on sub-millimetric heavily T2-weighted MR images.

Only MR can demonstrate the abnormal course, hypoplasia or aplasia of the vestibulocochlear nerve (or one of is three branches) and facial nerve.

HRCT and MRI temporal bone are complementary to each other in evaluating children for cochlear implantation as HRCT is excellent for demonstrating bony details but, lack in providing details of inner ear neural structures and MRI is better than CT in demonstrating vestibulocochlear nerves.

Combined HRCT and MRI studies are mandatory for evaluation of inner ear, the radiologist must be familiar with imaging findings that absolutely contraindicate implantation (Cochlear aplasia, cochlear nerve aplasia and labyrinthine aplasia), and with those that relatively contraindicate implantation (labyrinthitis ossificans, other inner ear dysplasia) and with other findings that could significantly alter or complicate surgery (hypo plastic mastoid process, facial nerve dehiscence, oto-mastoiditis)