Radiological and audiological features of a recently defined inner ear malformation: Cochlear Hypoplasia Type 4.

Objectives: To point out the radiological and audiological characteristics of a recently defined cochlear hypoplasia type: cochlear hypoplasia type 4. To describe this malformation in relation to adjacent anatomial structures within temporal bone. Material and Methods: This study was a reprospective chart review of the cochlear hypoplasia type 4 (CH-IV). Cases were selected from our departments' database and their CT and MRI images together with audiological evaluation data were revised.Inclusion criteria was having CH-IV diagnosis in at least one ear and having both CT and MRI carried out in hospital. Axial and coronal CT of the temporal bone were evaluated to delineate the features of the malformation appropriately. MRI images were reviewed to evaluate the statos of the cochlear nerve (CN). CN was evaluated on axial and especially on sagittal-oblique T2 weighted images and classified as aplastic, hypoplastic, or normal. Results: There were 26 ears with CH-IV in our database. In CH-IV; the cochlea has a normal basal turn, but middle and apical turns are severely hypoplastic and located anterior and medially rather than in their normal central position. The labyrinthine segment of the facial nerve is usually located antero-superior to the cochlea rather than in its normal location. Six (27.3%) out of 26 with CH-IV had cochlear nerve hypoplasia. Four ears had naroow IAC. On axial CT scan; facial recess was evaluated also. An imaginary line crosses from the mastoid segment of the facial nerve to the basal turn and round window was created. In CH-IV cases; mastoid segment was anterior to this line. In transmastoid approach; exposure of the round window might be difficult because of this rotational malformation. 22 ears (84.6%) anomalous route of the labyrinthine segment of the facial route. In those cases labyrinthine segment was anterosuperoriorly displaced. 7.7% of the cases had slight to mild hearing loss(HL), 46.2% had moderate to moderately severe HL and 46.2% had severe to profound HL. Some selected cases did not require hearing aids and cochlear implantation. As the number of turns are smaller with narrower scalae, it is strongly advisable to use thin and shorter electrodes. Thick and long electrodes may not be inserted fully into the cochlea. It is important to diagnose CH-IV since the visualization of round window may be challenging and this anomaly has diverse audiological features which directly affect the treatment plan. [ABSTRACT FROM AUTHOR]