A case report of vestibular dysfunction caused by cochleovestibular nerve deficiency in a patient with a normal internal auditory canal size

Abstract

　Recent advances in high-resolution MRI have enabled clear visualization of each nerve (cochlear nerve, vestibular nerve and facial nerve) in the internal auditory canal (IAC). Cochlear nerve deficiency (CND) is one of the known causes of bilateral or unilateral hearing loss in children. Although a narrow IAC was thought to be the cause of CND before the emergence of high-resolution MRI, recent studies have reported cases of CND associated with a normal IAC size as assessed by MRI; thus, CND is not able to diagnose by only existing a narrow IAC. There have been a few reports of CND associated with vestibular nerve deficiency, and most such cases were found to have a narrow IAC. However, there are only a few reports of assessment of vestibular function in patients with CND. In this paper, we report a rare case of a patient with CND and vestibular nerve hypoplasia, without a narrow IAC.

　An eight-year-old boy with congenital unilateral profound sensory neural hearing loss in his right ear presented to us with no reaction on auditory brain stem response (ABR) testing and a normal response on otoacoustic emission (OAE) testing. Heavily T2-weighted 3T-MRI showed absence of the cochlear nerve and hypoplasia of the vestibular nerve on the affected side. However, the internal auditory canal was normal in size on both the affected side and the normal side. Vestibular function tests, caloric testing and cervical vestibular myogenic potential (cVEMP) testing revealed vestibular dysfunction, canal paresis and decreased cVEMP reaction in the affected ear. In this paper, we would like to emphasize the necessity of considering the vestibular function in patients with CND, because of the possibility of associated vestibular dysfunction in these patients.