Arachnoid cyst of the cranial posterior fossa causing sensorineural hearing loss and tinnitus: a case report.

Arachnoid cysts are developmental collections of cerebrospinal fluid covered by layers of arachnoidal epithelium and are usually located in the middle cranial fossa. Localizations in the posterior fossa are uncommon and generally remain asymptomatic or cause vague and non-specific symptoms. We here report the unusual case of a young patient with an arachnoid cyst of the posterior fossa that had become manifest in the form of left-sided hypoacusia and tinnitus. Audiometric evaluation, auditory brainstem responses and transient-evoked otoacoustic emissions tests revealed a progressive monolateral sensorineural retrocochlear hearing loss with cochlear involvement. Magnetic-resonance imaging showed an arachnoid cyst of the cerebellar convexity compressing the cerebellar hemisphere and thus the homolateral cerebellopontine angle. Because of the progressive worsening of the retrocochlear impairment after a 6-month wait-and-see period, the patient finally underwent endoscopic cyst decompression. This was judged to be successful as it stopped the progression of hearing loss and the impairment of the auditory brainstem responses and made the tinnitus more tolerable.