An Internal Auditory Canal Arachnoid Cyst With MR Enhancement Mimicking Vestibular Schwannoma: A Rare Case Report.

Internal auditory canal (IAC) arachnoid cysts are rare lesions that can mimic vestibular schwannomas. A 39-year-old female presented with a 9-year history of left-sided tinnitus and hearing loss. MRI showed an expanding cystic lesion in the IAC with ring enhancement, mimicking vestibular schwannoma. However, complete cerebrospinal fluid signal suppression on FLAIR and no diffusion restriction on DWI suggested an arachnoid cyst. Intraoperative findings revealed a cyst enveloped by thickened arachnoid membrane. The diagnosis remains presumptive based on intraoperative findings, as histopathological confirmation was not obtained due to insufficient specimen. This case highlights the value of multimodal MRI, especially FLAIR and DWI, to distinguish these entities despite atypical enhancement.