Acoustic schwannoma and arachnoid cyst colocated in the cerebellopontine angle--case report.

A 50-year-old female presented with a right acoustic schwannoma colocated with a cerebellopontine angle arachnoid cyst. The arachnoid cyst was distinct from the arachnoid cap surrounding the acoustic schwannoma. Initial excision of the arachnoid cyst created the space required to excise the schwannoma. The acoustic schwannoma had surprisingly dense adhesions to the brainstem, probably due to the constant pressure exerted by the cyst displacing the tumor towards the brainstem. The acoustic schwannoma was excised by meticulous dissection. Such a coexisting lesion should be suspected when a large cystic collection surrounds an acoustic schwannoma. Initial excision of the arachnoid cyst will prevent excessive cerebellar retraction.