[A case of arachnoid cyst in the posterior fossa with lower cranial nerve palsy].

We experienced a rare case of arachnoid cyst in the right cerebellomedullary cistern. A 59-year-old female was admitted to our clinic because of lower cranial nerve palsy (deviation of uvula to the left side, swallowing disturbance, curtain sign, hoarseness, atrophy of the right sternocleidomastoid muscle, and deviation of the tongue to the right side). MRI demonstrated a mass lesion compressed towards the medulla oblongata in the right cerebellomedullary cistern. MRI was very helpful as a diagnostic tool, since there is no bone object in the posterior fossa. Although by CT cisternography, the arachnoid cyst was thought to have a communication with the surrounding subarachnoid spaces, the cyst wall was removed because of its compressive symptoms on the lower cranial nerves. After the operation, the cyst shrank, and the clinical symptoms were reduced.