[A surgical fenestration of an arachnoid cyst in the cerebellomedullary cistern].

Arachnoid cysts originating from the cerebellomedullary cistern are very rare, and their indications for surgical treatment remain controversial. A 41-year-old man with a cystic lesion in the left cerebellomedullary cistern presented with sudden perspiration, palpitations, and vertigo. Subsequently, he complained of numbness of the left extremities and a dull headache. Otolaryngological evaluation revealed no abnormality. Physical examination on admission showed no neurological manifestations, except for left sensory disturbance. MRI revealed a cyst in the left cerebellomedullary cistern. Radiological examination did not reveal direct compression of the brain stem by the cyst, but the left cerebellum was compressed by the cyst. The patient underwent surgical fenestration of the arachnoid cyst via the midline suboccipital approach. The symptoms immediately disappeared after surgery. Patients with an arachnoid cyst in the cerebellomedullary cistern usually present lower cranial nerve dysfunction as a symptom. In our case, there was no cranial nerve dysfunction, but anatomically unexplainable symptoms, such as perspiration, palpitations, dizziness, and numbness of the left limbs, were observed. In conclusion, even if a patient with an arachnoid cyst in the posterior cranial fossa presents unexplainable symptoms, surgical intervention, including fenestration, is one of the therapeutic options.