[Acute onset of visual disturbance in a patient with arachnoid cyst in the middle cranial fossa].

Intracranial arachnoid cyst occurs most frequently in the middle fossa. Most of them are asymptomatic with or without neurological involvement. However some develop neurological deficits such as increased intracranial pressure, which manifests as headache, epilepsy and focal neurological deficits. Here we present an adult case of arachnoid cyst in the middle cranial fossa: the patient demonstrated rapidly deteriorating visual field defect and decreased visual acuity. The symptoms were improved by an emergency surgery. This is the first case report describing optic nerve compression due to an arachnoid cyst. Result of the coronal and sagittal magnetic resonance imaging (MRI) scans showed effective optic nerve compression. Long-standing asymptomatic arachnoid cysts might progress rapidly to cause cranial nerve deficits thus meticulous MRI follow-up are important in these patients.