[Dilation of subarachnoid space around the optic nerve in a patient with subdural effusion: a case report].

A fifty-year-old man who had a history of minor head injury a month previously presented with headache, visual disturbance and papilloedema. Brain MR imaging showed bilateral subdural effusion and fat saturated orbital MR imaging demonstrated dilated subarachnoid space around the optic nerve. The diameter of the subarachnoid space behind the globe was 7.0 mm and that of the optic nerve was 3.5 mm. Bilateral simple drainage was performed to prevent deterioration of the visual disturbance. Light bloody fluid with a subdural pressure of 10.5 cmH2O was drained from the burr hole at the left side, and colorless fluid was drained from the right. Orbital MR imaging during continuous drainage revealed shrinkage of the subarachnoid space around the optic nerve. However, follow-up MR imaging 5 months after drainage showed disappearance of the subdural effusion and the reappearance of the subarachnoid space around the optic nerve, even though the size was smaller than before surgery. These findings suggest that the diameter of the optic subarachnoid space co-relates with the intracranial pressure, and may be an indication for increased intracranial pressure.