"One-and-a-half" syndrome after a resection of a midline cerebellar astrocytoma: case report and discussion of the literature.

This report describes a rare complication after the resection of a tumor of the posterior fossa, the "one-and-a-half" syndrome. The one-and-a-half syndrome is a disturbance of horizontal eye movements in which patients have lateral gaze palsy in one direction and internuclear ophthalmoplegia in the other direction. The patient was a 54-year-old woman who developed headaches, diplopia, and blurred vision over 6 months. Computed tomographic scans and magnetic resonance imaging demonstrated an enhancing, mixed density, midline mass of the cerebellum. After a resection of the mass, an anaplastic astrocytoma, the patient complained of more severe diplopia and facial weakness. An examination disclosed a left one-and-a-half syndrome, left peripheral facial paralysis, dysarthria, dysphagia, mild left hemiparesis, dysmetria of the left upper limb, and truncal ataxia. The brain stem showed no abnormalities on postoperative computed tomographic scans. After 4 months of follow-up, the one-and-a-half syndrome had not improved, even though other signs had improved or resolved. This syndrome is caused by damage to structures within the pontine tegmentum: the medial longitudinal fasciculus, the ipsilateral paramedian pontine reticular formation, or the ipsilateral abducens nucleus. Multiple sclerosis and brain stem infarction are the most common causes of the one-and-a-half syndrome. Less frequently, it is caused by primary and metastatic tumors of the brain stem and cerebellum. Rarely, the one-and-a-half syndrome can develop postoperatively after the removal of tumors of the posterior fossa. The mechanism of pontine tegmental injury remains unknown.