Summary

Perimesencephalic subarachnoid haemorrhage is usually asymptomatic other than meningeal irritation sign. The authors report a case of subarachnoid haemorrhage at the quadrigeminal cistern showing ipsilateral trochlear nerve palsy and discuss the pathogenesis. A 71-year-old man with a history of diabetes mellitus and acute myocardial infarction presented with diplopia. He underwent CT, which revealed subarachnoid haemorrhage at the left quadrigeminal cistern. Neurological examination revealed left isolated trochlear nerve palsy, with results otherwise normal. The diagnosis of perimesencephalic subarachnoid haemorrhage was established on neuroimaging. The amount of haemorrhage is related to symptoms. A dense clot in the quadrigeminal cistern might have been the cause of trochlear nerve palsy.