Pupillary enlargement caused by an acute extradural haematoma associated with a non-symptomatic arachnoid cyst, from compression of the optic rather than the oculomotor nerve.

We report the case of an anisocoric boy with a posttraumatic left frontal-temporal extradural haematoma associated with a previously asymptomatic arachnoid cyst of the left anterior and middle fossa. Usually, anisocoria after a head injury suggests a third nerve palsy caused by an uncal herniation. In our case, the pupillary enlargement appeared to be caused by indirect compression of the optic nerve by the extradural haematoma, because of the low resistance exerted by the middle fossa arachnoid cyst.