Non-haemorrhagic subdural collection complicating rupture of a middle cranial fossa arachnoid cyst.

Arachnoid cysts are a common incidental finding on routine brain imaging and, for the most part, their presence is uneventful. Occasionally they may be associated with haemorrhage into the subdural compartment. Rarer still is simple rupture of the contents of the arachnoid cyst into the extra-axial space. MRI can help distinguish between these two rare occurrences--an important distinction to make as this may assist in directing the treating clinician toward the most appropriate management plan.