Interdural haemorrhage of the posterior fossa due to infraclinoidal carotid artery aneurysm rupture.

Several anatomical studies indicate that the intracranial pachimeninges consist of two dural layers joined together, which divide while bordering the venous sinuses, therefore located in an interdural space. We present here an uncommon case of haematoma due to rupture of an infraclinoidal internal carotid artery aneurysm. The dome of the aneurysm leaned against the posterior wall of the cavernous sinus and, following laceration, pierced the inner dural layer and caused its detachment from the periosteal layer, thus determining a truly interdural haematoma which progressively involved the whole posterior fossa. A 42-year-old female was admitted to our institution with a recent history of thunderclap headache and right ophthalmoparesis. Two cerebral computerised scan tomographies performed elsewhere tested negative for subarachnoid haemorrhage. A cerebral magnetic resonance imaging (MRI) showed a thin collection of blood adjacent to the clivus and all along the wall of the posterior fossa and foramen magnum. A right infraclinoid internal carotid artery aneurysm was also diagnosed, subsequently better highlighted on angiography. The patient underwent surgery with aneurysm clipping. Post-operative course was uneventful, and control angiography showed complete exclusion of the aneurysm from blood circulation. The patient was discharged 5 days later. At 3 months follow-up ophthalmoplegia had disappeared, and the patient had fully recovered. The possibility of a truly interdural location, particularly in cases of non-traumatic parasellar or clival haematomas, must be included in the differential diagnosis of posterior fossa extra-axial haemorrhages. MRI is the test of choice for diagnostic purposes.