BREAKDOWN OF THE MENINGEAL BARRIER SURROUNDING THE INTRAORBITAL OPTIC-NERVE AFTER EXPERIMENTAL SUBARACHNOID HEMORRHAGE

PURPOSE: The intraorbital optic nerve sheath meninges contain a perine ural subarachnoid space lined by meningeal cell layers and intercellul ar fibrous tissue. We sought to determine whether functional or struct ural characteristics, or both, of the optic nerve sheath are influence d by the increased intracranial pressure after the rupture of cerebral aneurysms. METHODS: We infused the great cisterns of cats with either x-ray contrast medium or autologous blood. The cisternal infusions we re done under the experimental condition of a sudden; 2.5-minute incre ase in intracranial pressure similar to that recorded after the ruptur e of cerebral aneurysms in humans. RESULTS: Digital subtraction radiog raphs of the optic nerves taker! during the cisternal infusion of cont rast medium at the start showed the opacification of the optic nerve s ubarachnoid space, After 2 minutes, the contrast medium leaked into th e orbit, indicating the breakdown of the meningeal fluid barrier, Ultr astructural investigation of the optic nerve sheath after high-pressur e cisternal infusions showed the arachnoid cell layers scattered. The flattened arachnoid cells displayed mainly intracellular and some inte rcellular, porelike openings. After infusion of blood into the great c istern, erythrocytes were found within porelike openings of the arachn oid cells, CONCLUSIONS: The meningeal fluid barrier of the optic nerve sheath can be destroyed by pressure changes associated with subarachn oid hemorrhage. This disruption might be regarded as a natural optic n erve sheath fenestration that allows outflow of cerebrospinal fluid in to the orbit to protect the optic nerve from increased intracranial pr essure after aneurysmal rupture.