Object: To explain the spontaneous CSF outflow into the orbit, the ultrastructure of the perineural meningeal layers at the distal and the proximal portions of the optic nerve were compared.
Methods: Ten cats were perfusion fixated and the orbital content removed for transmission and scanning electron microscopy. In five animals a 60-min cisternal infusion of contrast medium at low intracranial pressure was performed before perfusion fixation.
Results: In the contrast-infused animals it was possible to demonstrate the leakage of contrast medium in the distal portion of the optic nerve sheath (ONS) from the subarachnoid space (SAS) into the orbit and find it in the conjunctival lymphatics. Electron microscopy revealed that in the distal portion of the ONS the neurothelial layers are significantly thinner, some consisting of only one layer. Pore-like openings in the neurothelial covering are seen in the distal portion. Excavations of the SAS are far more numerous in the distal portion of the ONS. The excavations reach the neurothelial layer. Intracellular and extracellular filaments are more numerous in the distal portion of the ONS. There is no significant difference in the dura mater between the distal and proximal ONS. The results show the existence of an arachnoid window area in the distal portion of the ONS. It is characterised by a continuous, but thinned neurothelial barrier layer, with few pore-like openings.
Conclusions: The main differences between distal and proximal ONS are a thinned neurothelial barrier layer and an increased number of intercellular filaments and pore-like openings. The findings explain the lymphatic CSF outflow pathway along the optic nerve.