Endoscopic anatomy of the outer arachnoid. (A) Outer arachnoid cover of the left oculomotor trigone through left supraorbital approach. Note the outward protrusion of the outer arachnoid along the oculomotor nerve into the cavernous sinus (blue arrow-head). (B) Transition of the outer arachnoid from the tentorial surface of the cerebellum to the basal surface of the temporal lobe along the tentorial edge. Endoscopic view through left subtemporal approach. The blue arrow-head points to the trochlear nerve between its uncovered intracisternal segment and the beginning of the fixed part in the wall of the outer arachnoid. (C) Outer arachnoid relations of the tentorial apex viewed through a left supracerebellar-infratentorial approach. The arachnoid of the occipital lobe, cerebellum, and the splenium of the corpus callosum converges around the junction between the great vein of Galen and the straight sinus and follows the vein with an inward protrusion to the direction of the third ventricle. (D) Endoscopic view of the suprapineal arachnoid body through the same approach. (E) The outer arachnoid cover around the great vein of Galen follows its tributaries to the direction of the velum interpositum. The blue arrow-head points to this strong outer arachnoid cover around the left internal cerebral vein. Left supracerebellar-infratentorial approach. (F) Membranous sleeves around the deep veins in continuation with the meningeal structures of the velum interpositum in the top of the third ventricle. The approach is the same as on the previous picture. (G) Dural entrance of the right vertebral artery viewed through a right retrosigmoid approach. The blue arrow-head points to the inward protrusion of the outer arachnoid from the inner surface of the dura to the vertebral artery. The endoscope was introduced between the cranial and spinal roots of the accessory nerve. (H) Transition of the outer arachnoid between the intracranial and the intraspinal spaces. Through the small opening, the right posterior inferior cerebellar artery is visible (blue arrow-head). Median suboccipital approach. (I) After the introduction of the angled-endoscope through this small opening (blue arrow-head) the most caudal part of the cerebellomedullary cistern and the upper portion of the ventral spinal subarachnoidal space is visible.
a.=artery, ant.=anterior, arachn.=arachnoid, cer.=cerebral, cerebell.=cerebellum, chor.=choroid, CNIII=oculomotor nerve, CNXI=accessory nerve, ICS=intermediat cervical septum, int.=internal, med.=medial, occ.=occipital, petrocl.=petroclinoid, PICA=posterior inferior cerebellar artery, plex.=plexus, post.=posterior, SPAB=suprapineal arachnoidal body, str.=straight, subd.=subdural, tent.=tentorial, tons.=tonsil, trig.=trigone, v.=vein, VA=vertebral artery.