The venous system of the notochord and corresponding sclerotome extends from the basisphenoid (cavernous plexus) to the sacrum and gives rise to the ventral epidural drainage group. It collects the blood from spongious bony structures and has no primary role in the drainage of the central nervous system. It includes the basioccipital bone, the petrous pyramid, the basisphenoid with its adjacent sphenoid wings, and their related dural structures. The venous afferents of these regions are closely related to the bony structures, and they drain outside the bony limits, thus resulting in no subarachnoid (spinal or cortical) venous reflux. However, some factors may precipitate development of CVR in these regions such as thrombosis within the epidural space surrounding, remote, or distal to the shunt. This will produce reflux in the lateral epidural space as seen in rare cases of ventral epidural spinal shunts with perimedullary venous reflux. CVR can also be encountered in high-flow shunts forcing the emissary-bridging vein opening after reflux in the lateral epidural space. Ventral epidural shunts are characterized by a strong female predominance and absence of CVR unless extensive thrombosis of epidural drainage or high-flow shunts.

The dorsal epidural venous space includes the superior sagittal sinus (SSS), torcular, transverse sinus, medial occipital sinus, and posterior marginal sinus at the foramen magnum. It is poorly developed at the spinal level, and the presence of dorsally located dural sinuses intracranially is therefore the major difference between the venous systems of the brain and spine. Their formation is linked to the appearance, during evolution, of the dural falces and tentorium and associated with the development of the paleo- and neopallial structures. These sinuses result from the confluence in the epidural space of two different venous systems: the osseous system draining the cranial vault and the leptomeningeal system draining the brain. The venous pressure within the dural sinuses is usually low; thus, antegrade and craniofugal flow of the shunts is normally observed. CVR may occur after associated venous outflow restriction or with high-flow shunts; in case of variations of the venous opening into this system such as presence of an accessory epidural sinus, distal restriction of the sinus opening will rapidly produce CVR. Dorsal epidural shunts have no sex predominance, present at lower mean age, and can be multiple. Usually no leptomeningeal venous reflux is seen, and craniofugal flow is normally observed. Leptomeningeal reflux may occur following associated venous outflow restriction or with high-flow shunts. Variations of the venous opening into this system or distal restriction of the sinus opening will also be responsible for cortical venous reflux.

The veins draining the central nervous system (both spinal and cranial) are not related to the peripheral nerves but use “emissary” or “bridging veins.” These latter join the lateral epidural venous spaces as connecting drainage system. The leptomeningeal venous drainage has no direct confluent communication with the ventral and dorsal epidural venous plexuses, which drain the skull and spine. While the typical lateral epidural shunts correspond to spinal dural fistulas that drain into emissary or radicular veins joining the perimedullary venous plexuses, intracranial pathological locations involve emissary-bridging veins of the brainstem and their homologs draining deep cerebral structures such as the condyloid vein at the foramen magnum, the superior petrosal vein, the basal vein, the vein of Galen, the veins of the anterior cranial fossa, and the orbit. Thus, the following DAVS locations are encountered: marginal sinus, falco-tentorial angle, petrosal and basi-tentorial areas, Breschet sinus, paracavernous and intraorbital regions, and lamina cribriformis. The drainage is always directed towards leptomeningeal veins, and this type of DAVS is therefore always considered to be aggressive. Lateral epidural shunts are characterized by a male predominance and present at later ages.