The brain drains into two principle venous outlets. One route is the internal jugular veins. The other route is the vertebral venous systems. The openings to both drainage routes are located in the bottom of the posterior fossa of the cranial vault.
The vertebral veins are actually part of a large network of veins called the vertebral venous plexus or VVP which is one aspect of the vertebral venous systems. The VVP is located alongside the spinal vertebra within the spinal canal. Even though the veins are small in size they are many in number and extend from the head all the way down to the tail end of the spinal cord. Thus, the drainage capacity of the VVP is huge.
The Waterfall in the Brain
Upright posture causes a waterfall in the brain that follows a steep descent back to the heart. It starts in the superior sagittal sinus at the top of the brain. From there it flows down into the transverse sinus to the sigmoid sinus and then into the internal jugular or into the vertebral venous plexus (VVP) seen in the picture below behind the jugular veins.
The suction of the waterfall increases blood flow to the brain due to a siphoning affect. The increase in blood flow requires a proportionate increase in venous drainage capacity in order to prevent a backup of blood in the brain. In this regard, over time as humans were becoming more erect the force of the steep downhill stream eventually whittled away and carved out extra holes in the base of the skull the same way a river wears away and carves out stones that block its path.
The emissary veins are the extra veins that pass through the additional outlets in the base of the skull. One typical location is behind the ear. More importantly, there was a trend for emissary veins to pass through the foramen magnum and into the hypoglossal and condylar canals (located inside the foramen magnum). This is an important point because certain of these canals, such as the hypoglossal and condylar, are located below the level of the internal jugular foramen.
This is relevant to CCSVI research as these canals are located at the lowest point, the basement, in the skull. Therefore, when they remove the stenosis in the jugular they are not affecting all of the blood in the base of the cranium. According to a paper published in 2002 in the AJNR The Craniocervical Venous System in Relation to Cerebral Venous Drainage by Ruiz, Gailloud, Rufenacht, Delavelle, Henry, and Fasel “Passing from the supine to the upright position favors cerebral venous ouflow into vertebral systems rather than into the internal jugular veins.”
All of the extra venous outlets drain into the vertebral venous plexus, located inside the spinal canal and outside surrounding the spine.
Chronic Craniocervical Venous Back Pressure
The design of the upper cervical spine and skull makes the connections of the extra veins of the brain and their connection to the vertebral veins of the spine highly susceptible to compression. Blockage or compression of the emissary vein outlets by malformations, misalignments and deformation of the upper cervical spine can cause chronic craniocervical venous back pressure and drainage problems during upright posture (CCVBP). This may play a role in neurodegenerative processes and subsequent diseases.
The list of skull design problems that can impact the vertebral veins is long and includes conditions such as platybasia, basilar invagination, craniosynostois, and cleidocraniodysostosis to name a few. Design problems of the spine that can affect blood flow in the vertebral veins include Klippel-Feil, stenosis, scoliosis and a whole host of other genetic design problems.
The VVP and Inversion Flows
In contrast to most veins, the VVP like the cranial veins, have no valves to check or prevent back flow, known as inversion flows. This allows blood to flow in either direction depending on posture. Standing on one’s head, for example causes inversion flows in the VVP toward the brain.
Because the VVP lack valves circulatory and respiratory fluctuations, called cardiorespiratory waves are, likewise, transmitted back to the brain.
In addition to inversion flows, upright posture and bipedal motion cause blood to oscillate back and forth within the brain and skull bones similar to walking with a cup full of water.
The movement of venous blood within the skull shapes the special joints of the skull called sutures. They were called sutures by early anatomists because their shapes resemble stitches but they also resemble waves. In this regard, the large amplitude waves seen in the shapes of the sutures at the rear of the skull are a reflection of those large stresses as seen in the picture below.
Lastly, venous pressure in the brain also affects the pressure gradient and thus the flow of cerebrospinal fluid (CSF), which is produced in the ventricles depicted in red in the picture under “The Waterfall in the Brain” above. Thus venous flow can affect CSF flow. This is important because poor or low CSF flow can also lead to normal pressure hydrocephalus. Also, low CSF flow has been associated with the worst most progressive type of Alzheimer’s disease, which shares a comorbidity with glaucoma. It is possible, therefore, that blockage or compression of the VVP may play a role in neurodegenerative diseases such as Alzheimer’s, Parkinson’s and multiple sclerosis.