The upper cervical spine plays an important role in the venous drainage system of the brain, brain blood flow and brain cooling. Back pressure against the vertebral venous outlets in the upper cervical spine can thus be a cause of CCSVI, decreased blood flow and decreased cooling capacity of the brain. An overview of the cranial veins will make the connection clear.
The cranial veins include the veins of the face and scalp, the diploic veins, the emmisary veins and the dural sinues. The diploic veins, seen in the picture above, and mentioned in the previous post, sit between the inner and outer plates of the membranous bones of the skull that cover the cranial vault.
The dural sinuses seen in the pictures below are the main drainage routes of the brain inside the cranial vault. They are called dural sinues because they are not true veins. Instead they are tunnels formed by the outer coat of the brain itself, the dura mater. The inside walls of the dural sinuses are lined with the inner walls of veins. The dura mater, which means tough mother or material, makes the dural sinus drainage system much stronger than typical veins. As a result, they are better able to withstand stress and resist deformation from the pressure and movement of the brain, which sits on top of and presses down against them.
The cranial veins of the face and scalp, diploe and dural sinuses are all interconnected by the emissary veins. In contrast to the rest of the body, none of the cranial veins have valves to check or prevent reverse flows. That’s an important fact when it comes to discussing MS lesions, which I won’t go into here.
If you click on the picture to the left and look closely, you will see that the dural sinuses are depicted by stripes inside the skull. You will also see little black semicircles on the top and the bottom of the skull. The semicircles represent emissary veins, which link the face and scalp veins to the diploic veins and to the dural sinues.
The emissary veins play an important role in draining the head and brain. The ones located toward the back and bottom of the skull seen behind the outline of the ear, drain into the vertebral veins of the spine. In addition to drainage, the emissary veins also play a critical role in cooling the brain. They do so by delivering blood, that has been cooled by conduction and sweat evaporation at the surface of the face and scalp, to the diploe and to the dural sinues.
Besides cooling the diploe and dural sinuses, the brain also uses two counter current heat exchanger tunnel systems in the dural sinuses to cool incoming arterial blood before it enters the brain. The two cavernous (dural) sinuses are located inside the cranial vault. If you click on the picture to the right you will see the internal carotid depicted passing through the cavernous sinus before it enters the brain.
The other tunnel is called the suboccipital cavernous sinus, which is also known as the atlantooccipital membrane as depicted in the picture below. The suboccipital cavernous sinus is located just outside the skull between the first cervical vertebra and the occipital bone at base of the skull.
Even though it is outside the skull, studies have shown that the suboccipital cavernous sinus is constructed of nearly identical materials, in the same way and serves the same function as the cavernous sinus. For this reason, some scientists now consider it to be part of the dural sinuses of the brain. The suboccipital cavernous sinus contains and cools the two vertebral arteries before they enter the brain.
Thus, the brain is surrounded by cooled venous blood in the cranium and incoming arterial blood keeping the brain about two to three degrees cooler than the rest of the body. Some physical anthropologists attribute the extra large size of the human brain more to its exceptional cooling capacity than to the increase in arterial blood flow that comes with upright posture. Anthroplogists refer to human encephalization due to enhanced cooling capacity as the “radiator theory.”
Both the cavernous and suboccipital cavernous sinuses also play a role in maintaining blood flow and pressure in the brain. Their inner walls contain pressure sensors called baroreceptors that detect pressure in the tunnels. When pressure goes up they send signals that cause the muscles in the incoming arteries to constrict and decrease blood flow. When pressure drops they signal the blood vessels to open up and increase blood flow. Technically it is called the “neurovascular myogenic autoregulatory reflex mechanism.” As an aside, similar important pressure receptors and blood flow regulators are located in the carotid sinuses near the Adams apple of the throat.
The cranial veins drain into two extracranial venous drainage routes. One route is the jugular veins. The other is the vertebral veins. Interestingly, in contrast to the jugular veins, the vertebral veins have no valves making them similar to the cranial veins. Thus, back pressure against the vertebral veins can affect both the drainage and cooling capacity of the brain. This is interesting in light of the fact that in addition to evidence of CCSVI, MS patients often experience symptoms of heat intolerance.
An increase in pressure in the suboccipital cavernous sinus can also decrease blood flow through the vertebral arteries that pass through it before supplying the inner rear and lower most parts of the brain. It can do so by either direct compression of the vertebral arteries, or by stimulating the pressure sensors in the sinus walls thereby causing the arteries to constrict.
Decreased blood flow through the vertebral arteries can cause a wide variety of symptoms such as fatigue, dizziness, loss of balance and coordination to name a few. The complete list of symptoms is too long to discuss here so I will save it for future posts.