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Pressure and Shear Stress in MS Lesions
Posted on October 7, 2010 by uprightdoctor
Image by Reigh LeBlanc via Flickr
According to Schelling one of the likely causes of MS lesions is venous back jets into the brain. He proposes that one of possible sources arise from normal cardiorespiratory waves. The other is from trauma. In either case venous blood flows backwards and into the brain.
One route of back jet into the brain is through the jugular veins. Schelling proposes that certain people are born or acquire incompentant valves in the jugulars that fail to check the reverse flows. The other route is through the vertebral veins which have no valves to prevent reverse flows. In the picture above, the left the jugular veins are the large veins in the front of the neck. The vertebral veins are the smaller veins in the back of the neck. The large veins inside the skull are called dural sinuses.
Typically, most, not all veins have valves to check and prevent back flow. Technically speaking none of the dural sinuses have valves. The alignment of some of the cerebral veins however serves to prevent reverse flows to a limited degree, which I won’t go into here. So disregarding that piece of information, the large veins of the brain have no valves.
Following the veins backwards, upstream and counter current to normal flow, the jugular and vertebral veins next connect to the sigmoid sinus, the S shaped sinus directly above them. The trasverse sinus is the short flat sinus that runs from the sigmoid sinus to the back of the skull. Right at that little circle junction at the back of the skull is another sinuse that runs forty-five degrees upwards and to the left toward te front of the brain. It’s called the straight sinus. Going straight up, beyond the junction of the transverse and straight sinus is the largest sinus of the brain that runs up to the top of the brain. It’s calle the superior sagittal sinus.
Now if you follow the straight sinus inward you will see it connects to the Great Vein of Galen, the Basal Vein of Rosenthal and the internal cerebral veins. These veins all drain the core of the brain. More importantly, regarding this discussion they go to the periventricular areas of the brain.
Now if you click on the picture and enlarge it, you will see a large shadow in the middle of the brain. The shadow you see is the lateral ventricle. The ventricles of the brain produce water called cerebrospinal fluid (CSF) which cushions, protects and supports the brain. If you look down by the vertebral veins you will see another shadow. That shadow is the spinal cord. Among other things it contains the subarachnoid space surrounding the cord. The subarachnoid space is part of the protective covering of the brain and cord called meninges. The ventricles, subarachnoid space and CSF pathways likewise have no valves.
I would like to put aside cardiorespiratory waves, which I disagree with as a source of injury to the brain, and focus on trauma. In contrast to cardiorespiratory waves, trauma produces massive uncontrolled forces that most certainly can reflux into the brain under significant pressure. In this regard, the vertebral veins contain a large volume of unchecked blood. The lumbar cistern also contains a significant volume of CSF. Lastly, the valves of the jugular veins have physical limitations which may be easily overwhelmed by massive forces.
According to Schelling venous back jets and massive refluxes from trauma simply follow the course of the dural sinuses. It makes sense then that the lesions in MS tend to show up around the larges veins in the brain and get progressively smaller as you follow the veins upstream counter-current to flow. MS lesions also tend to show up in the periventricular areas, which, as mentioned above, drain into the straight sinus system.
In other words venous blood back jets into the brain stretch the largest veins of the brain that take the brunt of the force, causing them to balloon out and strain nearby surrounding myelinated nerves. Researchers studying normal pressure hydrocephalus surmized many years ago that stretching from something as simple as edema alone was enough to break myelin. So basically speaking, myelin, is simply no match for the massive forces generated by whiplash and other similar type traumas. Trauma can generate significant pressure in the veins of the entire body never mind the brain. What’s more, pressure assoicated with trauma often cause acute rapid rises in surrounding tissue pressures.
For example, normal inversion and Valsalva maneuvers create reverse flows into the brain. Hanging upside down or standing on your head causes inversion flows. A Valsalva maneuver is performed by blowing real hard against maximum resistance. Pilots and scuba divers use Valsalva maneuvers to control pressure in the brain and ears respectively. The famous horn player “Satchmo” Louis Armstrong performed Valsalva maneuvers when he played powerful long sustained high notes. His neck and facial veins would buldge and his eyes would nearly pop out. Technically he should have blown his cork if inversion flows were a problem. In addition, Olympic style competition weight lifters do Valsalva maneuvers when they lift heavy weights. In fact, Valsalva maneuvers help shore up, stabilize and strengthen the spine.
In this regard, decades ago I decided to study bats, whales and giraffes because of the extreme inversion flows they face during head inversion and deep dives. As I expected, they appear to have developed compensatory mechanisms. The giraffe in the picture above uses extra large spaces inside the skull called diploe which I discussed in previous posts. This extra large handsome looking draft also has a rather distguished bump over the nasal area. The diploe and extra spaces in special strategically located bumps serve as a drip pan when the giraffe lower its head. Humans developed compensatory mechanisms to contend with upright posture. Interesting the valveless veins of the skull and spine play similar roles. In other words, moderate inversion flows don’t appear to be the problem. On the other hand, acute venous back jets are an entirely different story.
Unfortunately, back jets don’t explain the lesions you find in the cord. Again, that’s where Schelling theory makes the most sense. I will discuss cord lesions in MS in my next post. In contrast to massive pressure from venous back jets that stretch nearby myelin in the brain, according to Schelling the myelin in the cord simply snaps due to shear forces acting on attachments inside the cord itself. Those shear forces are amplified by a Tsunami of venous and CSF waves flowing through the subarchnoid space of the cord.