Cervical Spondylosis and Neurodegenerative Diseases
Cervical spondylosis refers to an abnormal condition of the structural elements of the neck. These elements include cartilage, ligaments and bones. Although it has yet to be recognized, as covered in my book called The Downside of Upright Posture, cervical spondylosis may play a role in multiple sclerosis, Alzheimer's and Parkinson's disease, as well as other neurodegenerative diseases, due to its impact on blood and CSF flow in the brain. In particular, degenerative conditions of the cervical spine, cervical spondylosis can lead to chronic ischemia, edema and hydrocephalic type conditions in the brain. They can also cause turbulance, backjets and standing waves of cerebrospinal fluid (CSF) to form in the basal cisterns of the brain. The basal cisterns are wells filled with CSF that surround and protect the brainstem.
As an aside, cervical spondylosis and abnormal conditions of the upper cervical spine may also play a role in neurodegenerative diseases of the cord such as amyotrophic lateral sclerosis. I will cover neurodegenerative diseases of the cord separately as this site develops.
The general term spondylosis literally means an abnormal condition of the spine. Spondylo means spine and osis means an abnormal condition of. It refers to an abnormal condition of any area of the spine associated with degeneration of the structural elements and labeled according to the area affected. There are several causes of degeneration of the spine, such as aging, wear and tear (micro trauma) and accidents (macro trauma). Osteoarthritis, which is another term for spondylosis is due to dry, degenerated and inflamed joints of the spine. Degenerated and herniated discs are, likewise, a type of spondylosis, as is degeneration of the joint capsules or ligaments of the spine.
In an xray the denser the structure the whiter it appears, the less dense appears grey to black. In the picture below the bones of the spine are whiter in color and the cartilage called discs and the connective tissues are shades of gray or black. The bones that make up the segments of the spine are called vertebra. Except for the first segment of the spine beneath the skull called atlas or C1, all the other segments of the spine are separated by discs. This lateral xray of the cervical spine is an example of a spine without cervical spondylosis.
The x-ray below is an example of cervical spondylosis associated with bone spurs called osteophytes and narrow degenerated discs in the lower vertebra.
Certain conditions are associated with spondylosis such as scoliosis and kyphosis due to abnormal curvatures creating greater stress and therefore wear and tear. Scoliosis and kyphosis affect the design of the spinal canal, which can cause functional stenosis. Scoliosis is typically diagnosed early on but adults can get it later in life due to degeneration of the bones, cartilage and connective tissues of the spine. Although these conditions can affect all areas of the spine they can also lead to cervical spondylosis.
Stenosis comes from the Greek term stenos meaning narrow. It is a condition in which the spinal canal, which houses the spinal cord or the openings on each side of the canal for the nerve roots to exit are too narrow, which can compress the cord and/or nerve roots. Stenosis can be due to congenital design problems, but more often it is the result of spondylosis due to aging and injuries to the bones and connective tissues of the spine. These tissues then invade the spine's neurovascular tunnels, which are the spinal canal and intervertebral foramen (the openings where the nerve roots exit).
Spondylosis and the Spinal Canal
Regardless of the type or cause, spondylosis alters the proper design and dimensions of the spinal canal. It can thus affect the contents of the spinal canal. The contents of the canal include the cord, blood vessels and cerebrospinal fluid. It also contains loose fat in the space between the spinal canal and cord called the epidural space. The space is called the epidural space because it surrounds the outer coat of the cord called the dura mater. It is important because, the epidural space contains the vertebral veins.
The Monroe-Kellie theory of the brain states that the cranial vault is filled to capacity with three elements, which are the brain, blood and cerebrospinal fluid. Because the cranial vault is a closed container an increase in the volume of one element causes a decrease in one or both of the other elements. Cervical spondylosis can have an impact on the body's ability to maintain this balance.
The Monroe-Kellie theory similarly applies to the spinal canal. The spinal canal like the cranial vault is a relatively closed container with a limited volume. The canal contains the spinal cord, blood, cerebrospinal fluid and fat. This means that if the contents of one of the elements within the spinal canal increases in volume, then one of the other elements must decrease in volume.
In contrast to the cranial vault, which is made of bone and separated into compartments by the dura mater of the brain, the spinal canal is comprised of bones, cartilage and connective tissues mentioned above. When cartilage and connective tissues degenerate (spondylosis) they often invade the spinal canal and compress its contents.
The first space and tissues to feel the effects from invasion of the space of the spinal canal is the epidural space, which contains the loose fat and vertebral veins. Since the walls of the dura mater are much stronger than the epidural fat and walls of the vertebral veins, which are relatively weak, the vertebral veins and fat become compressed. This increases venous pressure which is called venous hypertension.
Among other things, venous hypertension decreases the pressure gradient that determines blood flow to the cord. The pressure gradient for blood flow to the cord is the difference between arterial pressure and venous pressure. Therefore, compression of the vertebral veins and the subsequent venous hypertension it causes results in decreased blood flow to the cord in the area. According to neurosurgeon Dr. Wise Young, a leading expert in traumatic spinal cord injuries, venous hypertension is one of the most commonly overlooked causes of decreased blood flow and degeneration of the spinal cord. Consequently, venous hypertension and the subsequent ischemia (lack of blood flow) it causes may, likewise, play a role in demyelination of the cord. Demyelination is damage to the myelin sheath that surrounds the cord.
In addition to venous hypertension and ischemia, spondylosis can affect CSF flow in the subarachnoid space of the cord. This is because the cord, blood and CSF in the spinal canal are under constant hydraulic pressure the same as the brain, blood and CSF in the cranial vault. Therefore, if venous pressure in the epidural space increases, it can impact the cord and CSF flow.
During the contraction phase of the heart a large volume of blood enters the cranial vault and spinal canal. This requires a proportionate decrease in either venous blood or CSF to make room for the incoming blood. In the cranial vault this is accomplished by pushing venous blood and CSF out of the vault and into the spinal canal. While this helps the brain to compensate for the increase in blood volume it further increases the volume of blood and CSF entering the spinal canal and cord.
The Expansion Tank of the Spinal Canal
In contrast to the capacity of the brain and cranial vault, which are relatively fixed the capacity of the spinal canal and cord can change according to circumstances similar to an expansion tank connected to a domestic hot water tank in a house.
In cold northern climates the domestic water supply coming into a house can be relatively cold. As hot water is used for washing, the hot water storage tank is refilled with the colder incoming water. As it mixes with the hot water in the tank the water volume rapidly expands. The rapid expansion in volume can cause increased pressure on the tank and joints in the domestic hot water pipes. Too much pressure can cause joints in the pipes to break.
To compensate for the increase in volume an expansion tank is added to the system. The expansion tank is a small tank with an elastic diaphragm inside. The smaller expansion tank is connected by a pipe to the main pipe of the larger hot water tank as in the diagram above. As the water volume expands the diaphragm stretches and moves down. This increases the capacity of the smaller tank. As the water temperature balances and water volume decreases, the diaphragm moves back up to where it was. In this way the elastic diaphragm and tank maintain water pressure and volume.
Similar to expansion tanks on domestic hot water supply lines the spinal canal and cord have expansion tanks called the lumbar cistern and dural sac. The lumbar cistern or fifth ventricle, as it is sometimes called, is located at the bottom of the central canal of the cord. The top end of the central canal is connected to the fourth ventricle of the brain. The lumbar cistern most likely evolved to accommodate upright posture, which increases CSF production and outflow from the brain. The lumbar cistern can help absorb the excess volume from the fourth ventricle to a certain degree. The problem is that its capacity is small and the central canal that connects it to the fourth ventricle typically closes with age.
In contrast to the lumbar cistern the capacity of the dural sac located beneath the bottom end of the cord is large. The spinal cord itself ends slightly below the lowest ribs at the top of the low back called the lumbar spine. The dural sac is part of the outer protective membrane of the cord. While the bottom of the cord stops in the upper part of the low back, the dural sac continues down through the remainder of the lumbar spine and into the upper portion of the sacrum of the pelvis. Many nerves branch off the bottom of the cord and travel through the dural sac to their appropriate destinations in the legs and lower body. The cluster of nerves is called the cauda equina. It comes from Latin cauda (tail) and quina (horse) because it resembles a horse's tail.
The conus medullaris is the end part of the cord and is the inner coat of the cord called the pia mater which twists into a thin filament. The filament of pia mater extends from the conus medullaris all the way down through the dural sac along with the cauda equina. When it gets to the end of the sac it pierces and blends with the dura mater to form the filum terminale. The filum terminale passes through the remainder of the sacral portion of the spinal canal and attaches to the tailbone called the coccyx.
Sometimes the length of the cord or the filum terminale are too short which causes tethering of the cord. Among other things, tethering of the cord may play a role in scoliosis and Chiari 1 type malformations. Furthermore, spondylosis can result in tethering of the cord due to traction from scar tissue and bone spurs called osteophytes.
The dura mater of the cord expands and contracts similar to the brain according to the inflow and outflow of blood during each cycle of the heart, as well as respiratory pressures. Its ability to expand is also affected by pressure surrounding it in the vertebral veins which, in turn are affected by spondylosis as mentioned above, as well as posture.
In contast to the brain, which uses the spinal canal to control volume in the brain, the spinal canal uses the large capacity of the dural sac to compensate for fluctuations in CSF volume. Additional expansion can be further accomplished by squeezing blood and water out of the veins and fats in the epidural space surrounding the cord. The venous blood is squeezed into the numerous networks of the vertebral veins inside and outside the spinal canal and spine. Some scientists suspect that some CSF is forced out of the cord and into the epidural nerve root spaces as well. The dural sac thus serves as a large expansion tank that extends from the lumbar spine all the way down through the top two segments of the sacrum.
Blocked Blood and CSF Flow
Similar to the connection pipe between the expansion tank and water supply line and the central canal to the lumbar cistern, the spinal canal connects the CSF flow in the brain to the dural sac expansion tank in the cord. If the pipe or canal becomes blocked (spondylosis) the expansion tank and cistern will not function properly. Likewise, cervical spondylosis affects CSF flow between the brain, cord and dural sac.
The MRI scan below shows cervical spondylosis and stenosis. If you follow the brainstem and spinal cord down through the cervial spine, you see large indentations that press on the cord. This is a fairly severe form of cervical spondylosis associated with stenosis of the spinal canal that touches the cord. When it contacts the cord, cervical spondylosis affects CSF flow.
Most cases of cervical spondylosis are not associated with direct contact with the cord but they can still cause problems with blood and CSF flow. Spondylosis, stenosis and scoliosis alter fluid mechanics in the spinal canal with or without direct contact of the cord. Regardless of whether there is contact with the cord or not, compression of the vertebral veins surrounding the cord causes venous hypertension. As mentioned above, venous hypertension decreases arterial blood flow to the cord. It may similarly decrease CSF flow.
In addition to its impact on the spinal cord, cervical spondylosis can affect blood and CSF flow in the brain resulting in chronic ischemia, edema and increased CSF volume. Moreover, cervical spondylosis blocks the connection between the brain and the dural sac that serves as the expansion tank of the cord. Blockage of the connection can affect CSF flow through the cord and decrease CSF outflow from the brain, which would increase CSF volume in the cisterns and spaces surrouding the brain. It can also cause turbulance, backjets, also known as venous inversion flows and standing waves to form in the cisterns and surrounding spaces of the brain. What's more, standing waves may be as destructive to the brain as chronic ischemia, edema and hydrocephalus and may play a role in neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease and multiple sclerosis.
Cervical spondylosis can be treated in many ways using manual and mechanical therapies such as used in chiropractic, osteopathy and physical therapy. Special spinal decompression tables can also be very effective. When it becomes severe enough and conservative methods fail surgery is an option. When it comes to choosing surgery for cervical spondylosis, the latest methods use an endoscopic approach that is far safer and less destructive and cause far fewer complications. I will be discussing manual and mechanical approaches as well as endoscopic surgery to treat cervical spondylosis as this site develops.
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