Many neurodegenerative diseases of the spinal cord may share common causes and degenerative processes similar to the neurodegenerative diseases of the brain. The symptoms are different because of the different areas of the cord or brain that are affected.
In many cases, neurodegenerative diseases of the cord are most likely due to the design of the spinal canal and the circulatory system of the cord. Diseases of the spinal cord as a result the design of the canal and circulation of the cord include motor neuron diseases such as amyotrophic lateral sclerosis (Lou Gehrig’s disease) and primary lateral sclerosis, as well as transverse myelitis, optic spinal multiple sclerosis and Devic’s disease. There is a great deal of overlap between these conditions and multiple sclerosis. These and other neurodegenerative conditions of the cord will be discussed separately on this website.
The cord is an extension of the brain and together are considered to be the central nervous system. They contain the primary motor and sensory nerve pathways. The primary pathways run between the brain and the different levels of the cord. They are central nerve pathways because they do not leave the cord. It is the secondary nerve pathways that connect the cord to the body. The secondary nerve pathways exit the cord and central nervous system and go to the body. They make up what is called the peripheral nervous system.
The cord is housed within a relatively closed container called the spinal canal similar to the limited confines of the brain in the cranial vault. It is, therefore, subject to the principle of the Monroe-Kellie Doctrine(see below)in the skull.
The picture below demonstrates the confines of the canal. It is a slice of the canal looking from above down. The gold structure in the middle is the cord.
In brief, the Monroe-Kellie doctrine states, the cranial vault contains three elements: brain, blood and CSF. If the volume of one element goes up then the volume of one or both of the other two elements must go down proportionately. It is thus affected because, like the brain within the cranial vault the spinal canal is a relatively closed container and contains the cord, blood and CSF.
There is one significant difference between the cranial vault and the spinal canal however. In contrast to the dimensions of the cranial vault, the dimensions of the spinal canal are much more subject to deformation from spondylosis and stenosis. Spondylosis is degeneration of the vertebra and discs of the spine. It can be due to injuries, disease or simply aging. Degenerative changes in the spine can result in stenosis. Stenosis is narrowing of the spinal canal. Some people are born with naturally tight spinal canals. Degeneration of cartilage, bones and connective tissues however can encroach into the spinal canal and cause stenosis making the canal even tighter. Furthermore, abnormal curvatures of the spine such as scoliosis have been shown to cause functional stenosis by compressing the contents of the canal against the inside of the curves.
Deformation of the spinal canal by any means can compress its contents causing shifts in fluids and pressure. Shifts in fluids and pressure are transmitted to the contents of the canal such the anterior and posterior spinal arteries and the vertebral venous plexus. Pressure changes are also transmitted to cerebrospinal fluid (CSF) in the subarachnoid spaces, which in turn transmits it to the spinal cord causing actual or functional compression.
Lastly compression of the cord can also occur as a consequence of a tethered cord syndrome which is when the cord is too short relative to the spine. Tethered cord syndrome may play a role in the cause of scoliosis. It can also cause Chiari malformations by creating a downward drag on the brain pulling it toward the base of the skull and foramen magnum.