Syringomyelia

In syringomyelia, syrinx means cyst or cavity, and myelia means a condition of the spinal cord. So, syringomyelia is a cystic enlargement of the spinal cord, and it typically starts medially and enlarges outwards, eventually damaging the spinothalamic tract, which is a part of the spinal cord that sends sensory signals about pain, pressure, temperature, and touch to the brain.

Now, the spinal cord itself is composed of both grey and white matter. Grey matter is found within medial portion of the spinal cord and is shaped like a butterfly. This is where the cell bodies of different neurons can be found. In the center of the grey matter there’s a small cavity the central canal which is filled with cerebrospinal fluid. Cerebrospinal fluid helps provide nutrients and mechanical support to the brain and spinal cord. Surrounding the grey matter is white matter, which consists of the axons of various neurons. The spinal cord has different neural tracts that carrying sensory information to and from the brain. The corticospinal tract is a descending pathway which carries motor information from the brain to various muscles. The dorsal column, located in the posterior portion of the spinal cord, is responsible for sensing pressure, vibration, fine touch, and proprioception, or the awareness of one’s body position in space. Then there’s the spinothalamic tract, which is divided into two distinct tracts - the lateral spinothalamic tract which is responsible for sensing pain, pressure, and temperature, as well as the anterior spinothalamic tract which senses crude touch. The spinothalamic tract carries all of this sensory information from the spine up to the thalamus of the brain where the information is processed. This happens through three neurons that synapse with one another. First, a primary neuron carries sensory information, such as pain, from the skin to the dorsal horn of the spinal cord, where it synapses with a secondary neuron. The secondary neuron ascends 1-2 vertebral levels and decussates or crosses to the opposite side of the spinal cord via an area of white matter called the anterior white commissure. The secondary neuron then ascends up the length of the spinal cord via the anterior or lateral spinothalamic tracts, eventually synapsing at the ventral posterior nuclei of the thalamus. Finally, from there, a third neuron arises from nuclei in the thalamus and carries the signal to the primary sensory cortex or the postcentral gyrus of the brain where the sensory signal is interpreted.

In syringomyelia, the central canal of the spinal cord that houses the cerebrospinal fluid gradually expands. This expansion may be caused by acquired factors, but the main cause of syringomyelia is a congenital condition called. In this condition the cerebellar and brainstem tissue slip down into the foramen magnum—the opening at the base of the skull. This malformation results in a variety of balance and movement symptoms related to the cerebellum and usually results in hydrocephalus—an abnormal accumulation of cerebrospinal fluid in the brain. Normally cerebrospinal fluid would flow through the four ventricles of the brain, and after the 4th ventricle, the fluid would have two options. It could exit the ventricular system through openings called the medial and lateral apertures and into the subarachnoid space where it’s reabsorbed. Alternatively, the fluid could go into the spinal canal. In Chiari malformations, however, the displacement of the cerebellum ends up blocking the openings where cerebrospinal fluid would exit into the subarachnoid space. As a result, the fluid backs up within the spinal canal, eventually causing the spinal canal to widen, and this leads to syringomyelia.

Aside from Chiari malformations, syringomyelia can also be caused by any acquired condition that blocks the flow of cerebrospinal fluid, like a tumor. Alternatively it can be caused by damage to the spinal cord, which results in an enlarged spinal canal. Examples of that include spinal cord trauma, spinal tumors, and spinal cord abscess.

Regardless of the cause, the expansion of the central canal in syringomyelia interferes with the fibers within the anterior white commissure of the spinothalamic tract, leading to the loss of pain, pressure, temperature, and crude touch. As the central canal cavity expands it damages the lower motor neurons connected to the corticospinal tract which leads to muscle atrophy, muscle weakness, and paralysis. Syringomyelia usually spares the dorsal column, so sensations of pressure, vibrations, fine touch, and proprioception typically remain in tact. Although it sounds nice to not feel pain, this can lead to several problems. One of these is neuropathic arthropathy or Charcot joints, which is when there’s repeated trauma and inflammation in a joint since there’s no pain response. In syringomyelia, this is particularly seen in the shoulders. Lastly, the widening spinal canal can also lead to changes in the spine like scoliosis, which is a sideways curvature of the spine.

Classically, the destruction of the spinothalamic tract leads to the bilateral loss of pain and temperature in the upper extremities and back, which is often described as a "cape-like" distribution due to damage to these tracts at the level of the cervical spine, usually C4-C6. However, this could also lead to dysesthetic pain, which is typically described as an abnormal, burning pain in the shoulder and neck regions.

Motor symptoms include weakness, muscle atrophy, and paralysis of when lower motor neurons are damaged.