Background

Syringomyelia is a broad term that is used to describe a condition in which a syrinx forms within the spinal cord resulting from disruption to normal cerebrospinal fluid (CSF) flow. This may lead to progressive myelopathy. As described by Milhorat, there are many different types of syringomyelia depending on the location of the syrinx within the spinal cord and the underlying etiology. In the case of the most common type of syringomyelia, associated with Chiari I malformations, CSF flow is disrupted by the protrusion of the cerebellar tonsils through the foramen magnum. The protrusion of the cerebellar tonsils occludes the subarachnoid space at the level of the foramen magnum and leads to increased cervical CSF pulse pressure waves. Over time this leads to the formation of a syrinx within the central canal as the walls of the central canal struggle to tolerate the higher than normal CSF pressures. Approximately 12% to 22.9% of patients with a Chiari I malformation develop a syrinx on magnetic resonance imaging (MRI). The syrinxes seen with Chiari I malformations are described as noncommunicating central canal syrinxes, meaning that they are cystic dilations of the central canal in which the syrinx does not communicate with the fourth ventricle.

Posttraumatic syringomyelia describes a condition in which a syrinx forms within the spinal cord after spinal cord injury. Syrinx formation often results in progressive myelopathy. In contrast to syringomyelia due to Chiari malformations, posttraumatic syringomyelia generally consists of noncommunicating extracanalicular syrinxes, with cystic dilations of the spinal cord that are outside of the central canal. In posttraumatic syringomyelia the cause of obstruction to CSF flow is not at the level of the foramen magnum, in contrast to the more common Chiari malformation, but instead at the site of the initial spinal cord injury. Specifically, in posttraumatic syringomyelia there is obstruction of CSF flow at the site of initial trauma due to increased subarachnoid adhesions and narrowing of the subarachnoid space due to scarring from the initial trauma. This obstruction to CSF flow at the level of the initial trauma results in increased CSF pulse pressure in the subarachnoid space.

According to the intramedullary pulse pressure theory, the increased CSF pulse pressure leads to the formation of a syrinx as the CSF flow is diverted by (1) traveling through Rudolf-Virchow spaces into the parenchyma of the spinal cord and (2) passing through the narrowing of the subarachnoid space with an increased velocity and decreased pressure due to Bernoulli’s equation. The CSF flow into the spinal cord contributes to the formation of a syrinx by directly increasing the extracellular fluid within the cord. The CSF that flows along the outside of the cord past the narrowing contributes to the formation of a syrinx by decreasing the pressure on the outside of the cord and thus allowing it to balloon from within ( Fig. 62.1 ).

Intramedullary Pulse Pressure Theory . Narrowing of the subarachnoid space causes an increase in the cerebrospinal fluid (CSF) pulse pressure superiorly. CSF flow is diverted near this area of narrowing either by traveling through Rudolf-Virchow spaces into the parenchyma of the spinal cord as seen in (A), or by passing through the narrowing with an increased velocity due to Bernoulli’s equation as seen in (B). The CSF flow into the spinal cord contributes to the formation of a syrinx by directly increasing the extracellular fluid within the cord. The CSF that flows along the outside of the cord past the narrowing contributes to the formation of a syrinx by decreasing the pressure on the outside of the cord and thus allowing the cord to balloon from within.

(Figure adapted from Greitz D. Unraveling the riddle of syringomyelia. Neurosurgical review. 2006;29(4):251-263; discussion 64.)

Posttraumatic syringomyelia typically presents in a delayed fashion from months to decades after spinal cord injury. The symptoms of posttraumatic syringomyelia typically progress gradually. Rarely, sudden deterioration has also been reported. The most common presenting symptoms are pain and sensory loss followed by motor weakness. Pain is typically intermittent at or above the level of the initial injury and may be described as burning, dull, or aching. Coughing, sneezing, and straining often exacerbate the pain due to increased venous pressure. Sensory loss is also typically at or above the level of the initial injury and can come in a variety of forms, from a combination of loss of pain and temperature sensation, to a combination of loss of pain and proprioception. Motor weakness typically begins after symptoms of sensory loss and presents as new loss of motor function above the level of previous injury. New loss of deep tendon reflexes may be an early sign of posttraumatic syringomyelia.

Posttraumatic syringomyelia is underappreciated as a source of morbidity after spinal cord injury. According to the National Spinal Cord Injury Database, between 183,000 and 230,000 individuals in the United States live with spinal cord injury. In this population, between 1% and 9% report symptomatic syringomyelia. Between 1 and 30 years after injury, 21% to 28% will be found to have a syrinx, and another 30% to 50% will have some degree of spinal cystic change.

Anatomic Insight

Syrinxes associated with posttraumatic syringomyelia are often irregular in shape and can occur anywhere along the spinal cord depending on the location and extent of the initial spinal cord injury. Most syrinxes are located near the initial trauma site with approximately 4% extending caudally, 81% rostrally, and 15% in both directions. Notably, posttraumatic syrinxes can communicate directly with the CSF in the subarachnoid space if found near the ventromedian fissure or a dorsal nerve root entry zone. Most posttraumatic syrinxes are found in the central and dorsolateral gray matter, whereas approximately 9% are found solely in the dorsal columns. In terms of size, the average syrinx length is between 6.5 and 7.8 spinal segments long.

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