Spinal Syrinx
June  2015

​​​​​​​​A 37-year-old female applied for life insurance. About three years prior to the application she presented with loss of sensation in her hands and back and some weakness in “the arms and legs.” Magnetic Resonance Imaging (MRI) of the brain was reported as normal.

An MRI of the spine reported a syrinx in the central canal between the level of the 4th cervical (C4) and 12th thoracic (T12) vertebrae, with the maximal dimension of 1.6 mm. “Patchy narrowing” was described at the cervico-thoracic junction. Annual MRIs since that time have remained unchanged. Neurological examination six months prior to application was reported as “normal.”   

What is a spinal syrinx and what are the mortality implications?
Syrinx is from the Greek word for “tube,” and in medicine it describes a glial-lined cavity within the spinal cord. It may extend into or wholly exist in the brainstem, in which case it is syringobulbia. Syringomyelia is the process of the syrinx expanding over time. Most lesions occur between C2 and T9. Most often syringomyelia affects the central canal of the spinal cord, but rarely a syrinx can form in the parenchyma of the cord.

central spinal cordThe central canal of the spinal cord derives from the neural groove at the seventh day of embryonic development. It closes into a neural tube on the 14th day and progressively forms a canal.

At birth the central canal communicates with the fourth ventricle of the brain and usually measures .05 mm-.1 mm in diameter. It is usually visible only in newborns as it undergoes stenosis with age, until the central canal is obliterated in the majority of adults (Figure 1).

In some studies 50% or more of cases of syringomyelia occur in association with Chiari malformation (CM), a condition where the basal portion of the brainstem or cerebellum is displaced through the opening in the skull (foramen magnum). A survey of neuro-imaging centers in Japan suggested an estimated prevalence of ambulatory syringomyelia of 1.94 per 100,000. The asymptomatic proportion was 22.7%.

A study in New Zealand found that the prevalence of symptomatic syringomyelia was 5.4 per 100,000 in Caucasian/East Asian people. In New Zealand the mean age of symptom onset was 27.5 years and the mean age of diagnosis was 33 years. The underlying cause of syringomyelia in the New Zealand study is illustrated in Figure 2.

Figure 2 – Underlying Causes of Syringomyelia (NZ Case)
Associated Condition* Percent of total affected (%)
Chiari I malformation alone 38.2
Chiari malformation and spinal cord abnormality 20.6
Other Posterior cord abnormality 2.9
Isolated spinal cord abnormality 2.9
Post-traumatic arachnoiditis 11.8
Non-traumatic arachnoidits 5.9
Idiopathic 17.6
*Patients with spinal cord tumors were excluded from this study

Chiari malformation - on its own or in concert with other complications - accounts for more than half of all cases of syringomyelia observed in a New Zealand study.

cervical syrinxSyringomyelia most commonly affects the central spinal canal which results in central cord syndrome. This syndrome consists of loss of pain and temperature sensation in dermatomes that are adjacent to the lesion.

Areas above and below the lesion are usually unaffected. As the syrinx expands muscle weakness can develop in the area of anesthesia with eventual loss of deep tendon reflexes in the affected area (Figure 3).

On a basic level, the etiology of most cases of syringomyelia is an interruption or imbalance in the flow of cerebrospinal fluid (CSF). Conditions that cause this interruption include CM, curvatures of the spine, spinal cord tumor, trauma and posttraumatic scarring, post-infectious arachnoiditis and post-inflammatory arachnoiditis.

When an underlying cause cannot be identified, the term idiopathic syringomyelia (IS) is generally used. However, as imaging techniques improve, it is more common for blockage or narrowing of the subarachnoid space to be detected. Reports of using cardiac-gated phase-contrast cine-mode MRI in multiple axial planes to precisely locate obstructions to CSF flow are appearing in medical literature.

Arachnoid webs or adhesions that are not visible on MRI or CT can often be identified at surgery and removed with favorable outcome in stopping the progression of the syrinx. Decompression of obstruction of CSF flow via cranio-caudal decompression or laminectomy is successful in some cases.

Shunts have been tried between the syrinx and various other spaces including the peritoneal space, the subarachnoid space and the third ventricle. Shunts frequently become blocked and some medical professionals would consider them only as a last resort. An etiology-driven approach to treatment is essential with conservative management often being successful.

As MRI and CT scanners have become more prevalent, there have been efforts to define an entity that consists of a widened central spinal canal in an adult that does not progress and does not appear to be pathologic. Some authors would refer to this entity as hydromyelia, while others use the terms syringomyelia and hydromyelia interchangeably.

Common features for the diagnosis of the non-progressive widening of the central canal are

  • relatively smaller diameter (mean transverse diameter in one study was 2.7 mm, range 1.5-5.8)
  • relatively short vertical extension (mean of 3.5 vertebrae in one study)
  • location in the thoracic, cervical or cervico-thoracic spine
  • presentation with diffuse pain as the primary symptom
  • lack of abnormal neurological findings.

When patients with these characteristics were followed over three years, there was no evidence of neurologic or radiologic progression.

The prognosis of syringomyelia is related to the underlying cause and the neurologic deficit. Spinal tumors or cysts requiring removal have the attendant mortality of the neoplasm. In the case of mechanical obstruction to CSF flow that has been relieved (CM, spinal deformities, arachnoid membranes or adhesions) the mortality will correspond to that of the residual neurological deficit after relief of the obstruction.

Early in the course of idiopathic syringomyelia it is difficult to determine the longer-term prognosis. I outlined the characteristics associated with non-progression (at least for three years) in the paragraph above. However, it may also be that a patent, dilated central canal is a precursor to the development of a syrinx in response to future insult. The finding of a widened central canal has socio-economic and psycho-social implications for the individual, and further research is ongoing to better define outcomes. 
 

Returning to the Case

This appears to be an idiopathic widening of the central canal with a relatively small diameter. There is no neurologic deficit reported, and annual MRIs indicate it has been stable. There would appear to be minimal, if any, excess short-term mortality attributable to the incidental finding of widened central spinal canal.      

References

Chang H, et al. “Dorsal spinal arachnoid web diagnosed with the quantitative measurement of cerebrospinal fluid flow on magnetic resonance imaging.”  J Neurosurg Spine, 2014. 20:227–233.

Roy A, et al. “Idiopathic syringomyelia: retrospective case series, comprehensive review, and update on management.” Neurosurg Focus, 2011. 31(6):E15.

Roser F, et al. “Defining the line between hydromyelia and syringomyelia. A differentiation is possible based on electrophysiological and magnetic resonance imaging studies.” Acta Neurochir, 2010.  152:213-219.

Blevgrad C, et al. “Syringomyelia: a practical, clinical concept for classification.” Acta Neurochir, 2014. 156:2127-2138.

Brickell K, et al. “Ethnic differences in syringomyelia in New Zealand.”  J Neurol Neurosurg Psychiatry, 2006. 77:989–991.

Sakushima K et al. “Nationwide survey on the epidemiology of syringomyelia in Japan.”  J Neurological Sciences, 2012. 313(1-2):147-152.