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Pathogenesis of syringomyelia associated with Chiari type 1 malformation: review of evidences and proposal of a new hypothesis

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Abstract

The exact pathogenesis of syringomyelia associated with Chiari type 1 malformation is unknown, although a number of authors have reported their theories of syrinx formation. The purpose of this review is to understand evidences based on the known theories and to create a new hypothesis of the pathogenesis. We critically review the literatures on clinicopathological, radiological, and clinical features of this disorder. The previously proposed theories mainly focused on the driven mechanisms of the cerebrospinal fluid (CSF) into the spinal cord. They did not fully explain radiological features or effects of surgical treatment such as shunting procedures. Common findings of the syrinx in clinicopathological studies were the communication with the central canal and extracanalicular extension to the posterior gray matter. Most of the magnetic resonance imaging studies demonstrated blockade and alternated CSF dynamics at the foramen magnum, but failed to show direct communication of the syrinx with the CSF spaces. Pressure studies revealed almost identical intrasyrinx pressure to the subarachnoid space and decreased compliance of the spinal CSF space. Recent imaging studies suggest that the extracellular fluid accumulation may play an important role. The review of evidences promotes a new hypothesis of syrinx formation. Decreased absorption mechanisms of the extracellular fluid may underlie the pathogenesis of syringomyelia. Reduced compliance of the posterior spinal veins associated with the decreased compliance of the spinal subarachnoid space will result in disturbed absorption of the extracellular fluid through the intramedullary venous channels and formation of syringomyelia.

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  1. Department of Neurosurgery, Sapporo Medical University School of Medicine, South 1, West 16, Chuo-ku, Sapporo, 060-8543, Japan

    Izumi Koyanagi & Kiyohiro Houkin

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  1. Izumi Koyanagi
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Correspondence to Izumi Koyanagi.

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Comments

Fumio Suzuki and Kazuhiko Nozaki, Shiga, Japan

The authors are to be congratulated for this comprehensive overview on the pathophysiology of syringomyelia. They made a modification of the theory proposed by Greitz D. in that a decreased compliance of the large veins in the subarachnoid space, which results from the reduced compliance of CSF below the obstruction, decreases the absorption of the extracellular fluid from intramedullary venous channels, resulting in the accumulation of extracellular fluid in spinal cord. This phenomenon might contribute partly to the development of syringomyelia but does not seem to be a main cause of syrinx formation. Although Greitz D. reported in his review that venous congestion might contribute to syrinx formation, venous congestion is not so obvious in Chari malformation type 1 as in spinal dural AVFs, in which large veins of the spinal cord are congested severely and compromised veins should reduce their compliance. These abnormal venous conditions may induce necrotizing myelitis but do not necessarily accompany syrinx formation. The authors referred to the report by Heiss J. et al. as an evidence of reduced CSF compliance, but the data was not statistically significant. More data about the changes in compliance of CSF space in Chiari Type 1 should be needed before establishing their modified theory.

Comments

Ricardo V. Botelho, São Paulo, Brazil

The authors performed a comprehensive review of mechanisms and concepts related to the pathogenesis of syringomyelia in Chiari malformation and have designed a hypothetical model of pathogenesis for syringomyelia.

Some of the factors reviewed are well established and others are hypothetical:

1. Patients with CM and sirirngomielia have smaller posterior fossa than those who did not have syringomyelia.

2. In patients with Chiari malformation, smaller tonsillar herniations are associated more frequently with syringomyelia than larger herniations.

The combination of these two features, small and shallow posterior fossa and small herniation of the tonsils might suggest a lower compliance of the foramen magnum, at the same time, prevents the descent of the tonsils and produces an early and intense blockage of free flow of craniocervial CSF in patients with syrinx.

3. Patients with syringomyelia have a blockage of subarachnoid CSF flow and less complacency of the subarachnoid space and posterior spinal veins.

4. The reduced absortion mechanism from the extracellular fluid from the spinal Cord parenchyma would result in syringomyelia in Chiari type 1 malformation, as speculated by the authors.

One real and observed effect in patients with syringomyelia and MC is that decompression of the posterior fossa often decreases syringomyelia cavity, probably by restoring the caniocervical flow of CSF.

The importance of reducing capacity venos absorption of extracellular fluid is an interesting suggestion posed by the authors that future works will confirm or not these suggestions.

Comments

Jörg Klekamp, Quakenbrück, Germany

In this paper, Koyanagi and Houkin present a hypothesis that was supposed to explain the development of syringomyelia in patients with a Chiari type I malformation. The authors correctly summarize in their paper that previous theories trying to explain syringomyelia by cerebrospinal fluid (CSF) entering the spinal cord via the 4th ventricle or other avenues have failed to demonstrate such a communication and are not able to explain several observations in these patients. Even though several thoughts and conclusions by the authors are well founded, I do have some reservations against this paper.

In table 1, the authors provide a list of previous theories and disqualify each of these as speculative. This statement is grossly negligent. Gardner's and Williams' theories, for instance, may no longer be tenable but were based on careful clinical tests, pressure recordings in patients and several animal studies. Given the technical conditions at the time, these works were state of the art and well founded on the observations made. Likewise, the theories of extracellular origin relating syringomyelia to edema formation are based on animal experiments and clinical observations and by no means just the result of a literature review.

The concept of syringomyelia as a spinal cord edema is by no means new. Tannenberg in 1924 and Liber and Lisa in 1937 were the first to propose this view. Taylor and Byrnes in 1974, Aboulker in 1979, and Yamada et al. in 1996 further elaborated on this theory and already emphasized the importance of venous obstruction, which they thought to cause syrinx formation in combination with CSF flow obstruction.

I do not agree with the authors' initial statement, that theories concerning the pathophysiology of syringomyelia on this basis do not apply to patients with a Chiari malformation. Several experimental studies have provided new insights into the physiological exchange between extracellular fluid (ECF) of the spinal cord and CSF under normal conditions as well as with CSF-flow obstructions. It appears that any pathology causing a CSF-flow obstruction and/or spinal cord tethering as well as certain intramedullary tumors are able to disturb the balance between ECF und CSF in the spinal canal, which may then lead to syrinx formation. This concept applies to patients with a Chiari malformation just as well as to those with posttraumatic syringomyelia, for instance. After all, syrinx formation in Chiari patients is the result of CSF-flow obstruction at the foramen magnum as it is in posttraumatic syringomyelia with CSF-flow obstruction at the level of the posttraumatic arachnopathy. With their hypothesis, Koyanagi and Houkin simply add a reduced compliance of posterior spinal cord veins to this concept of ECF/CSF imbalance. Spinal cord veins may turn out to contribute to syrinx formation in this setting but this assumption does not imply a completely novel hypothesis.

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Koyanagi, I., Houkin, K. Pathogenesis of syringomyelia associated with Chiari type 1 malformation: review of evidences and proposal of a new hypothesis. Neurosurg Rev 33, 271–285 (2010). https://doi.org/10.1007/s10143-010-0266-5

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