Pathogenesis of Syringomyelia

I read with great interest the article, The Presyrinx State: A Reversible Myelopathic Condition that May Precede Syringomyelia, by Nancy J. Fischbein et al (1). Although their theory of pathogenesis for the formation of the presyrinx state and syringomyelia is provocative, several areas need further clarification. The authors propose, as have others, that increased pressure in the subarachnoid space of the spinal canal in patients with Chiari malformation or other causes of relative blockage of CSF flow at the foramen magnum would cause the CSF to flow along the perivascular spaces and accumulate either in the central canal, causing syringomyelia, or in the cord substance itself, causing a “presyrinx” state. Nonetheless, as has been pointed out by others, increased pressure applied to a fluid (CSF) surrounding a distensible semisolid structure (the spinal cord) would have a tendency to compress that structure rather than force fluid into it (2). The only way there can be net flow of fluid into the spinal cord from the CSF surrounding it is if there is a pressure differential from the subarachnoid space to the central canal or to the spinal cord substance.

The authors state it has been proved that CSF flows from the subarachnoid space into the perivascular spaces of the spinal cord and from there along the interstitial spaces toward the central canal. For support of this, the authors reference Stoodley et al, among others (3). To summarize the experiment of Stoodley et al briefly, they injected horseradish peroxidase into the subarachnoid space of sheep and, by reducing arterial pulsations by ligating the brachiocephalic trunk, they demonstrated a reduction in the distribution of the horseradish peroxidase tracer through the perivascular spaces and central canal. They felt this experiment supported the hypothesis of arterial-driven flow of fluid from the subarachnoid space into the perivascular spaces across the interstitial space and into the central canal. In my opinion, however, this does not prove that there is bulk flow of fluid in that direction. It only proves that the cerebrospinal fluid is anatomically continuous with the central canal through a series of perivascular and interstitial spaces (4) and that when you impart energy (arterial pulsations) to a solid (horseradish peroxidase tracer) in solution (CSF) that solid will be distributed more rapidly and over a greater volume than if no energy is imparted to the mixture.

Liquids flow along the path of least resistance and from regions of high pressure to regions of low pressure. In order for there to be a net flow of fluid from the subarachnoid space through the very small (high resistance) perivascular and interstitial spaces into the very small (high resistance) central canal of the cord, and then possibly out the central canal at the level of the obex, there must be a significant pressure differential between the cerebrospinal fluid surrounding the spinal cord and the pressure in the central canal. Or, the resistance to flow from the subarachnoid space surrounding the spinal cord to the cisterna magna or basilar subarachnoid space must be greater than the combined resistances of the perivascular spaces, interstitial spaces, and central canal. This seems unlikely.

As we have previously proposed, we believe that the spinal cord is a net producer of extracellular fluid (5), and that this fluid normally flows along the perivascular spaces either into the subarachnoid space or possibly even into the central canal and is ultimately absorbed at the arachnoid villi with the rest of the CSF (6). We feel that increased resistance to flow at the foramen magnum by either Chiari I malformations or other causes prevents this extracellular fluid from exiting, and it accumulates in the spinal cord. This explanation appears more reasonable to us than the hypothesis that cerebrospinal fluid is somehow forced from the subarachnoid space through channels of very high resistance into the spinal cord and accumulates there.