Article Review: Physical Mechanisms That Lead to Spinal Syrinxes

I recently read “Computer simulation of syringomyelia in dogs” by Cirovic and colleagues, and I found it particularly interesting. George, my dog, has the Chiari Malformation (CM) and Syringomyelia (SM), so I have taken a special interest in learning about all aspects of the conditions. I am also a veterinary student, so I am a huge nerd and love learning the intricacies of medicine. I hope to one day contribute to research in my career, as I find it fascinating. 

The article delves into the “how” of spinal syrinx formation as a result of the CM/obstruction of the craniocervical junction. 

To the left is an example of the Chiari Malformation with the cerebellum being extruded into the spinal canal - from George’s MRI. Syrinx & directional terms are labeled for clarity.

The researchers excited the cranial end of the spinal cord/subarachnoid space (SAS) in three models to simulate how CSF and the spinal cord move when blood volume shifts in the cranium in coordination with the cardiac cycle. They simulated movement in three extreme examples of CKCS spinal cords: the normal (cranial end of spinal cord is tethered to the surrounding neural tissues, restricting abnormal movement, CSF is allowed to flow freely as normal at cranial end; no syrinx), with CM but no syrinx yet (cranial end of spinal cord can move freely because it is not anchored to the brainstem, CSF blocked at cranial end; no syrinx), and with CM and SM/syrinx formed (cranial end of spinal cord can move freely, CSF blocked at cranial end; syrinx present).

When the cranial end of the spinal cord/SAS was excited, CSF moved at a lower maximum speed (a little below 0.01m/s) for both of the “Chiari” conditions. The “normal” condition’s speed was about double the “Chiari” ones, with a roughly 0.02m/s maximum speed. The simulation also calculated the radial, axial, and shear stress on the spinal cord for the three conditions. It showed that there was much more axial stress (longways up and down the spinal cord) and shear stress (when parts of the spinal cord shift horizontally in relation to one another) in the “Chiari” conditions than for the normal, but the radial stress was about the same for all three conditions.

The diagram above, from the paper, shows the stress values graphically with pressure units in Pascals. a) radial, b) axial, and c) shear. The blue line, “CSF movement,” correlates to the normal condition. The black line, “Cord movement,” correlates to the Chiari without syrinx condition. The orange line, “Cord/syrinx movement,” correlates to the Chiari with syrinx condition. The lines start at 0 meters and goes to over 0.3 meters (the approximate length of a CKCS spinal cord), running cranial to caudal down the spinal cord. Notably, the peaks for the Chiari without syrinx condition for axial and shear stress (B & C) peak twice around the C2/C3 vertebrae and the cervico-thoracic junction, the places where the cord bends the most.

Above is a diagram from the paper. The red regions are the ones under the highest amount of stress, they are also the curved portions of the spinal column. The syrinxes in the CKCS with Chiari form in the red regions first - usually first in the cranial-cervical portion (C1-C4) and then in the thoracic-spinal portion (T12-L2). One notable difference is that red regions in the simulation formed on the periphery/edges of the spinal column model, whereas in most patients the syrinxes form in the center of the spinal cord. This finding correlates with their stress simulation, as the highest pressure is on these two regions, and these regions are where the cavities/syrinxes start forming first in CKCS dogs with syringomyelia.

For reference I have included a diagram (above) that labels where the Cervical (C), Thoracic (T), and Lumbar (L) portions of the spine are, from toegrips.com. In Cavaliers, the syrinxes usually form first in the neck and then in the middle of the back.

The data from this study tells us that when the CSF is blocked and the cord is allowed to move freely, more axial and shear stress is put on the spinal cord, and this could be the reason for syrinx formation. This simulation is not definitive proof of this, and the stress values are not strong enough to do immediate damage to the spinal cord; however, overtime the stress might be enough to do this mechanical damage.

Cirovic et al. BMC Veterinary Research (2018) 14:82

https://doi.org/10.1186/s12917-018-1410-7