Lynne E. Bilston, Marcus A. Stoodley and David F. Fletcher
The mechanisms of syringomyelia have long puzzled neurosurgeons and researchers alike due to difficulties in identifying the driving forces behind fluid flow into a syrinx, apparently against a pressure gradient between the spinal cord and the subarachnoid space (SAS). Recently, the synchronization between CSF flow and the cardiac cycle has been postulated to affect fluid flow in the spinal cord. This study aims to determine the effect of changes in the timing of SAS pressure on perivascular flow into the spinal cord.
This study uses a computational fluid dynamics model to investigate whether the relative timing of a spinal artery cardiovascular pulse wave and fluid pressure in the spinal SAS can influence CSF flow in the perivascular spaces.
The results show that the mass flow rate of CSF through a model periarterial space is strongly influenced by the relative timing of the arterial pulse wave and the SAS pressure.
These findings suggest that factors that might alter the timing of the pulse wave or the fluid flow in the SAS could potentially affect fluid flow into a syrinx.
Elizabeth C. Clarke, Marcus A. Stoodley and Lynne E. Bilston
The pathogenesis of syringomyelia in association with Chiari malformation Type I (CM-I) is unclear. Studies of patients with CM-I have shown alterations in the CSF velocity profile using cardiac-gated cine phase-contrast MRI, and computational simulations have demonstrated that temporal features of the CSF pulse could contribute to syrinx development or enlargement. Few studies have reported temporal characteristics of the CSF profile, and few studies have reported on CM-I patients with and without syringomyelia separately. This study was performed to determine whether specific temporal features of the CSF flow profile may underlie the development or enlargement of a syrinx in patients with CM-I.
Ten healthy volunteers and 18 patients with CM-I with (8 patients) and without (10 patients) syringomyelia were studied using cardiac-gated cine phase-contrast MRI, measuring the maximum CSF velocities in the cranial and caudal directions, the timing of these maximums relative to the cardiac cycle time, the timing of caudal flow onset, timing of cranial flow onset, and the duration of caudal flow.
The caudal CSF flow was significantly faster (p ≤ 0.01) and earlier (p < 0.02) in patients without syringomyelia than in healthy volunteers and patients with syringomyelia. There were no significant differences in the CSF velocities between patients with syringomyelia and healthy volunteers. Patients with CM-I who had syringomyelia had a significantly later start of caudal CSF flow (p < 0.01) and earlier maximum cranial velocity (p = 0.03) than healthy volunteers, but the relative durations of caudal and cranial flow were not significantly different between any of the groups.
The significantly earlier arrival and earlier peak velocity of caudal CSF flow may underlie the development of syringomyelia in patients with CM-I, and after a syrinx develops the CSF flow profile appears to stabilize.
Vannessa Leung, John S. Magnussen, Marcus A. Stoodley and Lynne E. Bilston
The pathogenesis of syringomyelia associated with Chiari malformation type I (CM-I) is unclear. Theories of pathogenesis suggest the cerebellar tonsils may obstruct CSF flow or alter pressure gradients, or their motion might act as a piston to increase CSF pressure in the spinal subarachnoid space. This study was performed to measure cerebellar tonsillar and hindbrain motion in CM-I and assess the potential contributions to syrinx formation.
Sixty-four CM-I patients and 25 controls were retrospectively selected from a clinical database, and all subjects had undergone cardiac-gated cine balanced fast-field echo MRI. There were a total of 36 preoperative CM-I scans, which consisted of 15 patients with and 21 patients without syringomyelia. Nineteen patients underwent paired pre- and postoperative imaging. Anteroposterior (AP) and superoinferior (SI) movements of the tip of the cerebellar tonsils, obex, fastigium of the fourth ventricle, pontomedullary junction, and cervicomedullary junction were measured. The distance between the fastigium and tip of the tonsils was used to calculate tonsillar tissue strain (Δi/i
CM-I patients had significantly greater cerebellar tonsillar motion in both the AP and SI directions than controls (AP +0.34 mm [+136%], p < 0.001; SI +0.49 mm [+163%], p < 0.001). This motion decreased after posterior fossa decompression (AP −0.20 mm [−33%], p = 0.001; SI −0.29 mm [−36%]; p < 0.001), but remained elevated above control levels (AP +56%, p = 0.021; SI +67%, p = 0.015). Similar trends were seen for all other tracked landmarks. There were no significant differences in the magnitude or timing of motion throughout the hindbrain between CM-I patients with and without syringomyelia. Increased tonsillar tissue strain correlated with Valsalva headaches (p = 0.03).
Cerebellar tonsillar motion may be a potential marker of CM-I and may have use in tailoring surgical procedures. The lack of association with syringomyelia suggests that tonsillar motion alone is not the driver of syrinx formation. Tonsillar tissue strain may play a part in the pathophysiology of Valsalva headaches.
Sarah J. Hemley, Lynne E. Bilston, Shaokoon Cheng and Marcus A. Stoodley
Noncommunicating canalicular syringomyelia occurs in up to 65% of patients with Chiari malformation Type I. The pathogenesis of this type of syringomyelia is poorly understood and treatment is not always effective. Although it is generally thought that syringomyelia is simply an accumulation of CSF from the subarachnoid space, the pathogenesis is likely to be more complex and may involve cellular and molecular processes. Aquaporin-4 (AQP4) has been implicated in numerous CNS pathological conditions involving fluid accumulation, including spinal cord edema. There is evidence that AQP4 facilitates the removal of extracellular water following vasogenic edema. The aim of this study was to investigate AQP4 expression and the structural and functional integrity of the blood–spinal cord barrier (BSCB) in a model of noncommunicating canalicular syringomyelia.
A kaolin-induced model of canalicular syringomyelia was used to investigate BSCB permeability and AQP4 expression in 27 adult male Sprague-Dawley rats. Control groups consisted of nonoperated, laminectomy-only, and saline-injected animals. The structural integrity of the BSCB was assessed using immunoreactivity to endothelial barrier antigen. Functional integrity of the BSCB was assessed by extravasation of systemically injected horseradish peroxidase (HRP) at 1, 3, 6, or 12 weeks after surgery. Immunofluorescence was used to assess AQP4 and glial fibrillary acidic protein (GFAP) expression at 12 weeks following syrinx induction.
Extravasation of HRP was evident surrounding the central canal in 11 of 15 animals injected with kaolin, and in 2 of the 5 sham-injected animals. No disruption of the BSCB was observed in laminectomy-only controls. At 12 weeks the tracer leakage was widespread, occurring at every level rostral to the kaolin injection. At this time point there was a decrease in EBA expression in the gray matter surrounding the central canal from C-5 to C-7. Aquaporin-4 was expressed in gray- and white-matter astrocytes, predominantly at the glia limitans interna and externa, and to a lesser extent around neurons and blood vessels, in both control and syrinx animals. Expression of GFAP and APQ4 directly surrounding the central canal in kaolin-injected animals was variable and not significantly different from expression in controls.
This study demonstrated a prolonged disruption of the BSCB directly surrounding the central canal in the experimental model of noncommunicating canalicular syringomyelia. The disruption was widespread at 12 weeks, when central canal dilation was most marked. Loss of integrity of the barrier with fluid entering the interstitial space of the spinal parenchyma may contribute to enlargement of the canal and progression of syringomyelia. Significant changes in AQP4 expression were not observed in this model of canalicular syringomyelia. Further investigation is needed to elucidate whether subtle changes in AQP4 expression occur in canalicular syringomyelia.
Johnny H. Y. Wong, Xin Song, Sarah J. Hemley, Lynne E. Bilston, Shaokoon Cheng and Marcus A. Stoodley
The pathogenesis of posttraumatic syringomyelia remains enigmatic and is not adequately explained by current theories. Experimental investigations require a reproducible animal model that replicates the human condition. Current animal models are imperfect because of their low reliability, severe neurological deficits, or dissimilar mechanism of injury. The objective of this study was to develop a reproducible rodent model of posttraumatic syringomyelia using a spinal cord impactor that produces an injury that more closely mimics the human condition and does not produce severe neurological deficits.
The study consisted of 2 parts. Seventy animals were studied overall: 20 in Experiment 1 and 48 in Experiment 2 after two rats with severe deficits were killed early. Experiment 1 aimed to determine the optimal force setting for inducing a cystic cavity without neurological deficits using a computer-controlled motorized spinal cord impactor. Twenty animals received an impact that ranged from 50 to 150 kDyn. Using the optimal force for producing an initial cyst determined from Experiment 1, Experiment 2 aimed to compare the progression of cavities in animals with and those without arachnoiditis induced by kaolin. Forty-eight animals were killed at 1, 3, 6, or 12 weeks after syrinx induction. Measurements of cavity size and maximum anteroposterior and lateral diameters were evaluated using light microscopy.
In Experiment 1, cavities were present in 95% of the animals. The duration of limb weakness and spinal cord cavity size correlated with the delivered force. The optimal force chosen for Experiment 2 was 75 kDyn. In Experiment 2, cavities occurred in 92% of the animals. Animals in the kaolin groups developed larger cavities and more vacuolations and enlarged perivascular spaces than those in the nonkaolin groups.
This impact model reliably produces cavities that resemble human posttraumatic syringomyelia and is suitable for further study of posttraumatic syringomyelia pathophysiology.
Elmira Najafi, Lynne E. Bilston, Xin Song, Andre Bongers, Marcus A Stoodley, Shaokoon Cheng and Sarah J. Hemley
Syringomyelia pathophysiology is commonly studied using rodent models. However, in vivo studies of posttraumatic syringomyelia have been limited by the size of animals and lack of reliable noninvasive evaluation techniques. Imaging the rat spinal cord is particularly challenging because the spinal cord diameter is approximately 1–3 mm, and pathological lesions within the spinal cord parenchyma are even smaller. The standard technique has been histological evaluation, but this has its limitations. The aim of the present study was to determine whether syrinx size could be reliably measured using a preclinical high-field MRI animal system in a rat model of posttraumatic syringomyelia.
The authors used an existing rat model of posttraumatic syringomyelia, which was created using a controlled pneumatic compression device to produce the initial spinal cord injury, followed by a subarachnoid injection of kaolin to produce arachnoiditis. T2-weighted MRI was performed on each animal using a 9.4-T scanner at 7, 10, and 13 weeks after injury. Animals were killed and syrinx sizes were calculated from in vivo MRI and histological studies.
MRI measurements of syrinx volume and length were closely correlated to histological measurements across all time points (Pearson product moment correlation coefficient r = ± 0.93 and 0.79, respectively).
This study demonstrates that high-field T2-weighted MRI can be used to measure syrinx size, and data correlate well with syrinx size measured using histological methods. Preclinical MRI may be a valuable noninvasive technique for tracking syrinx formation and enlargement in animal models of syringomyelia.