Pathophysiology of primary spinal syringomyelia

Abstract

Object: The pathogenesis of syringomyelia in patients with an associated spinal lesion is incompletely understood. The authors hypothesized that in primary spinal syringomyelia, a subarachnoid block effectively shortens the length of the spinal subarachnoid space (SAS), reducing compliance and the ability of the spinal theca to dampen the subarachnoid CSF pressure waves produced by brain expansion during cardiac systole. This creates exaggerated spinal subarachnoid pressure waves during every heartbeat that act on the spinal cord above the block to drive CSF into the spinal cord and create a syrinx. After a syrinx is formed, enlarged subarachnoid pressure waves compress the external surface of the spinal cord, propel the syrinx fluid, and promote syrinx progression.

Methods: To elucidate the pathophysiology, the authors prospectively studied 36 adult patients with spinal lesions obstructing the spinal SAS. Testing before surgery included clinical examination; evaluation of anatomy on T1-weighted MRI; measurement of lumbar and cervical subarachnoid mean and pulse pressures at rest, during Valsalva maneuver, during jugular compression, and after removal of CSF (CSF compliance measurement); and evaluation with CT myelography. During surgery, pressure measurements from the SAS above the level of the lesion and the lumbar intrathecal space below the lesion were obtained, and cardiac-gated ultrasonography was performed. One week after surgery, CT myelography was repeated. Three months after surgery, clinical examination, T1-weighted MRI, and CSF pressure recordings (cervical and lumbar) were repeated. Clinical examination and MRI studies were repeated annually thereafter. Findings in patients were compared with those obtained in a group of 18 healthy individuals who had already undergone T1-weighted MRI, cine MRI, and cervical and lumbar subarachnoid pressure testing.

Results: In syringomyelia patients compared with healthy volunteers, cervical subarachnoid pulse pressure was increased (2.7 ± 1.2 vs 1.6 ± 0.6 mm Hg, respectively; p = 0.004), pressure transmission to the thecal sac below the block was reduced, and spinal CSF compliance was decreased. Intraoperative ultrasonography confirmed that pulse pressure waves compressed the outer surface of the spinal cord superior to regions of obstruction of the subarachnoid space.

Conclusions: These findings are consistent with the theory that a spinal subarachnoid block increases spinal subarachnoid pulse pressure above the block, producing a pressure differential across the obstructed segment of the SAS, which results in syrinx formation and progression. These findings are similar to the results of the authors' previous studies that examined the pathophysiology of syringomyelia associated with obstruction of the SAS at the foramen magnum in the Chiari Type I malformation and indicate that a common mechanism, rather than different, separate mechanisms, underlies syrinx formation in these two entities. Clinical trial registration no.: NCT00011245.

Fig. 1

Illustration of the proposed mechanism of syringomyelia formation and progression. A: Brain expansion during cardiac systole creates a CSF pressure wave (long, straight arrows), which is normally dissipated throughout the length of the spinal canal, particularly in the lumbar thecal sac. B: Obstruction of the SAS prevents normal damping of the CSF pressure wave, reduces compliance, and directs CSF into the Virchow-Robin spaces of the spinal cord by excess CSF pulsation (arrows). C: Over time, CSF within the central gray matter of the spinal cord coalesces into a syrinx. After formation of a syrinx, enlarged spinal CSF pressure waves (curved arrows) also act on the spinal cord to propel syrinx fluid and elongate the syrinx.

Fig. 2

Midsagittal T1-weighted MRI studies of the cervical and thoracic spine in a 35-year-old paraplegic man with previous fracture-dislocation at the T8-9 level. The patient developed posttraumatic syringomyelia (A, arrow) above the level of his injury (B), which is obscured by metal artifact. One year after T-8 and T-9 laminectomies, intradural exploration with reestablishment of the subarachnoid space, duraplasty, and removal of instrumentation, MRI demonstrated resolution of the cervicothoracic syrinx (C), myelomalacia at the level of the previous spinal cord injury, and enlargement of the subarachnoid space (D, arrowheads).

Fig. 3

Sagittal phase-contrast cine MRI scans of the cervical spine before (A and B) and 1 year after surgery (C and D) obtained in the patient described in Fig. 2. Pulsatile flow is present in the syrinx before surgery. Fluid flow in the inferior direction (during systole) is white, and in the superior direction (during diastole) is black. Before surgery inferior flow is present within the upper cervical subarachnoid space (arrowhead) and the syrinx (arrows) during systole (A) and superior flow is present in the same locations during diastole (B). After surgery, the syrinx has resolved, and fluid flow during systole (C) and diastole (D) is confined to the SAS (arrowheads).

Fig. 4

Graphs demonstrating the pulse pressure in the cervical subarachnoid space in patients with PSS (before and after surgery) and in healthy volunteers (normal subjects) (A). Before surgery the cervical pulse pressure was significantly (*p < 0.004) increased in the PSS group compared with healthy controls (B).

Fig. 5

Graphs demonstrating the changes in lumbar subarachnoid space pressure in response to jugular compression before (left) and after (right) surgery in the patient featured in Figs. 2 and 3. Before surgery the rise in lumbar pressure is slower and much less than the cervical pressure. After surgery, lumbar pressure rises more rapidly and to a higher level than before surgery. (Cervical pressure recording was not performed after surgery at the patient's request.)

Fig. 6

Intraoperative axial cardiac-gated ultrasonographic images obtained after thoracic laminectomy in a patient with PSS. Frames in late diastole (A and B) are followed by frames in systole (C–F) and early diastole (G and H). Fibrous tissue bands are seen (A, black arrowheads) that traverse the subarachnoid space and pass from the spinal cord surface to the dura (white arrowheads). These bands isolate the dorsal subarachnoid space (B, short black arrows) from the remainder of the subarachnoid space. The syrinx (B, long arrow) is located slightly to the right of the center of the spinal cord and is at its maximal diameter during cardiac diastole. During cardiac systole, the syrinx diameter becomes smaller (F, long arrow) as the anterior and lateral surface of the spinal cord flatten in response to the adjacent CSF pulse wave.

Fig. 7

Graph showing the change in syrinx diameter (ΔS) that resulted from surgical treatment in relation to the number of spinal levels (L) that were explored in an attempt to open the SAS. In patients with more extensive disease, there was less reduction in syrinx diameter (mm) after surgery. The dotted line indicates a linear regression (R² = 0.42; p < 0.0001) of this relationship, as expressed in the following formula: ΔS = 1.2L − 8.4 mm.