Tapering of the Cervical Spinal Canal in Patients with Distended or Nondistended Syringes Secondary to Chiari Type I Malformation

Discussion

Syringomyelia has been reported to accompany CMI 20%–85% of the time and, if left untreated, may lead to progressive deterioration of the spinal cord function.^22⇓–24 Despite decades of research, the maintenance, propagation and, in particular, the pathogenesis of SM in association with CMI remain virtually obscure. As highlighted in the literature, dysfunctional CSF hydrodynamics arising from overcrowding of the caudal cranial fossa and downward displacement of the cerebellar tonsils may, in essence, be held responsible for the development of SM, with steeper pressure differentials across the foramen magnum or spinal levels precipitating accentuated pulsatile CSF subarachnoid waves during systole and diastole.^8⇓⇓–11 No consensus, however, has hitherto been reached as to what primarily contributes to this pressure gradient.

Recently, the cervical spinal anatomy as a key component in the pathogenesis of CMI-associated SM has received increasing concern.^16,17 Hirano et al,¹⁶ in their retrospective review of 21 patients, noted a statistically steeper taper ratio of the upper cervical spinal canal in patients with CMI and SM. Concerning patients with CMI but no syrinx, however, taper ratio was found to be similar to that of their age- and sex-matched control subjects. These authors, therefore, surmised that steeper tapering of the spinal canal, in conjunction with obstruction of the subarachnoid space at the level of foramen magnum, could substantially increase the pressure gradient and subsequently accelerate the CSF velocities, leading to syrinx formation. Of note, their studies took an important first step in trying to elucidate the role of the cervical spinal canal anatomy in CMI, and brought to light an interesting and under-recognized phenomenon that may contribute to a better understanding of the pathophysiology of SM. Unfortunately, the small sample size, to some extent, restrained the validity of their findings. Bearing in mind the documented correlation between age and taper ratio,¹⁷ confounding bias might also be potentially induced by their relatively wide age range (from 4 to 60 years of age). To mitigate this bias and further clarify their findings, we analyzed the sagittal profiles of the cervical spinal canal on a larger but homogeneous cohort of adolescents with CMI. In addition, given that patients with a distended syrinx are more inclined to have a regional enlargement of the spinal canal than those with less extensive cavitation, we sought to determine whether disparities in the spinal geometry exist between the 2 subgroups.

In our series, the average taper ratios of the cervical spinal canal in patients with distended and nondistended syringes were both steeper than in the healthy controls reported previously (−0.2 ± 0.3 mm/level, on average),¹⁶ corroborating the conjecture of an abnormal development of the cervical spine in patients with CMI. Based on established hydrodynamics principles, the intrathoracic pressures are transmitted toward the spinal and cranial compartments and further to the subarachnoid space.^25,26 Hence, one can hypothesize that in patients with CMI, the physiologic pressure transmission that normally exists between the intrathoracic and CSF compartments is exaggerated as a consequence of the steeper tapering of spinal canal, which, in turn, exacerbates the pressure differentials between the cranial and spinal compartments as well as their possibility of equalization, both of which are already altered by the herniated cerebellar tonsils. Intuitively, the elevated pressure differential may precipitate hyperkinetic CSF flow, and once a critical threshold is surpassed, a syrinx could be created. In addition, the anteroposterior diameter of the cervical spinal canal was found to be narrowest at C4 in both groups (Fig 4), corresponding with the results of prior studies that the peak CSF velocity increased progressively from the foramen magnum to C4 in patients with CMI.^11,13,27 In light of these findings and the well-recognized association of syrinx with cervical canal stenosis,^28,29 the pivotal role of cervical spinal canal dimensions in determining CSF circulatory features seems to be evident.

Theoretically, CSF circulatory disturbances may not only be associated with syrinx formation but also could influence the progression of SM, because its maintenance rests upon CSF being forced into the spinal cord by pulsatile pressure waves.^6,30 Based on this assumption, the steeper the tapering is, the more abnormal the CSF dynamics and thus the larger the syrinx should be. Our results, however, demonstrated that patients with nondistended syrinx (S/C ratio ≤0.5) had steeper taper ratios than those with distended syrinx (S/C ratio > 0.5). In a previous study of 22 patients with CMI, Hammersley et al¹⁷ demonstrated that the taper ratio for C1–C7 was similar between patients with CMI plus syrinx and those with CMI only; however, the fact that only 2 patients were included in the latter group made this finding somewhat less convincing. Given that the local bone growth may be influenced by the size of the spinal cord in early development, as evidenced by the experimental work from Holtzer,³¹ a plausible explanation for our results is that the regional enlargement of the spinal cord caused by SM may exert an impact upon the development of the cervical spinal canal during skeletal maturation. Clarke et al⁹ reported that as a syrinx forms and enlarges, characteristics of the spinal subarachnoid space are altered, resulting in the CSF flow profile being stabilized or reverting to closer to normal, which indirectly supports our findings.

The correlation between age/sex and taper ratio has been documented in healthy adults and patients with CMI.^16,32 Hirano et al¹⁶ reported that each increasing year of age was associated with a 0.03 mm/level decrease in the taper ratio in patients with CMI. In the present study, however, no significant difference in taper ratio was noted between older patients (>14 years) and younger patients (≤14 years) (Table 2), which presumably reflected the small sample sizes of the subgroups and also the narrow age range of subjects. We also looked into the possible relationship between sex and spinal canal diameters; however, no statistical correlation was found. Notably, a marginally significant correlation was demonstrated between taper ratio and the anteroposterior diameter of the foramen magnum (r = −.221, P = .053), and this probably reflects the close anatomic relationship between the cervical spinal canal and the foremen magnum. In addition, no significant differences in taper ratio were found between patients with different grades of cerebellar tonsillar descent.

Importantly, the current study has for the first time unraveled the disparities in cervical spinal anatomy between patients with CMI with different syrinx morphologies, which, we anticipate, will shed further light on the integral role of cervical spinal canal dimensions in the pathophysiology of CMI-associated syringomyelia. Limitations of our study lie in its cross-sectional design and the lack of a control group. Consequently, future longitudinal studies involving both patients with CMI and healthy adolescents are warranted to expand our understanding of the reciprocal interactions between syringomyelia and the cervical spinal canal geometry. Subjects of this study were limited to adolescents to mitigate potential confounding bias. Tapering of the cervical spinal canal in the pediatric and adult population, therefore, still merits further investigation. In addition, the fact that more clinically severe syrinx cases are more likely to present for medical evaluation at an earlier time than those with less severe syrinx tends to induce a selection bias. Moreover, whether the relationship between syrinx size and spinal canal tapering observed in this study is an early developmental finding that remains relatively static or an evolving dynamic process has yet be explored by future studies.