In this study of patients with right thoracic scoliosis, the outcomes of spinal instrumentation and fusion are compared between those with syringomyelia-related deformity and those with adolescent idiopathic scoliosis. The syringomyelia group included all patients treated for that condition at a given institution and a given time period, whereas the idiopathic cases were selected to match the syringomyelia group with respect to age, sex, and curve characteristics. The outcomes of surgery, including deformity-related measures of curve severity, balance, flexibility, overall correction, and loss of correction, were quite similar in the two groups. A minor difference was that more patients with syringomyelia had proximal junctional kyphosis (14% versus 6% in the idiopathic group), which the authors related to the need to correct excessive kyphosis in the syringomyelia group. Patient-reported outcomes were also equivalent in the two groups.
The selection of the groups for comparison deserves some comment. Syrinx-related scoliosis, as reported, usually affects boys as often as girls, whereas idiopathic scoliosis is much less common in males. Surgical correction has been reported to be more difficult in males, which is probably related to body size and curve flexibility. Thus, a more typical, female-dominated idiopathic group may have fared better than the reported cohort, in which 45% of the patients were male. In addition, left thoracic curves are almost the norm for syrinx-related scoliosis. We surgeons often find the left thoracic curve a bit more challenging as we have to reverse many of our corrective maneuvers. I would be interested in an analysis of the authors’ surgical results in left thoracic curves in comparison with the results of “normal” right thoracic surgery done for idiopathic deformities.
A frustrating issue is our lack of understanding of the basic causal mechanisms of either idiopathic or syringomyelia-associated scoliosis. There is some evidence that idiopathic scoliosis is initiated by the loss of physiologic kyphosis. There is also some recent evidence that genes related to spinal muscle development are possibly etiological factors. On the other hand, syrinx-related scoliosis is often suspected on the basis of a finding of normal to increased apical kyphosis. As the authors stated, we call syrinx-related scoliosis “neurogenic,” as if we knew something about the mechanism. In truth, I am unaware of an evidence-based hypothesis that explains how the central canal dilation brings about a spinal deformity. Zhu and coworkers reported a significant correlation between curve convexity and the dominant side of a deviated syrinx, with an 83% concordance rate. Strahle et al.noted that scoliosis was not independently associated with a Chiari-I malformation without a syrinx when other variables were controlled. In contrast, Godzik et al. found that Chiari-I malformation without a syrinx was associated with scoliosis and that these patients had fewer atypical curve features, including fewer left curves, fewer kyphotic curves, and fewer related neurological abnormalities compared with patients who had a Chiari-I malformation with a syrinx. The actual etiological mechanisms for this remain obscure.
Sha et al. noted that the presence of apical hypokyphosis distinguishes idiopathic scoliosis from syrinx-related deformity, a very important finding in the diagnosis of non-idiopathic deformity. It is unfortunate that while they reported findings related to overall kyphosis, they failed to note the presence of, or surgical effect on, the apical hypokyphosis.
This brings us to a remarkable observation that, in the syrinx-related cases, the extent of the dilation of the spinal cord receded following the spinal correction and fusion. This occurred even in the patients who did not have decompression of the Chiari malformation. I, like the authors, would be foolish to venture an explanation of this phenomenon given our lack of understanding of the basic mechanisms of the disorder. Does straightening and immobilizing the spine alter spinal fluid dynamics? Let us consider this food for thought and, more importantly, a stimulus for further investigation. (Sha and coworkers also noted syrinx resolution in 45% of their patients following surgical correction and fusion of scoliosis in a previous study.)
Another important finding was the lack of neurological events in the syringomyelia group. The cohort is too small to allow us to generalize and assume that corrective surgery presents no greater risk in this group. Godzik et al. also reviewed a matched cohort to compare surgery for idiopathic and syrinx-related curves, and they found that the syringomyelia group had more neurological complications (11% versus 0%) and more neurophysiologic changes shown by neuromonitoring during surgery (28% versus 3%). While the current report by Sha et al. is encouraging, it is still prudent to approach this surgery with a degree of extra caution with regard to the amount of correction and to carefully interpret neuromonitoring changes.
The authors concluded that surgical correction of right thoracic curves, using current methods that employ pedicle screw fixation and that allow almost complete correction of the spinal deformity, has satisfactory results in patients with syringomyelia that are equivalent to those in idiopathic cases. I believe that their findings support this conclusion.
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