Posterior reconstructive or fixation surgery has been reported to have a higher incidence of C5 palsy than conventional decompression surgery, and an excessive kyphosis correction is closely related to a high rate of C5 palsy (tethered effect).8,11-14 On the other hand, some authors have insisted that C5 palsy is a disorder of the intrinsic spinal cord, not the root lesion.15,16 Moreover, some other hypotheses have been presented for C5 palsy development, including that it is a central cord disorder (cord ischemia and reperfusion injury) or some other peripheral root disorder (iatrogenic thermal injury), but a definitive consensus has yet to be reached, and the precise mechanism remains unknown.16,17
The demographics of our population might provide considerable information about the clinical and radiological features of C5 palsy patients. First, C5 palsy occurs within two days after surgery with a weakness grade of 2.7, and about four months later, the paralysis has recovered to around a grade 4. Second, our results show that the occupying ratio was high in C5 palsy patients, which is similar to the previously published reports. Third, the lamina open side was more closely related to C5 palsy than the hinge side during open-door laminoplasty in this study.8,18 Generally, it was previously reported that the range of bilateral C5 palsy occurrence was very wide (up to 42%). However present study’s result (12/85, 14%) was similar or slightly higher than that in a previous meta-analysis report (8%).2,18
In recent studies, the C4–5 foramen stenosis has been considered the most relevant risk factor for C5 palsy during cervical operations.3,8,11,17,19-22 Previous clinical data suggested a reference point for stenosis (C5 palsy group: 1.99–2.70mm, non-C5 palsy group: 2.76–3.20mm) that can cause C5 palsy. Lee at al. insisted that severe stenosis of the C4–5 FD (less than 2mm) can induce C5 palsy at a rate of about 16 times higher than seen with non-severe C4–5 stenosis.19 Our results showed that patients with severe paralysis (MMT ≤3) were more likely to belong to group A, which suggested a close correlation between the degree of paralysis and the degree of C4-5 foraminal stenosis. In other words, those with a severe state of C5 palsy had smaller absolute dimensions for C4–5 FD and a smaller C4–5 FD on the affected side than the non-affected side. The above results could be confirmed once again through univariate and multivariate analysis.
Generally, the larger the cord rotation, the higher the probability of C5 palsy.23-25 However, there was no content on the clinical course of C5 palsy by cord rotation in the previous studies. Unexpectedly, the present study revealed that the degree of C5 palsy is more serious, when there is little cord rotation in pre-operative. Among the pathophysiology of C5 palsy, the tethered effect may be most commonly accepted theory. Uneven and asymmetric cord shift by cord rotation may be reasonable theoretical grounds for explaining the relationship between the C5 palsy side and lamina hinge side in laminoplasty. Considering the above results, it can be inferred that the C5 palsy, which was not mainly caused by cord rotation (tethered effect), could be more serious.
T2HIZ was reported to be associated with C5 palsy occurrence and prognosis in some papers.7,26 We did not find T2HIZ relevance to the degree of C5 palsy when we simply evaluated its presence or absence. However, according to clarity, when we subdivided it into three groups, we found that the degree of clarity may be related to the severity of C5 palsy symptoms.8 On univariate analysis, the severity of C5 palsy was closely related to T2HIZ grade. However, there was no statistical significance in multivariate analysis in the present study.
In terms of the onset of C5 palsy, differences were closely related to the primary pathology type in the present study. The OPLL group had a high probability of belonging to the early onset group, and the delayed onset group had a statistically high HNP/spondylosis patient composition ratio (Table 4). The above result was re-confirmed through multivariate analysis. OPLL pathology (OR 13.7 [95% CI 1.33-140.77], P=0.02) was closely related to the early-onset of C5 palsy in the present study. Previously, Takenaka et al. showed that the prevalence rates of C5 palsy (OPLL) were higher in both early and late-onset in their meta-analysis.27 However, our multi-centered cohort study revealed that OPLL pathology was more closely related to the early onset of C5 palsy.
The following five factors also were identified as a potential factor that can predict early onset of C5 palsy: (1) old age, (2) radiculopathy, (3) posterior approach (4) pre-operative C2-7 angle and (5) post-operative C2-7 angle. Generally, reperfusion injury (myelopathy) can explain the pathophysiology of the late onset of C5 palsy and lag correction effect (hypercorrection) may explain the phenomenon of early onset of C5 palsy.12,28 From this point of view, depending on the main symptom and the difference of C2-7 angle could sufficiently explain the above results. A recent meta-analysis also showed that the posterior approach was closely associated with the early onset of C5 palsy. The above previous results are consistent with our present finding.27
Limitation and Interpretation
First, this study was a retrospective design. Second, this study lacks an analysis of a matched control cohort without post-operative C5 palsy. Therefore, a matched-control study could proceed in the future. Third, it may be worth recalling that the definition of C5 paralysis can significantly impact study results, as the absolute definition of C5 palsy has not been established. Fourth, there can be measurement error in various aspects from the incorrect measurement method (ex. C4-5 foramen diameter in axial view vs oblique sagittal view).