|Year : 2017 | Volume
| Issue : 1 | Page : 80--82
Multimodal intraoperative neuromonitoring during surgery for correction of spinal deformity: Standard of care or luxury?
Department of Neurosurgery, Madurai Medical College, Madurai, Tamil Nadu, India
Prof. Natarajan Muthukumar
Department of Neurosurgery, Madurai Medical College, Madurai, Tamil Nadu
|How to cite this article:|
Muthukumar N. Multimodal intraoperative neuromonitoring during surgery for correction of spinal deformity: Standard of care or luxury?.Neurol India 2017;65:80-82
|How to cite this URL:|
Muthukumar N. Multimodal intraoperative neuromonitoring during surgery for correction of spinal deformity: Standard of care or luxury?. Neurol India [serial online] 2017 [cited 2020 Aug 15 ];65:80-82
Available from: http://www.neurologyindia.com/text.asp?2017/65/1/80/198221
Spine surgeons have started performing increasingly complex procedures involving aggressive deformity correction and tumor resection. Such complex spinal surgery entails the risk of neurological deterioration. In order to avoid adverse outcomes, intraoperative neuromonitoring (IONM) has been developed to assess, in real-time, the function of the spinal cord and nerve roots. For IONM to be beneficial, (1) it must provide indications of impending neurological damage early enough to permit the surgical team to take appropriate measures to reverse or minimize the damage; (2) it should be easily interpreted; (3) it should be readily available; and, (4) it should be cost effective.
In this issue, Krishnakumar et al., present their 2 year, single-centre experience with “Multimodal intraoperative neuromonitoring in scoliosis surgery.” The key messages from this study are as follows: (1) Baseline values are lower in patients with neuromuscular scoliosis vis-à-vis adolescent idiopathic scoliosis. (2) The authors did not have any false negative results. (3) All the false positives (10%) were either due to malfunction of the leads or due to changes in blood pressure. (4) No patient had a fresh deficit, either temporary or permanent, postoperatively. Finally, (5) no patient required or was subjected to the Stagnara wake-up test. The authors did not highlighted the preoperative degree of scoliosis and the extent of correction postoperatively. This is especially important as all patients underwent single-stage procedures. On the basis of the abovementioned facts, it is presumed that the patients in this cohort had relatively simple deformities, which might be responsible for the excellent results reported by these authors. In this study, there were no false negative results, i.e., patients who had a normal IONM but developed a fresh neurological deficit postoperatively. This might be attributed to either careful case selection or the small number of cases studied because a large, single-centre study of 1017 patients by Sutter et al., showed a false negative rate of 0.8%. It is also important to note that certain relative exclusion criteria for IONM are used by experienced groups and include epilepsy, stroke within 6 months, and infectious diseases of the central nervous system (CNS), skull defects, cortical lesions, etc., The authors of this study have not highlighted their exclusion criteria. It is important for the readers of this journal to be aware that not all potential candidates for deformity correction or other spinal procedures are neurophysiologically eligible to undergo IONM. Unlike studies with a large number of patients, the index study has a 100% sensitivity and specificity, which obviously is due to the small number of patients.
Multimodality intraoperative neuromonitoring and the nature of the underlying pathology
7.5% of patients in the index study had neuromuscular scoliosis. It is important to remember that, in pediatric patients with neuromuscular scoliosis, MEPs (Motor Evoked Potentials)have limitations, including an unacceptably high rate of false positives, which may be as high as 27.1%. This is important as false positives may compel the surgeons to unnecessarily change the operative plan. Hence, false positives also have important limitations. Another problem with neuromuscular scoliosis is that when somatosensory evoked potentials (SSEPs) are used as the only IONM modality, it has been found that if there are changes intraoperatively in SSEPs, recovery of such changes after institution of appropriate corrective maneuvers will not necessarily imply that the patient will not suffer a postoperative neurological deficit because there is still a 50% to 60% risk of neurological deficit in spite of restoration of SSEP values to baseline levels. In the index study being discussed here, there was a time lag of 15 minutes between the changes that were picked up by motor evoked potential (MEP) and SSEP. Other studies have noted a time lag of 5 minutes between MEP changes and SSEP changes during IONM. Therefore, MEP changes are more sensitive than SSEP changes. It is important for us to remember that, while SSEPs are being monitored continuously, MEPs are done intermittently either during high risk maneuvers or at fixed time intervals during surgery because MEPs involve patient movements, and therefore, the surgery should be briefly interrupted to perform MEP.
In the literature, there is considerable variation in the criteria used for “alerts.” In the index study, an “alert” was raised when the MEP was decreased by at least 65%, whereas in certain other studies such as the large single centre study of 1162 patients by Zhuang et al., an “alert” was raised only when all the three following criteria were met: (1) More than 80% amplitude loss, (2) synchronously and logically associated with high-risk surgical maneuver, and (3) systemic and anesthetic factors were ruled out. In another large study by Bhagat et al., the following were considered as “alerts”: More than 50% decrease in the amplitude of SSEPs, or more than 10% increase in the latency of SSEPs and more than 80% decrease in the amplitude of MEPs. In another study by Langloo et al., three different criteria were compared for MEPs, i.e., more than 80% decrease in amplitude in one of six recordings, two of six recordings, and two instead of six recordings from tibialis anterior; they showed that using different criteria resulted in different false positive and false negative rates. Feng et al., in a review of 176 patients undergoing deformity correction surgery, used an amplitude decrease of more than 75% in MEP and 50% decrease in amplitude in SSEP as alerts. Therefore, the stringency of the criteria for raising an “alert” is also an important factor that is responsible for the different false-positive and false-negative rates in different studies, and the lack of uniformity in the criteria for “alerts” makes comparison between different studies difficult. If a low threshold is used for raising an alert, then there is increased chance of false positives, with surgical procedures being unnecessarily altered or abandoned by these false positives. On the contrary, if a high threshold is used for raising an alert, then the chance of false negatives with resultant postoperative neurological deficits is a problem. Therefore, as stated by Padberg and Bridwell, “Further research is definitely needed from the clinical sector to more fully define parameters for determining the significance of response decrement.”,
Complexity of the spinal deformity
The nature and severity of the spinal deformity is also important while analyzing the rates of false positives, and especially, false negatives. For example, in a series of 1162 patients undergoing deformity correction surgery, Zhuang et al., reported that 80% of alerts occurred in patients with congenital scoliosis. Therefore, they considered this pathology to be a high-risk diagnosis. Feng et al., noted that procedures involving an osteotomy, a curve Cobb angle of more than 90 degrees, and a significant kyphosis were the major risk factors for neuromonitoring changes. Intuitively, patients with complex spinal deformities, especially those with significant kyphoscoliosis with complex curves requiring complex, anterior and/or posterior procedures have a higher risk of postoperative neurological deficits than patients with simple deformities that are easy to correct by a single-stage procedure. Most studies that deal with multimodalilty IONM do not address these issues by providing data such as preoperative and postoperative Cobb's angle, etc.
Why is multimodality monitoring necessary?
It has been shown that no single modality sufficiently monitors all spinal cord pathways so as to use it as a proxy for spinal cord function. Hence, a combination of testing methods that includes SSEPs, transcranial MEPs and triggered EMGs (to detect individual root function), and EMG of the sphincters (to assess sphincter function) is necessary. Thus, multimodality IONM, if used appropriately, can provide information about the function of neural structures at risk. However, multimodality IONM is not without problems, which include increase in the operative times, cost, etc.
Another practical issue that has to be considered in the context of multimodality IONM is the following scenario; where there is an alert in one modality and the other modality does not show any change, as has been reported earlier. In this scenario, the surgical team is left with the difficult decision of whether to reverse a potentially risky surgical maneuver or proceed further. If the procedure is abandoned and such a patient wakes up without neurological deficits then a potentially useful surgical procedure would have been abandoned unnecessarily. Such issues need clarification in future studies.
Is there a role for Stagnara wake-up test in contemporary spine surgery practice?
In the index study being discussed, Stagnara wake-up test was not performed. Certain authors such as Sutter et al., consider the Stagnara wake-up test to be obsolete in contemporary spine surgical practice. However, others such as Malhotra and Shaffrey suggest that, if IONM demonstrates a suspicious abnormality, a normal wake-up test can reassure the surgeon that no permanent neurological damage has occurred. This is especially so if such IONM changes occur during what the surgeon considers as low risk maneuvers, or if one modality shows significant changes and the other does not. To avoid precipitating a fresh neurological deficit by ignoring the changes or to abandon a potentially useful surgical procedure, in appropriate situations, it might still be reasonable to use the Stagnara wake-up test until the IONM technology evolves further.
Is multimodality intraoperative neuromonitoring cost-effective to be used in resource-constrained environments?
Even in centres without resource constraints, IONM is not used routinely for all spinal procedures. Sutter et al., used IONM in 9% (1017 patients) out of 11536 spine procedures. Even in resource-constrained environments, it might still be cost-effective if IONM is used selectively. As suggested by Sala et al., it is extremely difficult to analyze the cost-effectiveness of procedures such as IONM. However, considering the enormous costs of health care and the human suffering related to the development of postoperative paraplegia/quadriplegia, there is enough evidence to prove that the cost of performing IONM does not exceed that of providing health care to the injured patients. Another recent study has shown that IONM during spinal surgery is not only useful to prevent neurological complications but is also cost-effective. Therefore, in my personal opinion, at least in high risk surgical procedures such as deformity correction, intramedullary tumors, etc., where there is a substantial risk of postoperative neurological deficits, IONM should be used. It is imperative that authorities in administrative cadres of health care institutions, especially those that perform high risk spinal surgery, should be made aware of the implications of having a dedicated IONM team both for patient care as well as the potential medicolegal implications.
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