Perioperative neurological damage is a highly feared complication of spine surgery and can lead to significant postoperative motor and sensory impairments. In recent years, there has been a rise in the use of intraoperative neurophysiological monitoring (IONM) to prevent these neurological problems. With the help of this technology, spinal cord function may be evaluated during surgery using real-time data from the sensory, motor, and individual nerve roots. Currently, somatosensory-evoked potentials (SSEPs), motor-evoked potentials (MEPs), spontaneous and triggered electromyography (EMG), and D-wave are the various multimodal techniques being used for IONM.[2,3]
Signal latency, drop in the peak amplitude, and decline in waveform complexity all progressively increase during surgical trauma or ischemia and are all linked to escalating damage to ascending sensory and descending motor pathways. It is conceivable to offer real-time information on the spinal cord’s functioning condition by quantifying these changes. In this regard, the clinical value of IONM necessitates prompt identification of the warning signs in order to carry out a speedy surgical correction to avert or reverse any harm. Similar to this, SSEP and MEP monitoring may help surgeons determine how much surgery they can safely perform. For example, if signal changes are within the threshold’s limits, it may be safe to do optimal tumor excision and spinal deformity correction.
If interventional measures are immediately undertaken to address the problem in the operation room, the iatrogenic injury may be reversible. To find out if IONM can prevent neurological injury after spine procedures, we conducted a comprehensive literature review. The following were the key objectives of this review: (i) to systematically review the literature regarding IONM during spinal surgical procedures, (ii) to review the evidence on whether IONM sensitively and specifically detects intraoperative neurologic injury during spine surgery, and (iii) to review the evidence on whether IONM can prevent or reduce neurological complications in spine surgery.
MATERIALS AND METHODS
According to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, a systematic literature review was carried out.
In order to find relevant studies about the use of IONM during spine surgery, published between 1990 and 2022, a thorough search of the literature was conducted. According to PRISMA guidelines, PubMed/MEDLINE, Embase, Scopus, Cochrane, and Google Scholar databases were searched for medical literature on IONM used during spine surgery. Two researchers searched for the terms “neurophysiological monitoring,” “spine surgery,” “intraoperative neurophysiological monitoring,” “IONM,” “somatosensory evoked potential,” “SSEP,” “motor evoked potentials,” and “MEP,” in any possible combination, and the results were based on various inclusion and exclusion criteria. Additionally, we manually searched the references in the selected articles that reported IONM. To settle any disputes, we asked a third reviewer to participate.
To review the evidence on whether IONM could prevent or reduce neurological complications in spine surgery, the literature was also searched for comparative clinical studies comparing outcomes with the use of IONM and without the use of IONM.
There was no restriction of age and gender for the patient inclusion. The studies that matched the criteria below were included: (i) randomized controlled trial, (ii) prospective study, (iii) retrospective cohort study, (iv) conducted among a group of at least 25 patients, and (v) a study that included IONM of individuals having spinal or spinal cord surgery. The exclusion criteria were: (i) the operations required brain surgery, (ii) an incomplete record of IONM findings including an absence of records of intraoperative alarm of signal changes in MEP, SSEP, and/or EMG, (iii) no record of postoperative neurology or the occurrence of neurological deficit, and (iv) case reports.
Data extraction and recording
The articles were extracted independently by authors to ensure their consistency. The retrieved articles were initially assessed based on the titles, which were then re-evaluated on the based abstracts, and the full articles of the selected abstracts were assessed in detail. The search methodology is demonstrated in Figure 1 (flowchart—PRISMA guidelines). The extracted information included: the first author, publication year, study design, level of evidence, and study data (sample size, patient population undergoing surgery, IONM modalities used and their alarm criteria, changes in SSEP and transcranial MEP [TceMEP], and postoperative outcome data). Different alarm criteria that were described in the literature were used to categorize the TceMEP variations. According to various alarm criteria, TceMEP alterations were categorized as either a total loss of signal, amplitude drop of 50%, or 65%, or 80%, all of which were mutually exclusive. Postoperative deficits were defined as any new persisting neurologic deficit that was noticeable at the 24-h postoperative evaluation.
The number of the true positives (TP) and negatives (TN), and false positives (FP) and negatives (FN) for each study cohort was collected and tabulated: Patients who had SSEP/TceMEP alterations, but no fresh postoperative neurological deficits were considered FP. Patients who had SSEP/TceMEP alterations and a fresh postoperative neurologic deficit were considered TP. FN were patients who had a new neurological deficit following surgery, but no changes to their SSEP or TceMEP. TN are patients with no TceMEP/SSEP changes and no new postoperative neurological changes deficits. The information was utilized to create 2 × 2 contingency tables, which were then used to determine the study’s sensitivity and specificity as well as positive predictive value (PPV) and negative predictive value (NPV) for TceMEP and SSEP.
SPSS 20.0 software package was used to conduct data analysis. The pooled sensitivity and specificity of SSEP and MEP were presented as the mean, median, and interquartile range where applicable. A P value of 0.05 was used to define statistical significance. A pooled analysis of sensitivity, specificity, PPV, and NPV of SSEP and TceMEP records of various studies was performed. T-test was used to compare sensitivity and specificity of various alarm criteria used for MEP changes during surgery.
For the comparative analysis of neurological events in IONM vs. non-IONM studies, the random-effects model was used. The neurological events reported for all patients, including those who used IONM, were compared in a pooled analysis of these studies, and the odds ratio (OR), confidence interval (CI), and P value were determined and analyzed.
Does IONM sensitively and specifically detect intraoperative neurologic injury during spine surgery?
A literature search of several databases generated a total of 1253 articles. After initial screening and removing duplicates, and excluding studies not fitting into the inclusion criteria, 452 spine-related studies were analyzed further. A total of 50 full-text articles,[5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54] which met the inclusion/exclusion criteria, were analyzed in-depth for a qualitative review [Table 1].
The analysis included data from 44 retrospective studies, five prospective studies, and one cohort study, which were published between 1990 and 2022. The procedures included scoliosis correction, spinal deformity correction in various regions of the spine, cervical, thoracic, and lumbar spine surgeries, and spinal tumor surgeries.
We conducted a systematic review of all the studies that reported the diagnostic accuracy of these criteria to predict postoperative neurologic outcomes in spine surgery to assess the performance of these warning criteria [Table 2]. Most studies just employed the amplitude factor.
It is interesting to note that several studies revealed very low pooled sensitivity, median (interquartile range, IQR) = 75% (25%–91.5%),[16,19,21,37,41,43] showing a failure of SSEP monitoring to identify postoperative neurologic deficits based on a warning threshold of 50% decrease in amplitude. Intriguingly, the pooled specificity was maintained, and the median (IQR) was 97% (94.75%–99.85%), despite the low occurrence of postoperative neurologic deficit. The pooled mean sensitivity and specificity for all SSEP changes were 67.7% (25%–100%) and 94.1% (53%–100%), respectively, with the pooled mean PPV and NPV of 79.3% (19%–100%) and 92.1% (62%–100%).
Nevertheless, despite an uneventful course of SSEP monitoring throughout surgery, several patients experienced postoperative neurologic weakness (i.e., FN). The prevalence of FN results has been estimated to range from 0.1% to 4.1% at this time.[35,50,51] According to these studies, SSEP amplitude threshold changes may be significantly lower than 50% of baseline, and the current warning criteria’s inaccuracy may too be responsible for the FN outcomes.
Tables 3 and 4 list the sensitivity, specificity, PPV, and NPV corresponding to four alarm criteria found in the analyzed studies: 50% or more amplitude loss, 65% or more amplitude loss, 80% or more amplitude loss, and total signal loss.
The following number of studies was found to report the diagnostic accuracy of TceMEP changes concerning amplitude loss: 14 studies for ≥50% amplitude loss excluding total signal loss, two studies for 65% amplitude loss excluding total amplitude loss, nine studies for ≥80% amplitude loss excluding total signal loss, and two studies for total signal loss.
The sensitivity and specificity for all TceMEP changes were 93.45% (43%–100%) and 94.54% (83%–100%), respectively, with the pooled PPV and NPV of 61.52% (17%–100%) and 97.81% (69%–100%), respectively.
The pooled estimates for sensitivity, specificity, PPV, and NPV for ≥50% amplitude loss were 87.37% (43%–100%), 94.78% (83%–100%), 67.01% (0.21%–100%), and 96.13% (69%–100%), respectively. The pooled summary estimates for ≥65% amplitude loss were 100% sensitivity, 97% (96%–98%) specificity, 84.61% PPV, and 100% NPV, while the same for ≥80% amplitude loss: 100%, 94.52% (87.9%–100%), 51.15% (17%–100%), and 99.77% (98.2%–100%), respectively. The pooled estimates for total signal loss for sensitivity, specificity, PPV, and NPV were 100%, 90.5% (90%–91%), 50%, and 100%, respectively.
T-test was performed to assess the statistical significance of the differences in the mean sensitivities and specificities. The test revealed a statistically significant difference in sensitivity between 50% amplitude reduction and 80% amplitude reduction (87.37% vs. 100%, P value = 0.004). However, there was no statistically significant change in specificity between 50% and 80% amplitude reduction (94.78% vs. 94.52%, P value = 0.45).
With the pooled sensitivity, median (IQR) = 100% (91%–100%), and pooled specificity, median (IQR) = 96% (90%–98.8%), MEPs are regarded as the gold standard [Tables 3 and 4].[20,22,37,43]
Does IONM prevent neurological injury during spine surgical procedures?
Using the PRISMA guidelines, we reviewed clinical comparative studies that evaluated the rate of new neurological events in patients undergoing spine surgery with and without IONM. Following the evidence hierarchy, randomized controlled trials were sought; however, in the lack of these, comparative case–control or comparative cohort studies were included following the same MeSH terms, inclusion, and exclusion criteria. Of the 1253 articles found in the literature search from the same databases, only six articles were identified and included for analysis, which included two cohort studies and four cross-sectional comparative database types of research. The characteristics of the studies used for analysis are displayed in Table 5.
Pooled analysis of the data from these six studies was performed. These six studies were further assessed by the random-effects model. Table 6 shows a meta-analysis of the postoperative neurological events occurring in these six studies. The pooled OR was 0.5746 (CI = 0.48–0.67), z = 6.637, P value < 0.0001.
Does IONM sensitively and specifically detect intraoperative neurologic injury during spine surgery?
This systematic review identified 50 studies that reported exact changes in SSEP or MEP and postoperative neurological outcomes. Our results affirm that SSEP changes have a low sensitivity (67.7%) and a high specificity (94.1%) and NPV (92.1%). For the MEP, the sensitivities and specificities of the assessed alarm criteria, ≥50% reduction in amplitude, ≥65% reduction in amplitude, ≥80% reduction in amplitude, and total signal loss were 87.4%, 100%, 100%, and 100%, and 94.8%, 97%, 94.5%, and 90.5%, respectively. In the order of the highest to the lowest, ≥65% and ≥80% reduction in amplitude and total signal loss had 100% sensitivity with ≥50% amplitude loss having 87.4% sensitivity. The amplitude loss of ≥65% was most specific followed by ≥50% and ≥80% decrease in amplitude. MEP changes had a high NPV, with total signal loss, ≥80% and ≥65% amplitude loss having NPV of 100%, 99.8%, and 100%, respectively. The overall MEP changes had a high pooled sensitivity (93.5%), specificity (94.5%), and NPV (97.8%).
The SSEP critical threshold related to the postoperative neurologic deficit has only been identified in a few investigations. Nordwall et al. found that paralysis and flaccidity were consistently associated with a full absence of the SSEP signals in a cat model, but there was a significant variance in the results with the changes in amplitude ranging from 72% to 96% drop from the baseline among cats with limb weakness. However, a reduction in SSEP amplitude of more than 70% was regarded as noteworthy. Less precise data exist for waveform latency changes. Surgery results of 300 patients undergoing orthopedic or neurosurgery treatments were reported by Brown et al. Neurological impairments following surgery were found in three patients (1%) who experienced an amplitude reduction of 50%. According to this study, a peak-to-peak amplitude reduction of at least 50% or an increase in latency of at least 10% would necessitate rapid surgical intervention. The warning alert of SSEP is usually believed to be a 50% amplitude reduction and a 10% latency increase, despite early suggestions emphasizing the lack of scientific data and the inability of determining an absolute limit of abnormality.[57,58] The Scoliosis Research Society revealed that in their national survey between 1989 and 1990, 70.5% of the surgeons who took part in the study selected the amplitude criterion. A similar percentage of surgeons adopted the same amplitude (72%) and latency (44%) criteria in a subsequent poll conducted between 1991 and 1995. American Society of Neurophysiological Monitoring has released a statement supporting the idea.
The motor cortex, the corticospinal tracts, the nerve roots, and the peripheral nerves are all included in TceMEPs, which are EMG recordings of this entire motor axis. It is common to record several muscle groups in the upper and lower extremities. Langeloo et al. found a sensitivity of 100% while monitoring at six locations, as opposed to an 88% sensitivity when monitoring at only two sites, in a cohort of 145 consecutive patients. Neuromuscular inhibition cannot be used during surgery because of muscle MEPs because transcranial stimulation causes movement. Before surgery, it is essential to discuss this anesthetic issue with the anesthetists. Additionally, muscle MEPs are only sporadically assessed, in contrast to SSEPs, which are continually recorded throughout the procedure. This is a major muscle MEP flaw that might prevent the early diagnosis of neurological injury. Again, a protocol for checking MEPs during the evaluation of the preoperative checklist must be communicated to the neurophysiology team. This process includes everything from routine monitoring at predefined intervals to simply doing MEPs upon the surgeon’s request.
The MEP warning criteria are typically ambiguous, and many guidelines for making decisions have been offered.[62,63,64,65] Similar to SSEP monitoring, a 50% decrease in amplitude of MEP has been used [Table 3].[7,9,10,12,16,17,20,22,23,24,26,27,37,46] However, a large percentage of FP alarms were generated by this criterion, which caused premature and needless termination of the surgery. A 65% fall in MEP amplitude has also been proposed.[28,38] To avoid FP alerts, Langeloo et al. recommended a threshold of greater than 80% fall in amplitude as the warning signal in MEP monitoring [Table 3]. This suggested criterion showed a 100% sensitivity and a 91% specificity in an evaluation of 145 individuals who underwent corrective surgery for spinal deformity. However, 10 individuals (6.9%) in the group had aberrant MEP alterations but no postoperative neurologic impairments (i.e., FP). Given that MEP amplitudes might change in every individual, there are arguments that MEP should be considered an “all-or-none” response [Table 3].[41,42] This standard allows for the continuation of surgery until the initial MEP signal absence.[66,67,68,69,70,71,72] Although the process for selecting between the various amplitude criteria is not yet obvious, it appears possible that the option may be surgery-specific. For instance, the majority of researchers will select a 50% decrease in amplitude as the warning criterion when doing spinal corrective surgery. The all-or-none criterion, on the other hand, is widely employed for the excision of intramedullary tumors. However, there is currently a lack of high-quality data and disagreement over the MEP monitoring warning signs. Additionally, TceMEP monitoring has been shown to provide an earlier diagnosis of spinal cord ischemia than SSEPs and D-waves, which may enable prompt injury reversal.
Does IONM prevent neurological injury during spine surgical procedures?
Literature was analyzed to determine whether IONM reduces the rates of new neurological deficits or the deterioration of preexisting neurological deficits during spine surgical interventions. However, we were unable to locate any prospective, randomized, comparative trials that had been conducted with this objective. The inclusion criteria were only met by six retrospective comparative studies, which were then assessed. According to this meta-analysis, patients who underwent spine surgery with IONM had fewer intraoperative events (OR = 0.57, CI = 0.49–0.68, P value < 0.0001) than those who did not have IONM.
When IONM was used in intramedullary spinal cord tumor surgery, Sala et al. found that the IONM group had improved motor outcomes at discharge (P = 0.12) and considerably superior results at a 3-month follow-up (P = 0.001). Zielinski et al. reported similar results with lower neurological complication rates in IONM patients in the intramedullary group. According to Cole et al., the use of IONM in single-level surgeries reduced neurological problems only in lumbar laminectomies; no such differences were detected in anterior cervical discectomy and fusion (ACDF), lumbar fusions, or discectomies. However, Ajiboye et al. observed that when compared to cases without IONM, the use of IONM did not reduce the rate of postoperative neurological problems for ACDFs. Lee et al., who reported on cases undergoing posterior cervical spine surgery with and without IONM, discovered comparable outcomes. Lee et al. found that IONM is beneficial for detecting intraoperative neurological damage during posterior cervical operations and improves operative results, particularly in high cervical surgeries.
Both the patient population and the types of surgery and the associated complications in these six included studies were varied, ranging from straightforward lumbar laminectomies to intramedullary spinal tumor resections. Although IONM is able to diagnose intraoperative neurological injuries, the benefits of preventing new neurological deficits in spinal surgery are uncertain. Furthermore, it must be recognized that in order to assess the benefits of IONM in the prevention of new neurological events, more patients will likely be necessary for lower neurological risk operations such as lumbar laminectomy than for treatments with higher neurological risk such as deformity surgery. In addition, Ajiboye et al., Cole et al., and Ney et al.[6,13,33] presented research from large databases in which surgeries were coded. The accuracy of the diagnosis, the actual tabulation of cases, and the controls are just a few of the drawbacks of these case-controlled studies.
Prior studies have shown that multimodal IONM (MIONM) is sensitive and selective in detecting intraoperative neurological injury. Multimodality monitoring, which is now the norm for many spine surgeries, offers the ability to make up for the shortcomings of each monitoring modality alone. A combination of SSEP and MEP monitoring has long been used in scoliosis surgery to track both ascending and descending pathways. The detection of nerve root injury can be improved by combining spontaneous and induced EMG. Multiple studies have demonstrated that the integrated multimodal neuromonitoring technique has 100% combined sensitivities and specificities [Table 1]. We believe that despite the positive results of IONM, some spine procedures, particularly those that have higher risks for complications, such as surgery for deformities, still call for and mandate the use of this screening tool by surgeons. With IONM, surgeons are able to modify their surgical methods, such as reducing the degree of deformity correction, compression/decompression, and spinal decompression, as well as omitting the wake-up test, which was frequently employed in the past. Similar changes in the IONM, such as a decline in MEPs or D-waves, will cause excision to stop for intramedullary spinal tumors, such as those lesions without a clearly defined cleavage plan, and will signal the start of corrective measures such as warm irrigation, blood pressure monitoring, and local papaverine application, among others.
The lack of prospective studies poses a limitation to this systematic review and meta-analysis (only five prospective studies were included as per the inclusion criteria). There was no precise quantification of the SSEP response change in the waveform. There is no comprehensive documentation of postoperative neurophysiological examinations. Spontaneous/triggered EMG data were not analyzed.
The six examined comparative observational studies have major problems with bias: (i) selection bias, caused by considerable patient characteristic variations between the treatment and control groups, (ii) a lack of generalization, because there is large heterogeneity in patient characteristics, surgeries, and risks for postoperative deficits, and (iii) insufficient statistical power in the enrolled comparative studies, which are the only ones available in the literature. The results of our analysis should be cautiously interpreted because our study also includes research publications on a variety of diverse spinal surgeries with varying rates of neurophysiological events.
An increasing amount of research is showing the effectiveness of IONM in identifying adverse events during spine surgery. There is a lack of prospective trials with high-level evidence to support its efficacy, and its use is dictated by surgeon preference and regional institutional rules. To better understand the therapeutic effect of IONM in spinal surgery, more research is required. However, because of ethical constraints and medicolegal repercussions, prospective studies intended to assess the effectiveness of post-IONM alert strategies are highly unlikely to take place in the future. As a result, the need for sizable prospectively designed cohort studies is justified. Last but not least, there are still no evidence-based processes for responding to alarms in MIONM, leaving a crucial knowledge vacuum in the event management both intraoperatively and afterward.
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