Abstract

Background: Triple Intraoperative Neurophysiological Monitoring (IONM) should be considered the standard of care (SOC) for performing cervical surgery for Ossification of the Posterior Longitudinal Ligament (OPLL). IONM’s three modalities and their alerts include; Somatosensory Evoked Potentials (SEP: =/> 50% amplitude loss; =/>10% latency loss), Motor Evoked Potentials (MEP: =/> 70% amplitude loss; =/>10-15% latency loss), and Electromyography (loss of EMG, including active triggered EMG (t-EMG)).

Methods: During cervical OPLL operations, the 3 IONM alerts together better detect intraoperative surgical errors, enabling spine surgeons to immediately institute appropriate resuscitative measures and minimize/avoid permanent neurological deficits/injuries.

Results: This focused review of the literature regarding cervical OPLL surgery showed that SEP, MEP, and EMG monitoring used together better reduced the incidence of new nerve root (e.g., mostly C5 but including other root palsies), brachial plexus injuries (i.e., usually occurring during operative positioning), and/or spinal cord injuries (i.e., one study of OPLL patients documented a reduced 3.79% incidence of cord deficits utilizing triple IONM vs. a higher 14.06% frequency of neurological injuries occurring without IONM).

Conclusions: Triple IONM (i.e., SEP, MEP, and EMG) should be considered the standard of care (SOC) for performing cervical OPLL surgery. However, the positive impact of IONM on OPLL surgical outcomes critically relies on spinal surgeons’ immediate response to SEP, MEP, and/or EMG alerts/significant deterioration with appropriate resuscitative measures to limit/avert permanent neurological deficits.

Keywords: Anterior cervical surgery, Anterior corpectomy fusion (ACF), Anterior diskectomy/fusion (ACDF), Intraoperative neural monitoring (IONM), Surgical errors, Negligence, Somatosensory evoked potentials (SEP), Motor evoked potentials (MEP): Electromyography (EMG), Neurological injuries, Spinal cord deficits, C5 Palsy, Brachial plexus injury

INTRODUCTION

Triple Intraoperative Neurophysiological Monitoring (IONM) should be considered the standard of care (SOC) for performing cervical surgery for Ossification of the Posterior Longitudinal Ligament (OPLL). The three modalities of IONM and their alerts include; Somatosensory Evoked Potentials (SEP: =/> 50% amplitude loss; =/>10% latency loss), Motor Evoked Potentials (MEP: =/> 70% amplitude loss; =/>10-15% latency loss), and Electromyography (EMG: passive and active/triggered EMG (t-EMG)) [ Table 1 ]. Any combination of these 3 alerts can signal the onset of intraoperative surgical errors/impending neurological injuries that can be limited/reversed/averted by spinal surgeons’ immediate performance of appropriate resuscitative measures. This perspective focuses on how triple IONM (i.e., SEP, MEP, and EMG) should be considered part of the standard of care to limit/avert nerve root, brachial plexus, and spinal cord injuries from occurring during cervical OPLL surgery [ Table 1 ].[ 1 - 20 ]

Table 1:

Ossification of the Posterior Longitudinal Ligament (OPLL) and The Necessity for Intraoperative Neural Monitoring (IONM).

Early Evidence that MEP were Safe/Effective for Monitoring ACDF for Disk Disease/Spondylosis/ Cervical Spondylotic Myelopathy

Two early studies showed that MEP were safe and provided effective IONM for anterior cervical surgery (i.e., ACDF for disk disease/spondylosis/cervical spondylotic myelopathy (CSM)) [ Table 1 ].[ 3 , 11 ] Darden et al. (1996) monitored 49 patients undergoing anterior cervical surgery with MEP (i.e., 38 had monoradiciulopathy); none exhibited new postoperative neurological worsening.[ 3 ] Interestingly, for those with preoperative deficits for under 1 year in duration, MEP helped predict postoperative improvement of motor function. In 1997, Gokaslan et al. using transcutaneously placed midthoracic epidural electrodes to monitor MEP during 16 ACDF found that no patients developed significant intraoperative changes/alerts, and that all were motor intact postoperatively.[ 11 ]

Evolution of IONM Techniques for Cervical OPLL Surgery From 1993-1998 Monitored with SEP/EMG to Combined SEP/EMG and MEP Monitoring by 2002-2014

For cervical OPLL surgery, Epstein documented the progression of IONM techniques from SEP/EMG monitoring alone from 1993-1998 to SEP/EMG and MEP techniques used together starting in 2002 [ Table 1 ].[ 4 - 9 ] In 1993, Epstein performed 41 anterior cervical corpectomy/fusions (ACF) and 10 cervical laminectomies/fusion (LAM/F) for OPLL using SEP/EMG monitoring; these 2 modalities appeared to “... limit operative morbidity”.[ 4 ] By 1998, SEP/EMG alone were still used to monitor 22 circumferential cervical OPLL procedures (i.e. including multilevel ACF with Posterior Fusions); postoperatively, patients improved an average of 3.0 postoperative Nurick grades, and the patients sustained no new postoperative cord deficits.[ 5 ] As of 2002, SEP/EMG and the option for MEP monitoring was not being used to supplement OPLL surgery; “...the option of undergoing MEP: “...for OPLL patients under 65 years of age with kyphosis being treated with single/multilevel ACF vs. laminectomies/posterior fusions if over 65 years of age with multilevel OPLL and a preserved lordotic curvature”.[ 6 ] In 2003, Epstein’s recommendations for performing cervical OPLL surgery included; “...continuous intraoperative somatosensory-evoked potential monitoring...”, and EMG monitoring, but not yet routine MEP monitoring (i.e., along with awake intubation and awake positioning).[ 7 ] By 2014, Epstein’s two studies focused on performing cervical OPLL surgery with routine intraoperative; “...somatosensory, motor evoked potentials, and electromyographic monitoring...”, along with Total Intravenous Anesthesia (TIVA) to avoid interfering with IONM signals.[ 8 , 9 ]

Utility of SEP in Cervical OPLL Surgery

Roh et al. (2007) Used SEP as Part of the Standard of Care in Cervical Surgery, Especially for OPLL

In 2007, Roh et al. utilized SEP during 809 consecutive cervical operations [ Table 1 ].[ 16 ] Successful resuscitative maneuvers included removal of tape from the shoulders (8 patients) and release of traction; (4 patients). Two additional patients whose SEP changes failed to return to baseline prior to closing respectively recovered neurological function within 6 hrs, and 2 mos. postoperatively. The authors concluded that SEP monitoring; “....probably prevented neurological deficits in 15 (1.9%) of 809 patients...” They further commented that OPLL patients showed a; “...7-fold greater risk of intraoperative SEP degradation”. Additional experience with IONM in this patient series led the authors to routinely incorporate SEP monitoring as part of their standard of care for performing cervical spine surgery.

Prolonged Preoperative SEP Latency Correlated with Intraoperative SEP/MEP Amplitude Losses and “Iatrogenic” Intraoperative and Postoperative Neurological Damage (PND)

Yoo et al. (2022) used SEP to reduce the incidence of intraoperative “iatrogenic damage” resulting in postoperative neurological damage (PND) observed in a series of 265 myelopathic patients undergoing cervical OPLL surgery [ Table 1 ].[ 19 ] When they compared preoperative SEP (PreSEP) obtained 3 days prior to surgery with postoperative SEP obtained 48 hrs. and 4 weeks postoperatively, they found that; “... prolonged latency for PreSEP correlated with both intraoperative SEP (ioSEP) and intraoperative MEP (ioMEP) amplitude losses, that forecast PND at 48 hrs. and 4 weeks postoperatively”.

SEP and TCeMEP Alerts Averted Brachial Plexus Injury in Anterior Cervical Surgery

While positioning for an anterior cervical procedure, Jahangir et al. (2011) found that SEP decreased, but TCeMEP were lost in the left upper extremity during draping for an anterior cervical procedure; the surgeon’s removal of the shoulder tape, and placement of the arms on arm boards resulted in SEP improvement, and full TCeMEP recovery [ Table 1 ].[ 12 ] Because of this case, the authors; “...recommend that TCeMEP monitoring be considered as an adjunct to SEP for prevention of injury to the brachial plexus...” for cervical spine surgery.

TCeMEP Detect Cord Injuries/C5 Palsies in Cervical OPLL/CSM Surgical Patients, While EMG’s Were Less Accurate

Two studies documented TCeMEP detected cord/long tract motor deficits, and C5 palsies in patients undergoing cervical surgery for OPLL or CSM; EMG’s were less accurate [ Table 1 ].[ 2 , 10 ] Bhalodia et al. (2013) found that during 229 predominantly C4 and/or C5 ACF, most C5 root injuries were documented with TC-MEP (100% sensitive; 99% specific); however, spontaneous EMG were less accurate.[ 2 ] In 2022, Funaba et al. performed TCeMEP (i.e., alert > /= 70% loss of amplitude) in 336 OPLL patients vs. 840 operations for CSM, and found the; “Overall sensitivity was higher for OPLL (71.4%) vs. CSM (42.9%)”.[ 10 ] In short, the sensitivity of TCeMEP for lower extremity paralysis/cord injuries for both groups was 100%. However, it’s accuracy was reduced to 66.7% for segmental palsy/root injuries in OPLL patients, but was nil (i.e., 0%) for signaling root injuries in CSM patients. Interestingly, most TCeMEP changes occurred during decompressions (63.16%), or screw placements (15.79%).

Efficacy of IONM for ACDF in Patients with OPLL But Not Disc Disease/Spondylosis/CSM

IONM Not Effective in Anterior Diskectomy/Fusion (ACDF) Excluding OPLL Surgery

In 2019, Badhiwala et al. (2019) demonstrated that IONM did not limit/prevent neurological deficits when used in ACDF surgery performed for disc disease, spondylosis, or CSM patients; however, this series specifically excluded OPLL patients [ Table 1 ].[ 1 ] Their NIS Sample (Nationwide Inpatient Sample Healthcare Cost and Utilization Project 2009-2013) showed comparable frequencies of new neurological injuries for ACDF performed with (i.e., 1.7% out of 9380 monitored ACDF) vs. without IONM (i.e., 0.22% out of 9380 unmonitored ACDF). Further, IONM cost an additional average of $6843.00 per ACDF case.

IONM Effective in ACDF Performed for Cervical OPLL

Kim et al. (2021) documented that IONM using SEP, TcMEP, and continuous EMG monitoring effectively averted/limited surgical errors/new neurological injuries from occurring during ACDF performed in patients with cervical OPLL [ Table 1 ].[ 13 ] They found the; “...rates of postoperative neurological deficits, neurophysiological warnings...”, in 196 OPLL patients undergoing ACDF surgery (2009-2019) was reduced to 3.79% with IONM (23 warnings: 17 TcMEP, and 6 for EMG), vs. a significantly higher 14.06% incidence of new postoperative deficits in historical controls (i.e. ACDF without IONM performed 2003-2009). They concluded that utilizing all 3 modalities together; “...may be a useful tool to detect neurological damage in ACDF for high-risk conditions such as OPLL with pre-existing myelopathy”.

MEP Alerts Warn of Surgical Errors/Impending Neurological Injury in Cervical OPLL and Other Cervical Surgery

Multiple studies effectively utilized IONM MEP alerts (i.e., set at =/> 70% decrease in amplitude; typically set at >/= 10% (i.e., subset set at >/= 15% decrease in latency)) to warn of surgical errors/impending neurological injuries occurring during OPLL, CSM and/or other cervical procedures [ Table 1 ].[ 14 , 15 , 17 , 18 , 20 ] From 2010 to 2012, Kobayashi et al. (2014) set the Tc-MEP alert at =/> 70% decrease in amplitude for 959 patients undergoing cervical surgery for OPLL, spinal deformities, and spinal cord tumors.[ 14 ] This yielded a; “...high sensitivity (95%) and specificity (91%) for intraoperative spinal cord monitoring...”; there were just 2 false negatives (i.e. no intraoperative alerts occurring in 2 patients undergoing surgery for intramedullary cord tumors). In 2018, using Br[E]-MsEP alerts (Brain Evoked Muscle-Action Potentials; decrease =/> 70% in amplitude; =/> 10% to 15% increase in latency), Kobayashi et al. found that 9 of 83 patients undergoing spine surgery developed new postoperative motor deficits (i.e., true positives with a “sensitivity of 100% and specificity of 93%, with just 7% false positives, and 0% false negative rates) [ Table 1 ].[ 15 ] They concluded that combining amplitude and latency alerts for Br[E]-MsEP provided highly sensitive and specific alerts, and; “...reduces false positive rates”. Yoshida et al. (2019) confirmed the efficacy of TcMEP (i.e., loss of =/> 70% amplitude) in high-risk spine cases, including 622 with cervical OPLL [ Table 1 ].[ 20 ] Most alerts occurred during; “...posterior decompression and dekyphosis in OPLL cases”. The specific rescue rate (i.e., “...number of patients rescued with intervention after IONM alert/number of true positive cases + rescue cases...”) for cervical OPLL cases was 82.1% (32/39 cases). With these findings, they concluded; “...appropriate intervention immediately after an IONM alert may prevent neural damage even in high-risk spinal surgeries”. Takahashi et al. (2021) evaluated the impact of using Tc(E)-MEPs (=/> 70 decrease in amplitude) on 1934 patients undergoing high risk (HR) surgery (Group HR included cervical OPLL, deformity and spinal cord tumors) vs. common cervical surgery (Group C included more routine procedures); for OPLL, the sensitivity was 100%, and the specificity was 86.9%.[ 18 ] Shigematsu et al. (2021) studied the impact of non-surgery related factors resulting in TES-MEP changes (alert loss =/> 70% amplitude) for patients undergoing 1934 spinal surgeries including for OPLL, deformity, and spinal cord tumors (2017-2019).[ 17 ] There were 62 true positives, and 189 false positives; 150 of these were attributed to “non-surgical related factors” (i.e., “...22 (35.5%) true positives, and 136 (72%) false positive cases”). They concluded; “Although the surgeon should examine surgical procedures immediately after a TESMEP alert, surgical intervention may not always be the best approach...”

CONCLUSION

This review indicates that triple IONM (i.e., SEP, MEP, and EMG) should be considered the standard of care (SOC) for performing cervical surgery for OPLL. Notably, using all 3 IONM modalities together better signaled intraoperative surgical errors that, if immediately addressed with appropriate resuscitative maneuvers, could limit/avert the onset of intraoperative/postoperative new neurological deficits (i.e. C5 and/or other root palsies, brachial plexus injuries, and/or new spinal cord deficits).

Declaration of patient consent

Patients’ consent not required as patients’ identities were not disclosed or compromised.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Use of artificial intelligence (AI)-assisted technology for manuscript preparation

The author(s) confirms that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

Disclaimer

The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policy or position of the Journal or its management. The information contained in this article should not be considered to be medical advice; patients should consult their own physicians for advice as to their specific medical needs.

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