Intraoperative EMG during spinal pedicle screw instrumentation

Intraoperative Monitoring of Neural Function Handbook of Clinical Neurophysiology, Vol. 8 M.R. Nuwer (Ed.) # 2008 Elsevier B.V. All rights reserved

404

CHAPTER 28

Intraoperative EMG during spinal pedicle screw instrumentation Jeffrey R. Balzera,b,*, Donald Crammonda, Miguel Habeycha,1 and Robert J. Sclabassia,b,c,d a

Department of Neurological Surgery, University of Pittsburgh, Pittsburgh, PA 15213, USA b

Department of Neuroscience, University of Pittsburgh, Pittsburgh, PA 15213, USA c

Department of Psychiatry, University of Pittsburgh, Pittsburgh, PA 15213, USA

d

Department of Electrical, Mechanical and Biomedical Engineering, University of Pittsburgh, Pittsburgh, PA 15213, USA

28.1. Introduction The objective of performing intraoperative neurophysiological monitoring during procedures involving decompression and instrumentation of the cervical, thoracic, and lumbosacral spine is to detect insults to the central and peripheral nervous systems and the subsequent prevention of iatrogenic neurological injury. This notion is predicated on the fact that the modalities that we use in the operating room to perform neuromonitoring are able to provide for early detection of reversible injury to both the spinal cord and spinal nerve roots. To this end, we have developed and successfully implemented a number of different monitoring modalities including somatosensory evoked potentials (SEPs), dermatomal sensory evoked potentials (DSEPs), motor evoked potentials (MEPs), and free-run and stimulus evoked electromyography (EMG). Each of these modalities has its advantages and disadvantages as it relates to which segment of the spinal cord it assays or whether it monitors cord versus single nerve root function. The selection of the modality(ies) to be used during these procedures is based on the nature of the procedure and the potential and systems at risk for injury. We and *

Correspondence to: Jeffrey R. Balzer, Ph.D., Department of Neurological Surgery, University of Pittsburgh Medical Center, Suite B-400, 200 Lothrop Street, Pittsburgh, PA 15213, USA. Tel.: þ1-412-648-2570; fax: þ1-412-383-8999. E-mail: [email protected] (J.R. Balzer). 1 Present Address: Department of Neurological surgery, UPMC Presbyterian Hospital, University of Pittsburgh, Pittsburgh, PA 15213-2582, USA.

others routinely adopt a multimodality approach to intraoperative neuromonitoring to allow for complete coverage of the neural axis during these procedures. The use of SEP and MEP monitoring has been well documented during these types of procedures for the prevention of spinal cord injury while their usefulness in detecting and preventing single root injury has been scrutinized. In response to this perceived shortcoming, we and others have developed and adopted intraoperative EMG protocols that allow for single nerve root protection and identification. This chapter will review and discuss the free-run and stimulus evoked EMG literature in the context of other modalities and how these EMG modalities are implemented, performed, and interpreted during decompressive and instrumented surgical procedures in the cervical, thoracic, and lumbosacral spine. 28.2. Decompression and instrumentation of the spine Decompressive and instrumented surgical procedures in the cervical, thoracic, and lumbosacral spine date back to the late 1800s and early 1900s (Hadra, 1891; Albee, 1911; Hibbs, 1911; Knoeller and Seifred, 2000). These procedures are now commonplace and extensively utilized for the treatment of spinal stenosis, compression, degenerative disc disease, pseudarthrosis, congenital deformities, and spine trauma. In contrast to earlier surgical constructs, current instrumentation procedures commonly utilize pedicle screws either in place of wires and hooks or exclusively. The impetus behind the increased usage of pedicle screws is a result of the numerous advantages that screw affixation offers

EMG, REFLEX AND NERVE CONDUCTION MONITORING

over more traditional constructs such as hooks and rods. The two most important advantages in using pedicle screw fixation are improved deformity correction and the overall strength of the construct. These advantages are primarily due to the fact that pedicular screw placement allows for three-column control over the spine. Other advantages include the lack of a need to place instrumentation within the spinal canal as is the case for some hook placement and wires and that screw placement is independent of bony integrity which makes their use possible in cases such as trauma and destructive lesions of the spine. The benefits of using pedicle screws in the cervical, thoracic, and lumbosacral spine have been tempered by the potential for significant neurological injury due to the close proximity of the spinal cord, spinal nerve roots, and vascular structures to the pedicle. As a consequence, a misplaced screw could lead to significant iatrogenic intraoperative injury to the patient. As a result, surgeons have employed a number of techniques to ensure safe and accurate placement of pedicle screws. These include detailed anatomic landmarks used to determine pedicle location and screw trajectory, intraoperative imaging including neuronavigation, and neurophysiological monitoring. The implementation of neuromonitoring techniques, particularly as these pertain to the use of free-run and stimulus evoked EMG techniques has proven to allow for the safe and successful placement of cervical, thoracic, and lumbosacral pedicle screws. 28.3. Intraoperative EMG The earliest development and applications of intraoperative EMG were in surgical procedures performed in and around the facial nerve, particularly during acoustic neuroma resection (Delgado et al., 1979; Mller and Jannetta, 1985; Prass and Lu¨ders, 1986; Benecke et al., 1987). Activity, in the form of compound muscle action potentials (CMAPs), was recorded from the musculature innervated by the facial nerve using free-run and stimulus evoked EMG techniques. It was at this time that investigators began to classify the amplitude and frequency of intraoperative free-run EMG that posed a threat to nerve function as well as correlate these intraoperative spontaneous discharges with outcome. The use of terms such as asynchronous firing, waning discharges, bursting, and neurotonic discharge were used in the operating room as they relate to spontaneous EMG discharges.

405

Concomitantly, hand-held electrified probes were first used for direct monopolar stimulation of the nerve and recording of stimulus evoked EMG for purposes of identification, continuity, firing threshold acquisition, and prediction of neurological outcome. Each of these EMG techniques, which were found to be very easily performed in the operating room, provided for near-instantaneous feedback to the surgical team concerning the health of the facial nerve. These early techniques set the stage for the future development, modification, and adoption of EMG recording techniques for nerve root protection in the cervical, thoracic, and lumbosacral spine along with the use of evoked potentials. 28.4. Cervical spine procedures Complications in the form of upper extremity paresis are commonly associated with iatrogenic injury and subsequent pathology of the fifth cervical (C5) nerve root (Sakaura et al., 2003). While other nerve roots (C6–8) can succumb to the same type of injury, the report of paresis in these nerves is significantly lower than in the C5 root. Typically nerve root morbidity occurs subsequent to decompression for cervical myelopathy via both anterior and posterior surgical approaches. The incidence of postoperative C5 palsies has been reported to range from 0 to 30% depending on the approach which is utilized (Epstein, 2001; Sakaura et al., 2003). C5 nerve root palsies are most commonly attributed to direct nerve root injury secondary to manipulation or traction or a segmental spinal cord injury secondary to ischemia. In an attempt to reduce the incidence of both nerve root and spinal cord injury during these procedures, several groups have supported the use of SEP and MEP monitoring (Fan et al., 2002; Bose et al., 2004; Hilibrand et al., 2004; Khan et al., 2006) during these procedures. Additionally, these same groups and others (Jimenez et al., 2005) have suggested that the addition of free-run EMG monitoring can further reduce morbidity during these procedures particularly, injury to individual nerve roots. 28.4.1. Free-run EMG during cervical procedures In a very large series of retrospectively reviewing 508 patients undergoing anterior cervical corpectomies for myelopathy, Khan et al. (2006) found that SEP recording was a very sensitive measure of spinal

406

cord perturbation and injury, particularly associated with hypotension, but the modality lacked in its ability to predict isolated single nerve root injury, particularly the C5 nerve root. They concluded that in addition to SEP recording, these procedures should also include free-run EMG recording so as to be able to sensitively detect manipulation and potential injury to single cervical nerve roots. Confirming their suspicion, Fan et al. (2002) conducted a dual study looking both retro- and prospectively at the ability of intraoperative neurophysiological monitoring during cervical laminectomy to detect iatrogenic C5 nerve root injury. The study reviewed a total of 200 patients who underwent cervical laminectomy. The first 132 patients, who were monitored using SEP, DSEP, and MEP techniques, were reviewed retrospectively. In this group, six patients awoke with unilateral C5 nerve root palsies in the absence of any significant changes in any evoked potential modality. Subsequent to these findings, a prospective study was applied to the remaining 68 patients in the group using not only the techniques applied in the initial 132 patients but also now with the addition of MEPs and free-run EMG recordings specifically targeting the deltoid and biceps muscle groups. In this group, intraoperative C5 nerve root compromise was detected in two patients as a sudden onset of sustained asynchronous neurotonic discharge in the free-run EMG and eventually as a significant change (>75%) in the MEP recordings. In both cases, additional decompressive procedures were undertaken in an attempt to rectify the neurophysiological changes observed. In both cases, free-run EMG subsided and in one, MEP recordings improved. Both patients awoke with C5 nerve root palsies which ultimately improved. No C5 palsies were observed in the absence of significant free-run EMG discharge or MEP changes. To further support the notion of using free-run EMG in cervical procedures for the protection of single nerve roots, Jimenez et al. (2005) reviewed 161 patients who underwent 171 various cervical procedures. In any and all cases where spontaneous free-run EMG was observed, the surgeon took the appropriate operative action if warranted (Fig. 1). This study utilized a retrospective cohort as a control group, that is, one with no EMG monitoring. In this group of 55 patients, 4 (7.3%) had postoperative C5 nerve root palsies. In the prospective cohort of 106 patients, spontaneous free-run EMG events resulted in a change in either positioning or operative

J.R. BALZER ET AL.

technique in three patients (2.8%). Of the three patients who showed spontaneous free-run EMG activity, only one (0.9%) experienced a postoperative C5 nerve root palsy. Like the Fan et al. study, no postoperative nerve root palsies were observed in any patient who did not exhibit spontaneous freerun EMG activity. Taken together along with our current experience with free-run EMG during cervical corpectomy and laminectomy procedures, it appears that the addition of free-run EMG to a routine multimodality approach is effective at reducing single cervical nerve root morbidity during these procedures. 28.4.2. Stimulus evoked EMG during cervical procedures In addition to anterior and posterior decompressive procedures for myelopathy, posterior instrumentation using screws placed in the lateral mass and/or pedicles of the cervical spine is being utilized with higher frequency for both stabilization and fusion purposes. Along with these posterior instrumentation procedures comes the potential for morbidity related to the misplacement of screws (Ludwig et al., 2000; Reinhold et al., 2007). Screw placement in these structures poses risk to several important anatomical and vascular structures including the spinal cord, cervical nerve roots, and the vertebral arteries (Ebraheim et al., 1999). While intraoperative sensory and MEP techniques can provide information concerning cord function and brainstem ischemia secondary to vertebral artery injury, the addition of free-run EMG, as well as stimulus-triggered EMG may serve to detect screw malpositioning and prevent individual nerve root injuries. While the utility of stimulus-triggered EMG in the cervical spine has been described in procedures involving tumor resection and nerve root identification (Guo et al., 2006), little data exist describing the use of stimulus-triggered EMG in activating lateral mass and pedicle screws for the confirmation of proper placement. In the only paper to date, Djurasovic et al. (2005) describe a technique which utilizes the same principles of current flow in intact pedicles and threshold values utilized in the lumbosacral spine in the cervical region. In their investigation, 26 patients undergoing posterior spine procedures involving lateral mass or pedicle screw instrumentation were prospectively studied to determine the correlation between screw activation thresholds and the position of the screws

EMG, REFLEX AND NERVE CONDUCTION MONITORING

407

Fig. 1. Neurotonic discharge associated with nerve root manipulation using surgical instrument. Note that activity from contralateral root is quiet during this period. [Reprinted from Jimenez et al. (2005) with permission from the American Association of Neurological Surgeons.)

in the pedicle. One hundred and forty-seven screws (122 lateral mass and 25 C-7 pedicle screws) were electrically stimulated and their thresholds for activation of muscle EMG recorded. Postoperative CT scans were obtained in each patient and screw position was independently evaluated. EMG threshold data and CT scans were compared to assess accuracy of EMG in predicting misplaced screws. The authors found that a stimulation threshold of 15 mA or greater provided a 99% positive predictive value that the screw was within the lateral mass or pedicle. Thresholds of 10–15 mA provided a 13% predictive value that the screw was positioned correctly and a threshold value of <10 mA provided a predictive value of 100% that the screw was not positioned correctly. The authors recommended that if thresholds are <10 mA then the screw should be explored,

repositioned or possibly removed. Threshold values of 10–15 mA typically represented a properly placed screw but should always be explored and thresholds of 15 mA or greater reliably predict correct screw position. While upper and lower limit threshold values are defined in this study with a high degree of confidence, future studies defining cervical screw thresholds will need to be completed in order to more accurately define what is currently a relatively large range of unpredictable thresholds. 28.4.3. Muscle group selections and electrode placement Bipolar pairs of needle electrodes should be placed subdermally over the belly of each muscle of interest. Electrodes should be placed 1 cm apart with care

408

not to scissor the electrode shafts during placement. Because upper extremity musculature receives innervation from multiple roots and that one root can innervate multiple muscles, EMG should be recorded bilaterally from multiple muscle groups to increase specificity of root activation particularly in instances where free-run EMG is being utilized and specific nerve root identification may be requested. To this end, the deltoid (C5), biceps (C5–6), extensor carpi radialis (C6), triceps and flexor carpi radialis (C7), and abductor pollicis brevis (C8, T1) muscle groups should be considered for recording in each case depending on the levels being decompressed and/or instrumented. 28.4.4. Recording and stimulation parameters CMAPs should be recorded using filter settings of 30 to 1 K (low- and high-filter settings, respectively) and a gain of 500. A time base of 25–50 ms should be utilized to properly resolve the CMAP. Monopolar stimulation should be delivered using an insulated ball-tip probe or the like with the return electrode (needle) placed in or around the site of incision. Stimulation current should be slowly increased until either a stimulus evoked EMG response is observed or a maximum current is reached. It is always advisable to use a positive control when stimulating to verify proper conduction given that the desired result is a negative one. 28.5. Thoracic spine procedures With increasing frequency, pedicle screws are being used in entirety of the thoracic spine for procedures including the correction of spinal deformities and spinal fixation for thoracic and thoracolumbar fractures (Liljenqvist et al., 1997; Masferrer et al., 1998). With an increase in utilization of instrumentation, particularly pedicle screws, comes an increase in potential morbidity with the placement of these devices to the spinal cord, spinal nerve roots, and thoracic vasculature. This morbidity may occur for a variety of reasons. First, the pedicle size, particularly diameter, is smaller in the thoracic spine than in the lumbosacral spine, the second and probably the most problematic is the variation in medial and rostral angulation that is encountered in the thoracic pedicles (Krag et al., 1988; Vaccaro et al., 1995b), and lastly, major vascular structures as well as the esophagus are at risk for screw placement that may

J.R. BALZER ET AL.

breach the anterior cortex of the vertebral body (Vaccaro et al., 1995a). Correct pedicle screw placement is of great concern in light of the high incidence of misplacement evidenced in cadaveric studies. These studies have shown that cortical breaches in thoracic pedicles during placement of screws can occur in as many as 41% of screws (Vaccaro et al., 1995a; Xu et al., 1998; Cinotti et al., 1999). Clinically, incorrect screw placement and cortical pedicle breaches generally occur at a rate of greater than 8% (Liljenqvist et al., 1997) with rates of 40% or higher being reported (Roy-Camille et al., 1996; Merloz et al., 1998). Secondary to this high level of screw misplacement and potential clinical morbidity, a variety of electrophysiological measures have been successfully utilized (SEP and MEP) (Pelosi et al., 2002; MacDonald et al., 2003) as well as nonelectrophysiological techniques (laminotomies, plain film X-rays, fluoroscopy, image-guided technology) (Kalfas et al., 1995; Odgers et al., 1996; Ferrick et al., 1997; Xu et al., 1999) with varying degrees of success have been employed to aid in the guidance and confirmation of accurate pedicle screw placement. An additional electrophysiological modality, namely stimulus evoked EMG, may further aid in the reduction of iatrogenic nerve root injury secondary to screw misplacement in the thoracic spine region. The utilization of this technique has proven quite useful in surgical procedures involving screw placement in the lumbosacral spine (Calancie et al., 1994; Rose et al., 1997). In an attempt to validate the use of this technique in the thoracic spine for screw placement, several authors have investigated the application of the stimulus evoked EMG technique in animal studies (Danesh-Clough et al., 2001; Lewis et al., 2001). In the study performed by Danesh-Clough et al. (2001), a total of 91 screws were placed in the lower thoracic pedicles (T8–12) of sheep. The group recorded stimulus evoked EMG in response to pedicle screw placement from the abdominal and intercostal musculature and found that using a stimulus evoked EMG threshold of <10 V was highly effective in predicting pedicle breaches in the lower thoracic spine. Their established threshold provided for a sensitivity of 94% and specificity of 90% and supports the transition of this technique to a clinical setting. In an additional animal study, Lewis et al. (2001) also examined the ability of stimulus evoked EMG to improve the accuracy of thoracic pedicle screw

EMG, REFLEX AND NERVE CONDUCTION MONITORING

placement by using a porcine model. They recorded from the intercostal musculature and found that the threshold values that they obtained inconsistently identified medial thoracic pedicle wall breaches and concluded that this method could not differentiate screws that were properly placed in the thoracic pedicle from screws that broke through the medial wall. Subsequent to these animal studies, a number of clinical studies have been performed in an attempt to characterize stimulus evoked EMG threshold parameters in humans and determine the clinical utility of the technique in general in instrumented thoracic spine procedures (Reidy et al., 2001; Raynor et al., 2002; Shi et al., 2003). Reidy et al. (2001) prospectively studied 17 patients while stimulating 95 thoracic pedicle screws. The technique they used was one where, prior to screw insertion after a pilot hole was created, a K-wire was placed into the pedicle and electrified and thresholds obtained. Confirmatory CT scans were performed postoperatively to establish screw positions in the thoracic pedicles. In their series, there were eight unrecognized breaches of the pedicle. The authors used a 7.0-mA threshold as their indication for a misplaced screw and found the sensitivity of EMG to be 50% in detecting breaches in pedicles with specificity at 83% using this value. Overall, pedicle screws were properly placed in 90% of patients and along with the EMG results, they concluded that the stimulus evoked EMG did not significantly improve the reliability of screw placement. In 2002, a much larger prospective investigation was performed by Raynor et al. evaluating the sensitivity of stimulus evoked thoracic EMG recording to assess proper screw placement in thoracic pedicles. These authors reviewed stimulus thresholds and screw placement in 92 consecutive patients representing a total of

409

677 thoracic screws placed. Stimulus evoked EMG was recorded (and thresholds established) from the rectus abdominis muscles in response to stimulation of every screw placed from T6 to T12. In their study, the screw demonstrating the lowest threshold value in each patient was removed and the pedicle hole was reinspected. Screws were separated into three groups: Group A had thresholds >6.0 mA and were in the pedicle (n ¼ 650), Group B had thresholds <6.0 mA but pedicles had intact medial walls (n ¼ 21), and Group C had thresholds <6.0 mA and had medial pedicle wall breaches (n ¼ 6) (Fig. 2). Overall, 3.9% (27 of 677) of all screws in this study had thresholds <6.0 mA but only 22% of those (6 of 27) had breaches in the medial wall of the pedicle. Because of a lack of absolute threshold value consensus, each screw threshold that was <6.0 mA (Groups B and C) was further compared with an overall mean threshold value calculated by averaging all the mean stimulus evoked EMG values for every screw tested within each patient. The percent decrease from this mean was calculated for each individual screw with a low (<6.0 mA) threshold. Group B screws had a mean decrease of 54% from the mean of all other screws and Group C screws had a mean decrease of 68.9% from the mean of all other screws tested. The authors concluded that thresholds >6.0 mA indicated a properly placed screw but if values fell below <6.0 mA, further assessment needed to be done. If a threshold of <6.0 mA was obtained coupled with values that were 60–65% decreased from the overall mean threshold then the surgeon should be alerted and the medial wall of the pedicle inspected. These results stand in contrast to a somewhat definitive cut-off threshold value that has been established in the lumbosacral spine and has been used with great success

Group A: >6.0 mA threshold and completely in the pedicle. Group B: <6.0 mA threshold and completely in the pedicle. Group C: <6.0 mA threshold and medial wall perforations.

A and B

C

Fig. 2. Screw placement groups used for threshold classification in the thoracic spine. [Reprinted from Raynor et al. (2002) with permission from Lippincott, Williams and Wilkins.]

410

J.R. BALZER ET AL.

for at least 10 years now. While only a small percentage (5%) of screws fell into the <6.0 mA range, it seems somewhat impractical in the intraoperative environment to one, garner a threshold for every screw tested and two, if any one screw is <6.0 mA go on to calculate an average and subsequent deviation from that number. It is possible that with further experience with thoracic screw stimulation and the establishment of thresholds, a more comparable “cut-off” value might be established. In the only other study to date evaluating EMG thresholds and thoracic pedicle screw placement, Shi et al. (2003) retrospectively reviewed their experience with the placement of 87 screws in 22 patients placed at various levels in the thoracic spine ranging from T1 to T12. In all, five screws (5.7%) were misplaced in this series as confirmed by postoperative CT scan. Six screws (6.9%) were found to have stimulation thresholds of 11 mA. An example of stimulus evoked thoracic EMG can be seen in Fig. 3. Of these six screws, three were found to breach the pedicle on postoperative CT scans. Of the 81 screws with thresholds >11 mA, 79 were found to be entirely within the bony column of the pedicle (97.5% negative predictive value). These results are more consistent with results in the lumbosacral spine both with respect to the threshold cut-off values obtained as well as the consistency of the results. Taken together, these three studies are inconsistent at best and exemplify the need for stimulus evoked EMG screw testing in many more cases as well as an accepted protocol primarily as it relates to electrode

placement and stimulus parameters used for stimulus evoked EMG recording. 28.5.1. Muscle group selections and electrode placement To effectively record stimulus evoked EMG in response to the electrification of thoracic pedicle screws, one would need to place electrodes over the intercostal muscles (T2–6), the abdominal muscles (T7–12), and in the case of T1, the flexor carpi ulnaris. In practice, these muscles and more specifically their landmarks are not always easily identified especially in morbidly obese patients. A lack of proper electrode placement secondary to an inability to palpate the muscles of interest could lead to high false-negative rates due to a general lack of sensitivity and specificity. Several published reports have recommended recording electrode montages for thoracic EMG recording. Raynor et al. (2002) suggest using bipolar needle electrodes placed in each rectus abdominis muscle lateral to the linea alba. This technique would require individual palpation and identification of individual musculature which, in most cases, would be very difficult to achieve with a high degree of specificity. Shi et al. (2003) provide a detailed description of electrode placement for recording thoracic EMG. For stimulation of screws at T2–6, they record from intercostal muscles with pairs of needle electrodes placed in the various intercostal spaces along the nipple line. For recording Left Abdominal wall Quadriceps

Evolved EMG activity

Anterior tibialis Right Abdominal wall

Quadriceps Anterior tibialis

Fig. 3. Stimulus evoked EMG recorded from bipolar pairs of needle electrodes placed over the abdominal musculature in response to lower thoracic pedicle screw stimulation. Note that activity was unilateral and in response to the side of screw stimulation. [Reprinted from Shi et al. (2003) with permission from Lippincott, Williams and Wilkins.]

EMG, REFLEX AND NERVE CONDUCTION MONITORING

411

It is always advisable to use a positive control when stimulating to verify proper conduction given that the desired result is a negative one. The threshold (intensity needed to produce a repeatable, observable response) should be recorded for each screw stimulated. 28.6. Lumbosacral spine procedures

Fig. 4. Placement of electrodes for recording stimulus evoked EMG activity in response to lower thoracic (T7–12) screw stimulation. [Reprinted from Shi et al. (2003) with permission from Lippincott, Williams and Wilkins.]

stimulus evoked EMG in response to stimulation of screws placed from T7 to T12, bipolar pairs of electrodes were placed in the abdominal musculature at a point along the nipple line that equally divides the distance between the lower margin of the tenth rib and the iliac ridge (Fig. 4). Despite a detailed description of electrode placement, the same challenges exist with regards to accurate placement of electrodes as it relates to muscle specificity. 28.5.2. Stimulation parameters Like cervical studies, thoracic stimulus evoked EMG should be recorded using filter settings of 30 to 1 K (low- and high-filter settings, respectively) and a gain of 500. A time base at least 50 ms should be utilized to properly resolve the CMAP response. Monopolar constant current stimulation (1–3 Hz, 0.2 ms duration) should be delivered using an insulated ball-tip probe or the like with the return electrode (needle) placed in or around the site of incision. Stimulation current should be slowly increased either until a stimulus evoked EMG response is observed or a maximum current is reached.

The incidence of neurological injury during the insertion of lumbar and sacral pedicle screws can range as high as 15% (Aebi et al., 1987; Whitecloud et al., 1989; West et al., 1991; Blumenthal and Gill, 1993; Esses et al., 1993; Lenke et al., 1995; Lonstein et al., 1999). The iatrogenic injury that occurs during these procedures typically results from a breach of the pedicle wall resulting in screw penetration either into the spinal canal and thecal sac or into the neural foramen potentially injuring or irritating the lumbar and sacral nerve roots. For these reasons, it is of great importance to be able to place screws with great accuracy and assay nerve root function during these procedures. Pedicle screw misplacement has been reported by several authors to occur with some frequency despite the use of various intraoperative nonelectrophysiological methods (Weinstein et al., 1988; Gertzbein and Robbins, 1990; Steinmann et al., 1993; Farber et al., 1995; Castro et al., 1996; Ferrick et al., 1997; Laine et al., 1997; Schwarzenbach et al., 1997; Isley et al., 1997; Girardi et al., 1999). To this end, several electrophysiological techniques have been utilized with report of varying degrees of success and utility. These modalities include SEPs and DSEPs. 28.6.1. SEPs during lumbosacral spine procedures SEP monitoring has been reported to have intraoperative sensitivity and utility in lumbosacral procedures in both animal (Jou, 2004; Tsai et al, 2005) and clinical studies (Balzer et al., 1998; Norcross-Nechay et al., 1999; Weiss, 2001; Krassioukov et al., 2004; Tsirikos et al., 2004). In fact, Norcross-Nechay et al. (1999) in a study reviewing 90 patients reported that acute, unilateral unresolved intraoperative evoked potential deterioration was associated with long-term ipsilateral weakness and that the findings in their report support the need for monitoring SEPs during surgery in all patients undergoing invasive lumbar surgery. Additionally, Balzer et al. (1998) reported the high sensitivity of SEP recordings in detecting injury and subsequent neurological sequelae during lumbosacral pedicle screw fusion

412

procedures. There are also reports in the literature that have downplayed the utility of SEP monitoring during lumbar fusion procedures (Gundanna et al., 2003; Gunnarsson et al., 2004). Specifically, Gundanna et al. (2003) retrospectively reviewed 186 consecutive patients and found that none of the 186 had significant SEP changes using 50% decrease in amplitude and 10% increase in latency as their alarm criteria. Despite this lack of change, there were five patients with new postoperative radiculopathies all of which were associated with malpositioned pedicle screws confirmed using postoperative CT scanning or plain film radiographs. The authors concluded that the use of SEPs in evaluating pedicle screw placement or detecting single nerve root injury was of little utility and that methods with greater sensitivity should be explored. The potential limitation to SEP recording in these cases arises because stimulation of the posterior tibial nerve results in the activation of multiple lumbosacral nerve roots and the averaged SEP response may not change significantly secondary to a single root being damaged. Despite this physiological shortcoming, we routinely make SEP recording part of our multimodality approach to monitoring of lumbosacral spine decompressions and instrumentation procedures. 28.6.2. DSEPs during lumbosacral procedures In an attempt to record evoked potentials which might reflect the function of single lumbar and sacral nerve roots, DSEPs have also been utilized intraoperatively during lumbosacral spine procedures (Aminoff et al., 1985; Herron et al., 1987; Owen et al., 1993; Toleikis et al., 1993; Tsai et al., 1997). As with SEPs, the success of DSEPs in assaying single nerve root function during lumbosacral decompression has been mixed. In two separate animal studies performed in rodents (Jou, 2004; Tsai et al., 2005), DSEPs were shown to be valuable in identifying and detecting acute single nerve root injury. In an early clinical study performed by Herron et al. (1987), the sensitivity of intraoperatively recorded DSEPs was evaluated in patients with lumbar spinal stenosis. In 30 patients, they found that DSEPs could be routinely generated and in most patients found that DSEP measurement pre- and postdecompression was helpful in assessing the adequacy of the intraoperative decompression. Likewise, Toleikis et al. (1993) studied 81 patients undergoing lumbosacral intrapedicular fixation and was able to record repeatable responses in all but one of these patients. Moreover, they found that DSEP recording was sensitive to nerve root

J.R. BALZER ET AL.

compromise but pointed out that the prediction of postoperative deficits was dependent only on the presence of the responses at the end of the procedure and not on any changes that may have occurred during the procedure. In contrast to these findings, Tsai et al. (1997) found that intraoperative DSEP recording was not only technically challenging but intraoperative changes that were observed did not correlate to the patient’s clinical outcomes. Specifically, useful DSEP recordings were obtained in only a little over 50% of the patients. These findings of inconsistency, unreliability, and insensitivity of intraoperative DSEP recordings are also in agreement with other intraoperative investigations using DSEP recording (Aminoff et al., 1985; Owen et al., 1993). Because of the reported potential shortcomings of SEP and DSEP recording for the detection of single nerve root injury during lumbosacral procedures, EMG techniques have been developed which carry a significantly higher degree of sensitivity and specificity with regard to the identification and prevention of single nerve root iatrogenic injury during these procedures. Other advantages of EMG recordings include near-real-time recordings and instantaneous intraoperative feedback as opposed to averaged responses as well as the technical ease with which the modality is implemented and interpreted. Each of these, as well as other features of both free-run and stimulus-triggered EMG during lumbosacral spine procedures will be discussed in the following sections. 28.6.3. Free-run EMG during lumbosacral spine procedures Free-run or “spontaneous” EMG recording is not a new modality to be applied in the operating room for single nerve root protection and has been used extensively for the protection of cranial nerve function during a variety of intracranial procedures. As mentioned previously, a large body of literature exists concerning the successful application of this EMG modality to procedures conducted on or near the facial nerve (Mller and Jannetta, 1985, 1987; Harner et al., 1987; Dickens and Graham, 1991). This and other literature describing facial nerve EMG monitoring has helped define the alarm criteria that we currently use in the lumbosacral spine. Alarm criteria for free-run EMG are defined by the amplitude and frequency of spontaneous firing patterns

EMG, REFLEX AND NERVE CONDUCTION MONITORING

that are observed and how this change in firing correlates with the surgical maneuver being performed at that time, for example, nerve retraction, the patient being under anesthetized, or saline irrigation in the surgical site. In the case of free-run EMG, baseline EMG (that is recorded just after electrode placement) is defined by the lack of any spontaneous activity or a “quiet” recording. The key to utilizing free-run EMG successfully in the operating room is understanding the significance of a situation where the EMG baseline is no longer quiet. Spontaneous EMG activity can occur in many forms and as a result of a number of different manipulations. Identification of nonpathological spontaneous EMG versus spontaneous EMG which has been initiated via a mechanical source (traction, stretching, compression, or manipulation) is crucial. Nonpathologic EMG firing patterns are typically characterized by small amplitude, low frequency, or isolated discharges occurring at times which do not correlate with surgical manipulation of nerve roots. In these instances, while any and all activity is made audible to surgical team, a high level of alert is not established. Pathologic spontaneous EMG activity has been described by a number of groups (Hormes and Chappuis, 1993; Owen et al., 1994; Beatty et al., 1995; Holland and Kostuik, 1997; Balzer et al., 1998; Holland, 1998; Bose et al., 2002). These studies have

413

correlated pathologic EMG activity with poor outcome or have shown that corrective measures to lessen or eliminate this type of EMG activity have resulted in patients succumbing to no new neurological deficits. In an attempt to correlate surgical maneuvers with EMG discharge, Obi et al. (1999) methodically studied the discharge of lumbar nerve roots in response to different manipulations such as tapping the root, “hard” mechanical hits, and steady tensile force with retraction. In accordance with their results, it is generally agreed that sustained and prolonged “train” EMG activity which can last for many minutes (Fig. 5) and “bursting” asynchronous EMG patterns termed neurotonic discharges (Fig. 6) both can result in iatrogenic nerve root injury and portend poor neurological outcomes. In either case, these types of EMG activities are immediately reported to the surgeon so that corrective maneuvers may be taken to alleviate whatever might be causing these types of pathological discharges or in many cases, the surgeon will be asked to stop whatever he is doing until the EMG activity normalizes to a quiet background. Unlike SEP and DSEP monitoring which require responses to be averaged and consequently take time to collect and analyze and may not be sensitive to single nerve root damage, free-run EMG is a powerful and simple tool that provides a near-instantaneous assessment of individual nerve root irritation or injury. This type of “early-warning” feedback in the

Fig. 5. Sustained high-frequency spontaneous EMG discharge noted in response to traction of the right S1 nerve root during decompression. Columns 1–3 represent the right quadriceps, anterior tibialis, and gastrocnemius muscle groups, respectively.

414

J.R. BALZER ET AL.

Fig. 6. Neurotonic high-frequency bursting in response to traction of the left L5 nerve root during placement of interbody implant.

form of free-run EMG combined with other monitoring modalities such as SEPs provides the surgical team with an extensive intraoperative understanding of the patient’s electrophysiological status during lumbosacral spine procedures. This multimodality approach, as discussed above, continues to be invaluable in the prevention of iatrogenic injury during a variety of spine procedures. 28.6.4. Stimulus evoked EMG during lumbosacral procedures As described earlier, several nonelectrophysiological methods have been employed to prevent the misplacement of lumbosacral pedicle screws and subsequent neurological morbidity. Despite these tools, their employment has had limited success based on the significant numbers of misplaced pedicle screws that have been reported in the literature. To this end, several electrophysiological techniques have been employed including SEP monitoring. While the shortcomings of some of these techniques have already been discussed, of all of the electrophysiological techniques utilized during instrumentation in the lumbosacral spine, stimulus evoked EMG has been the most accurate in evaluating and predicting correctly placed pedicle screws (Calancie et al.,

1994; Glassman et al., 1995; Lenke et al., 1995; Maguire et al., 1995; Clements et al., 1996; Darden et al., 1998; Rose et al., 1997; Welch et al., 1997; Balzer et al., 1998). It was Rosen (1991) who first described the use of electrification of the implanted pedicle screw using an improvised stimulator in humans. While Rosen did not directly measure stimulus evoked EMG activity from the lower extremities, he did use observational muscle twitching as his measure of whether or not the screw was properly placed. Calancie et al. (1992) advanced the notion of the use of pedicle screw stimulation for accurate pedicle screw placement by recording muscle EMG in response to pedicle screw stimulation using electrified instruments in a porcine animal model. The entire principle of the pedicle screw stimulation technique is based on the determination and evaluation of stimulus evoked EMG thresholds subsequent to screw stimulation. If, after screw placement, the column of bone, that is, the pedicle is intact, then a high resistance pathway (i.e., bone) will exist between the medial screw threads and the nerve root and/or the thecal sac. The existence of this high resistance pathway results in higher thresholds for EMG activation to pedicle screw hole or pedicle screw stimulation. If, on the other hand, the pedicle is breached then a

EMG, REFLEX AND NERVE CONDUCTION MONITORING

415

Fig. 7. Lumbar spine model and CT scan used to depict pedicle breach. Note that if the screw is electrically stimulated, current will pass from the screw to the surrounding neural elements rather than encountering a high-resistance barrier such as bone.

low resistance pathway allows for current flow from the screw to the root and results in low EMG activation thresholds to screw stimulation (Fig. 7). After defining the principle of using pedicle screw stimulation and evoked-EMG thresholds to identify misplaced screws, many authors, including Calancie et al. (1994) began to systematically define stimulation and recording protocols as well as the threshold ranges which defined correctly versus incorrectly placed pedicle screws in the lumbosacral spine (Glassman et al., 1995; Lenke et al., 1995; Maguire et al., 1995; Young et al., 1995; Clements et al., 1996; Rose et al., 1997; Welch et al.,

1997; Balzer et al., 1998; Darden et al., 1998; Toleikis et al., 2000; Bose et al., 2002; Leppanen, 2005). While each of these contributions have defined different methods by which to activate pedicle screws and record stimulus evoked EMG (i.e., constant current versus constant voltage, stimulator type, stimulation parameters, muscle group selection), different thresholds for defining proper versus improper placement, and different means by which to interpret these thresholds, all have promoted the use of the technique as an effective means by which to guide pedicle screw placement in the lumbosacral spine (Fig. 8).

Fig. 8. Example of stimulus evoked EMG in response to stimulation of the right L5 pedicle screw at a low (5 V) intensity. Note that activity is not only recorded from the right anterior tibialis muscle group but also from the quadriceps and hamstrings suggesting current spread to the thecal sac and a pedicle breach.

416

The first comprehensive clinical study investigating the application and evaluation of this pedicle screw stimulation technique was performed by Calancie et al. (1994) where they evaluated 102 pedicle screws placed in 18 patients. It was in this study that Calancie first described the use of constant current stimulation delivered to a variety of surgical instruments during various portions of the surgical procedure, recording of stimulus evoked EMG from multiple bilateral muscle groups, the use of a “searching” stimulus intensity (7 mA), and the recommendations that, if possible, EMG thresholds to direct nerve root stimulation be established as a baseline recording and that no pharmacological paralytic agents be used during testing (i.e., patients have four of four twitches). Calancie et al. concluded that the technique was sensitive and reliable in the detection of perforations in the pedicle wall. They went on to define that an EMG threshold of 10 mA in response to pedicle screw stimulation was consistent with screw placement which was entirely within the confines of the pedicle bone. It was after the publication of this study that other authors began to further evaluate the technique. In 1995, Maguire et al. evaluated stimulus evoked EMG in 29 patients. Using both constant current and constant voltage stimulation, EMG thresholds were determined after electrification of 95 drill bits, 144 screws and 26 exposed nerve roots. In contrast to the findings of Calancie et al., they found that an EMG threshold of >6 mA (2 standard deviations above the mean for direct lumbar nerve root stimulation) was predictive of proper screw placement with a 98% degree of sensitivity. They also state that they performed much of their screw testing under partial paralysis and state that their goal was to maintain muscle paralysis at 50% or less using a continuous infusion technique. The last pertinent point they make is that constant current seems to be less variable than constant voltage which also was an observation that Calancie et al. made in their study. In two additional publications in 1995, both of which utilized constant current stimulation, Lenke et al., in a combined animal and human study, and Glassman et al., in the largest published series at that time, again explored the application and sensitivity of the pedicle screw stimulation technique in detecting pedicle breaches during and after screw placement. Lenke et al. (1995) studied 233 pedicle screws placed in 54 patients and found that 93% of correctly placed

J.R. BALZER ET AL.

screws had stimulus evoked EMG thresholds >8.0 mA under conditions where muscle relaxant was maintained at two to three twitches using train-of-four technology. If EMG thresholds were <4.0 mA, there was a strong likelihood of a pedicle wall breach. In a prospective study, Glassman et al. (1995) studied 90 patients undergoing lumbosacral fusion procedures, testing 512 screws. These investigators utilized postoperative CT scans to document screw placement. They found that stimulation thresholds >15 mA provided a 98% confidence that the screw was within the pedicle. Stimulus evoked EMG thresholds between 10 and 15 mA were usually associated with adequately placed screws (87% confidence level) although the authors recommended screw exploration. Stimulation thresholds <10 mA were associated with pedicle breaches in most cases. Clements et al. (1996) performed a prospective study that evaluated 112 pedicle screws in 25 patients. Using a monopolar stimulator under conditions where the patient had no detectable pharmacological paralytics in their system (four of four twitches), they used constant current stimulation which was increased until a consistent EMG response was obtained or to a maximum of 30 mA current intensity. One hundred of the 112 pedicle screws that were correctly placed had stimulus evoked EMG thresholds of 11 mA. The remaining 12 screws were found to have breached the pedicle wall and all were found to have thresholds of <11 mA. In a study performed at the University of Pittsburgh by myself and colleagues (Rose et al., 1997), we prospectively studied 42 patients and 173 pedicle screw insertions using a persistently electrified pedicle stimulation technique. Rather than using intermittent stimulation with electrified instruments, this method persistently electrifies any and all instruments being used by the surgeon and provides continuous feedback via electrophysiologic monitoring during preparation for, and placement of pedicle screws. This method allows for the detection of potential pedicle breaches during the dynamic phases of the surgery such as probing of the pedicle with a probe. Stimulation was performed using constant-voltage square-wave pulses with intensities ranging from 0.5 to 150 V under conditions of no muscle relaxation. When possible, for each site that a screw was to be placed, we determined voltage thresholds for intact bone at the pedicle roof, spinal nerve epineurium, and exposed cancellous bone once the pedicle was unroofed. For bony thresholds,

EMG, REFLEX AND NERVE CONDUCTION MONITORING

stimulation intensity was set at 100 V and was gradually decreased until evoked EMG activity was no longer observed. Subsequent “search” stimuli (persistent stimulation) were set at a value between the nerve and bone thresholds. If nerve thresholds were not determined, then intensity was set 20% below the bone threshold. Using this stimulus intensity, no evoked EMG activity was observed as long as the pedicle was intact. Typically, the intensity ranged from 20 to 40 V. We found that at each site, bone thresholds were typically at least three times as high as nerve root thresholds and, consequently, persistent stimulation was set at least two times as high as nerve threshold (just under bone threshold). We recorded nerve thresholds that ranged from 3 to 15 V and bone thresholds ranging from 25 to 130 V. Using this technique, no false-positive or false-negative findings were observed in this study. In the largest series to date, Toleikis et al. (2000) performed a prospective study that included 662 patients and placement and testing of 3,409 pedicle screws. Using multiple lower extremity muscle recordings under condition of minimal muscle relaxation, the authors utilized a hand-held insulated nasopharyngeal electrode to deliver constant current stimulation to electrically activate screws. Stimulation was begun at 0.0 mA and gradually increased until a stimulus evoked EMG response was observed or up to a maximum intensity of 50 mA. Of the 3,409 screws tested, 133 screws had stimulus evoked EMG thresholds of 10 mA. After inspection, 82 of these 133 screws were not removed or redirected, 18 were redirected, and 33 removed and not replaced. Twenty-one screws had EMG thresholds of 5 mA and of these, 19 were removed. Thirty-seven screws had EMG threshold intensities between 5 and 7 mA. Of these, 22 were left in place and 15 removed. Toleikis et al. found that even when stimulation threshold intensities were between 7 and 10 mA, close inspection typically found a crack in the pedicle or one or two threads of the screw breaching the medial wall of the pedicle and over 75% of these screws were left in place without redirection. The authors conclude that EMG threshold intensities of 10 mA represent a high degree of confidence that the pedicle screw is properly placed. Under any other circumstances (i.e., <10 mA), the screw placement should be inspected and scrutinized.

417

28.6.5. Sources for discrepancies in thresholds and false negatives As evidenced from the literature review above, a final consensus concerning an EMG threshold which reflects accurate screw placement has yet to be achieved, although a very narrow range of thresholds exists for what is considered a properly placed screw and activation thresholds <6 mA almost always indicate a pedicle breach. Thresholds indicative of proper screw placement in the lumbosacral spine range from 6 to 15 mA with varying degrees of sensitivity. At first glance, one might conclude that these studies were performed using very similar protocols but this was not the case. Numerous protocol differences existed as well as intangible circumstances which also could lead to the observed differences. Protocol differences included the stimulus frequencies used (range 1–5 Hz) and pulse durations (range 50– 300 ms) utilized as well as the general stimulation technique employed including when the assay was conducted, that is, prior to and after screw placement or only after the screw was placed, or whether or not baseline nerve root thresholds were obtained. False-negative results (and some of the discrepancies noted above) can occur secondary to a variety of factors. Some technical factors influencing threshold and consequently having the potential to result in false-negative results include current shunting secondary to blood and irrigation fluid in the wound or screw pilot hole (Skelly et al., 1999), the type of screw being utilized for the fusion (Anderson et al., 2002), and the degree of muscle relaxation present when the threshold testing is performed. With regard to confirming the degree of muscle relaxation, a train-of-four test from the limb of interest as well as a positive control (i.e., stimulating the thecal sac directly and inspecting for a response) is highly recommended. A major physiological variable that can result in false-negative results pertains to the health of the nerve at the level where the screw is being placed. Several authors (Maguire et al., 1995; Holland et al., 1998) have reported abnormally high nerve root thresholds in response to direct stimulation under conditions where chronic compression was suspected. If nerve root activation thresholds are high at baseline, then stimulus evoked EMG thresholds in response to screw stimulation also can be significantly elevated. In this scenario, even modest stimulus intensities may not result in muscle EMG even

418

J.R. BALZER ET AL.

in the face of a screw that is malpositioned. A second factor that may lead to elevated thresholds and, as a consequence, the same false-negative result, is diabetes (Toleikis et al., 2000). In both cases, it is advisable to garner single nerve root thresholds whenever possible to allow for adjustment of critical threshold values dictating accurate screw placement. 28.6.6. Muscle group selections and electrode placement Bipolar pairs of needle electrodes should be placed subdermally over the belly of each muscle of interest. Electrodes should be placed 1 cm apart and affixed with tape with care not to scissor the electrode shafts during placement. We recommend never placing needle electrode through compression stockings due to a high probability of electrodes “backing-out” during the procedure. Because lower extremity musculature receives innervation from multiple roots and that one root can innervate multiple muscles, EMG should be recorded bilaterally from multiple muscle groups to increase specificity of muscle activation. As was discussed in the previous sections describing electrophysiological techniques used in the cervical and thoracic spine, muscle selection for stimulus evoked EMG recording is dictated by the levels of the lumbar and sacral spine that will be instrumented during the surgical procedure. We routinely record from five different muscle groups bilaterally utilizing the adductor magnus (L2–4), rectus femoris (L3–4), tibialis anterior (L4–5), biceps femoris (L5–S1), and triceps surae (S1–2) muscle groups (Table 1). Additional Table 1 Muscle group options for recording stimulus evoked EMG during lumbosacral fusion L2–4 L3–4 L4–5 L5–S1 S1–2 S2–5

Adductor magnus Rectus femoris* (quads: extension) Vastus lateralis (abduction) Tibialis anterior* (dorsiflexion) Biceps femoris* (hamstrings) Gluteus maximus (S1, L5) Triceps surae (gastrocnemius* and soleus: plantarflexion) Perianal musculature*

We typically (*) record from at least four muscle groups per limb and often add the perianal musculature depending on the levels being decompressed and fused.

muscle groups should be considered (e.g., anal sphincter) for recording in each case depending on the levels being decompressed and/or the potential for increased morbidity such as a redo fusion. 28.6.7. Recording and stimulation parameters CMAPs should be recorded using filter settings of 30 to 1 K (low- and high-filter settings, respectively) and a gain of 500. A time base of 50 ms should be utilized to properly resolve the lower extremity CMAP. For direct screw stimulation, monopolar, cathodal, constant voltage stimulation is delivered using an insulated ball-tip probe or the like with the return electrode (needle) placed in or around the site of incision. For direct nerve root stimulation, a Prass monopolar stimulator is utilized, also utilizing constant voltage stimulation. At our institution, we also utilize electrified instruments throughout the procedure to dynamically assess pedicle integrity prior to screw placement and detect a breach before the screw is placed. Stimulation current, delivered at a 5-Hz rate and 200 ms duration, should be slowly increased either until a stimulus evoked EMG response is observed or a maximum current is reached. It is always advisable to use a positive control when stimulating to verify proper conduction given that the desired result is a negative one. 28.7. Anesthetic considerations As had been elucidated throughout, anesthetic considerations during decompressive and fusion procedures in the cervical, thoracic, and lumbosacral spine are critical particularly as they relate to the use of pharmacological paralytic agents. Because both spontaneous and stimulus evoked EMG are essential components to the monitoring approach in all of the procedures discussed in this chapter, the level of pharmacological paralytic needs to be closely regulated and measured. Additionally, SEP recording also is an integral component to the overall multimodality neuromonitoring approach. It is our recommendation that a typical balanced anesthetic technique be utilized except for continued administration of paralytic agents. We recommend that anesthesia be induced with thiopentalsodium, etomidate, or propofol and intubation be performed under the influence of short-acting muscle relaxants such as succinylcholine or rocuronium. No further muscle relaxant should be administered until all instrumentation is complete and closure begins.

EMG, REFLEX AND NERVE CONDUCTION MONITORING

In certain circumstances, additional short-acting agents can be used to aid with exposure of the spine but should be terminated and four of four twitches should be obtainable upon commencement of decompression and/or instrumentation and any time EMG is being recorded. We, like Calancie et al. (1994), highly recommend recording EMG only when four of four twitches are obtainable because of the potential confounds with regard to amplitude and threshold determinations if the patient is partially relaxed. We would further advocate the measurement and documentation of the number of twitches generated in response to train-of-four stimulation and recommend that the train-of-four testing occurs from the limb where EMG activity is being recorded. Anesthesia should be maintained with any of the halogenated agents and nitrous oxide with care not to exceed a combined total of 1 minimum alveolar concentration (MAC). Fentanyl or propofol infusions can be used to augment the anesthetic regimen. The levels of these gases can be manipulated depending on the point in the surgery. For example, during dynamic stimulus evoked EMG testing (i.e., placement of instrumentation), we typically are not continuously collecting SEP data so an increase in volatile inhalational agents is acceptable especially in the face of our request for no muscle relaxation at this time. 28.8. Conclusions Decompression and instrumentation of the cervical, thoracic, and lumbosacral spine can carry a significant risk of neurological injury due to a variety of circumstances including vascular injury, spinal cord compression, and contusion and direct nerve root injury. Each of these iatrogenic events can result in new neurological deficits in this patient population. In an attempt to reduce the incidence of these injuries, the implementation of a multimodality intraoperative neuromonitoring approach has been adopted. The use of SEP and MEP monitoring has proven to be quite useful in reducing morbidity associated with spinal cord iatrogenic injury. Additionally, it is clear that real-time EMG monitoring in the form of free-run and stimulus evoked EMG also can prevent neurological complications. Using freerun and stimulus evoked EMG techniques in the lumbosacral spine has been shown to significantly reduce morbidity and increase the accuracy of pedicle screw placement. These same modalities are also gaining popularity in the cervical and thoracic

419

spine and hold promise for the same effective application of the technique although further studies need to be performed in order to adopt consistent threshold ranges. The utilization of the above-discussed EMG methods, along with the more traditional evoked potential techniques, in spinal decompressive and instrumented procedures offers the surgeon a comprehensive intraoperative assay of the entire neural axis. Moreover, this neuromonitoring approach to the spinal cord and spinal nerve roots has clearly resulted in decreased surgical morbidity and better outcomes in this patient population. References Aebi, M, Etter, C, Kehl, T and Thalgott, J (1987) Stabilization of the lower thoracic and lumbar spine with the internal spinal skeletal fixation system. Indications, techniques, and first results of treatment. Spine, 12: 544–551. Albee, FA (1911) Transplantation of a portion of the tibia into the spine for Pott’s disease. JAMA, 57: 885–886. Aminoff, MJ, Goodin, DS, Barbaro, NM, Weinstein, PR and Rosenblum, ML (1985) y somatosensory evoked potentials in unilateral lumbosacral radiculopathy. Ann. Neurol., 17: 171–176. Anderson, DG, Wierzbowski, LR, Schwartz, DM, Hilibrand, AS, Vaccaro, AR and Albert, TJ (2002) Pedicle screws with high electrical resistance: a potential source of error with stimulus-evoked EMG. Spine, 27: 1577–1581. Balzer, JR, Rose, RD, Welch, WC and Sclabassi, RJ (1998) Simultaneous somatosensory evoked potential and electromyographic recording during lumbosacral decompression and instrumentation. Neurosurgery, 42: 1318–1324. Beatty, RM, McGuire, P, Moroney, JM and Holladay, FP (1995) Continuous intraoperative electromyographic recording during spinal surgery. J. Neurosurg., 82: 401–405. Benecke, JE, Jr., Calder, HB and Chadwick, G (1987) Facial nerve monitoring during acoustic neuroma removal. Laryngoscope, 97: 697–700. Blumenthal, S and Gill, K (1993) Complications of the Wiltse pedicle screw fixation system. Spine, 18: 1867–1871. Bose, B, Wierzbowski, LR and Sestokas, AK (2002) Neurophysiologic monitoring of spinal nerve root function during instrumented posterior lumbar surgery. Spine, 27: 1444–1450. Bose, B, Sestokas, AK and Schwartz, DM (2004) Neurophysiological monitoring of spinal cord function during instrumented anterior cervical fusion. Spine J., 4: 202–207.

420 Calancie, B, Lebwohl, N, Madsen, P and Klose, KJ (1992) Intraoperative evoked EMG monitoring in an animal model: a new technique for evaluating pedicle screw placement. Spine, 17: 1229–1235. Calancie, B, Madsen, P and Lebwohl, N (1994) Stimulusevoked EMG monitoring during transpedicular lumbosacral spine instrumentation: initial clinical results. Spine, 19: 2780–2786. Castro, WHM, Halm, H, Jerosch, J, Malms, J, Steinbeck, J and Blasius, S (1996) Accuracy of pedicle screw placement in lumbar vertebrae. Spine, 21: 1320–1324. Cinotti, G, Gumina, S, Ripani, M and Postacchini, F (1999) Pedicle instrumentation in the thoracic spine. A morphometric and cadaveric study for placement of screws. Spine, 24: 114–119. Clements, DH, Morledge, DE, Martin, WE and Betz, RR (1996) Evoked and spontaneous electromyography to evaluate lumbosacral pedicle screw placement. Spine, 21: 600–604. Danesh-Clough, T, Taylor, P, Hodgson, B and Walton, M (2001) The use of evoked EMG in detecting misplaced thoracolumbar pedicle screws. Spine, 26: 1313–1316. Darden, BV, II, Owen, JH, Hatley, MK, Kostuik, J and Tooke, SM (1998) A comparison of impedance and electromyogram measurements in detecting the presence of pedicle wall breakthrough. Spine, 23: 256–262. Delgado, TE, Buchheit, WA, Rosenholtz, HR and Chrissian, S (1979) Intraoperative monitoring of facial muscle evoked responses obtained by intracranial stimulation of the facial nerve: a more accurate technique for facial nerve dissection. Neurosurgery, 4: 418–421. Dickens, JRE and Graham, SS (1991) A comparison of facial nerve monitoring systems in cerebellopontine angle surgery. Am. J. Otol., 12: 1–6. Djurasovic, M, Dimar, JR, II, Glassman, SD, Edmonds, HL and Carreon, LY (2005) A prospective analysis of intraoperative electromyographic monitoring of posterior cervical screw fixation. J. Spinal Disord. Tech., 18: 515–518. Ebraheim, NA, Xu, R, Stanescu, S and Yeasting, RA (1999) Anatomic relationship of the cervical nerves to the lateral masses. Am. J. Orthop., 28: 39–42. Epstein, N (2001) Anterior approaches to cervical spondylosis and ossification of the posterior longitudinal ligament: review of operative technique and assessment of 65 multilevel circumferential procedures. Surg. Neurol., 55: 313–324. Esses, S, Sachs, B and Dreyzin, V (1993) Complications associated with the technique of pedicle screw fixation. Spine, 18: 2231–2239. Fan, D, Schwartz, DM, Vaccaro, AR, Hilibrand, AS and Albert, TJ (2002) Intraoperative neurophysiologic detection of iatrogenic C5 nerve root injury during laminectomy for cervical compression myelopathy. Spine, 27: 2499–2502.

J.R. BALZER ET AL. Farber, GL, Place, HM, Mazur, RA, Jones, DEC and Damiano, TR (1995) Accuracy of pedicle screw placement in lumbar fusions by plain radiographs and computed tomography. Spine, 20: 1494–1499. Ferrick, MR, Kowalski, JM and Simmons, ED (1997) Reliability of roentgenogram evaluation of pedicle screw position. Spine, 22: 1249–1253. Gertzbein, SD and Robbins, SD (1990) Accuracy of pedicle screw placement in vivo. Spine, 15: 11–14. Girardi, FP, Cammisa, FP, Jr., Sandhu, HS and Alvarez, L (1999) The placement of lumbar pedicle screws using computerized stereotactic guidance. J. Bone Joint Surg. Br., 81: 825–829. Glassman, SD, Dimar, JR, Puno, RM, Johnson, JR, Shields, CB and Linden, RD (1995) A prospective analysis of intraoperative electromyographic monitoring of pedicle screw placement with computed tomographic scan confirmation. Spine, 20: 1375–1379. Gundanna, M, Eskenazi, M, Bendo, J, Spivak, J and Moskovich, R (2003) Somatosensory evoked potential monitoring of lumbar pedicle screw placement for in situ posterior spinal fusion. Spine J., 3: 370–376. Gunnarsson, T, Krassioukov, AV, Sarjeant, R and Fehlings, MG (2004) Real-time continuous intraoperative electromyographic and somatosensory evoked potential recordings in spinal surgery: correlation of clinical and electrophysiologic findings in a prospective, consecutive series of 213 cases. Spine, 29: 677–684. Guo, L, Quinones-Hinojosa, A, Yingling, CD and Weinstein, PR (2006) Continuous EMG recordings and intraoperative electrical stimulation for identification and protection of cervical nerve roots during foraminal tumor surgery. J. Spinal Disord. Tech., 19: 37–42. Hadra, BE (1891) Wiring the spinous processes in Pott’s diseases. Trans. Am. Orthop. Assoc., 4: 206–210. Harner, SG, Daube, JR and Ebersold, MJ (1987) Improved preservation of facial nerve function with the use of electrical monitoring during removal of acoustic neuromas. Mayo Clin. Proc., 62: 92–102. Herron, LD, Trippi, AC and Gonyeau, M (1987) Intraoperative use of dermatomal somatosensory-evoked potentials in lumbar stenosis surgery. Spine, 12: 379–383. Hibbs, RA (1911) An operation for progressive spinal deformities. N.Y. Med. J., 93: 1013–1016. Hilibrand, AS, Schwartz, DM, Sethuraman, V, Vaccaro, AR and Albert, TJ (2004) Comparison of transcranial electric motor and somatosensory evoked potential monitoring during cervical spine surgery. J. Bone Joint Surg. Am., 86(A): 1248–1253. Holland, NR (1998) Intraoperative electromyography during thoracolumbar spinal surgery. Spine, 23: 1915–1922. Holland, NR and Kostuik, JP (1997) Continuous electromyographic monitoring to detect nerve root injury during thoracolumbar scoliosis surgery. Spine, 22: 2547–2550.

EMG, REFLEX AND NERVE CONDUCTION MONITORING Holland, NR, Lukaczyk, TA, Riley, LH III and Kostuik, JP (1998) Higher electrical stimulus intensities are required to activate chronically compressed nerve roots. Implications for intraoperative electromyographic pedicle screw testing. Spine, 23: 224–227. Hormes, JT and Chappuis, JL (1993) Monitoring of lumbosacral nerve roots during spinal instrumentation. Spine, 18: 2059–2062. Isley, MR, Pearlman, RC and Wadsworth, J (1997) Recent advances in intraoperative monitoring of spinal cord function: pedicle screw stimulation techniques. Am. J. END Tech., 37: 93–126. Jimenez, JC, Sani, S, Braverman, B, Deutsch, H and Ratliff, JK (2005) Palsies of the fifth cervical nerve root after cervical decompression: prevention using continuous intraoperative electromyography monitoring. J. Neurosurg. Spine, 3: 92–97. Jou, IM (2004) The effects from lumbar nerve root transection in rats on spinal somatosensory and motor-evoked potentials. Spine, 29: 147–155. Kalfas, IH, Kormos, DW, Murphy, MA, McKenzie, RL, Barnett, GH, Bell, GR, Steiner, CP, Trimble, MB and Weisenberger, JP (1995) Application of frameless stereotaxy to pedicle screw fixation of the spine. J. Neurosurg., 83: 641–647. Khan, MH, Smith, PN, Balzer, JR, Crammond, D, Welch, WC, Gerszten, P, Sclabassi, RJ, Kang, JD and Donaldson, WF (2006) Intraoperative somatosensory evoked potential monitoring during cervical spine corpectomy surgery: experience with 508 cases. Spine, 31: E105–E113. Knoeller, SM and Seifred, C (2000) Historical perspective: history of spinal surgery. Spine, 25: 2838–2843. Krag, MH, Weaver, DL, Beynnon, BD and Haugh, LD (1988) Morphometry of the thoracic and lumbar spine related to transpedicular screw placement for surgical spinal fixation. Spine, 13: 27–32. Krassioukov, AV, Sarjeant, R, Arkia, H and Fehlings, MG (2004) Multimodality intraoperative monitoring during complex lumbosacral procedures: indications, techniques, and long-term follow-up review of 61 consecutive cases. J. Neurosurg. Spine, 1: 243–253. Laine, T, Schlenzka, D, Makitalo, K, Tallroth, K, Nolte, LP and Visarius, H (1997) Improved accuracy of pedicle screw insertion with computer-assisted surgery. A prospective clinical trial of 30 patients. Spine, 22: 1554–1558. Lenke, LG, Padberg, AM, Russo, MH, Bridwell, KH and Gelb, DE (1995) Triggered electromyographic threshold for accuracy of pedicle screw placement: an animal model and clinical correlation. Spine, 20: 1585–1591. Leppanen, RE (2005) Intraoperative monitoring of segmental spinal nerve root function with free-run and electrically-triggered electromyography and spinal cord function with reflexes and F-responses. A positions statement by the American Society of Neurophysiological Monitoring. J. Clin. Monit. Comput., 19: 437–461.

421

Lewis, SJ, Lenke, LG, Raynor, B, Long, J, Bridwell, KH and Padberg, AM (2001) Triggered electromyographic threshold for accuracy of thoracic pedicle screw placement in a porcine model. Spine, 26: 2485–2489. Liljenqvist, UR, Halm, HF and Link, TM (1997) Pedicle screw instrumentation of the thoracic spine in idiopathic scoliosis. Spine, 22: 2239–2245. Lonstein, J, Denis, F, Perra, J, Pinto, MR, Smith, MD and Perra, JH (1999) Complications associated with pedicle screws. J. Bone Joint Surg. Am., 81: 1519–1528. Ludwig, SC, Kramer, DL, Balderston, RA, Vaccaro, AR, Foley, KF and Albert, TJ (2000) Placement of pedicle screws in the human cadaveric spine: comparative accuracy of three techniques. Spine, 25: 1655–1657. MacDonald, DB, Al Zayed, Z, Khoudeir, I and Stigsby, B (2003) Monitoring scoliosis surgery with combined multiple pulse transcranial electric motor and cortical somatosensory-evoked potentials from the lower and upper extremities. Spine, 28: 194–203. Maguire, J, Wallace, S, Madiga, R, Leppanen, R and Draper, V (1995) Evaluation of intrapedicular screw position using intraoperative evoked electromyography. Spine, 20: 1068–1074. Masferrer, R, Gomez, CH, Karahalios, DG and Sonntag, VK (1998) Efficacy of pedicle screw fixation in the treatment of spinal instability and failed back surgery: a 5-year review. J. Neurosurg., 89: 371–377. Merloz, P, Tonetti, J, Pittet, L, Coulomb, M, Levallee, S and Sautot, P (1998) Pedicle screw placement using image guided techniques. Clin. Orthop., 354: 39–48. Mller, AR and Jannetta, PJ (1985) Monitoring of facial nerve function during removal of acoustic tumor. Am. J. Otol., (Suppl.): 27–29. Mller, AR and Jannetta, PJ (1987) Monitoring facial EMG responses during microvascular decompression operations for hemifacial spasm. J. Neurosurg., 66: 681–685. Norcross-Nechay, K, Mathew, T, Simmons, JW and Hadjipaviou, A (1999) Intraoperative somatosensory evoked potential findings in acute and chronic spinal canal compromise. Spine, 24: 1029–1033. Obi, T, Mochizuki, M, Isobe, K, Mizoguchi, K, Takatsu, M and Nishimura, Y (1999) Mechanically elicited nerve root discharge: mechanical irritation and waveform. Acta Neurol. Scand., 100: 185–188. Odgers, CJ, Vaccaro, AR, Pollack, ME and Cotler, JM (1996) Accuracy of pedicle screw placement with the assistance of lateral plain film radiography. J. Spinal Disord., 9: 334–338. Owen, JH, Bridwell, KH and Lenke, LG (1993) Innervation pattern of dorsal roots and their effects on the specificity of dermatomal somatosensory evoked potentials. Spine, 18: 748–754. Owen, JH, Kostuik, JP, Gornet, M, Petr, M, Skelly, J, Smoes, C, Szymanski, J, Townes, J and Wolfe, F (1994) The use of mechanically elicited electromyograms to protect

422 nerve roots during surgery for spinal degeneration. Spine, 15: 1704–1710. Pelosi, L, Lamb, J, Grevitt, M, Mehdian, SM, Webb, JK and Blumhardt, LD (2002) Combined monitoring of motor and somatosensory evoked potentials in orthopaedic spinal surgery. Clin. Neurophysiol., 113: 1082–1091. Prass, RL and Lu¨ders, H (1986) Acoustic (loudspeaker) facial electromyographic monitoring: part I. Evoked electromyographic activity during acoustic neuroma resection. Neurosurgery, 19: 392–400. Raynor, BL, Lenke, LG, Kim, Y, Hanson, DS, WilsonHolden, TJ, Bridwell, KH and Padberg, AM (2002) Can triggered electromyograph thresholds predict safe thoracic pedicle screw placement? Spine, 27: 2030–2035. Reidy, DP, Houlden, D, Nolan, PC, Kim, M and Finkelstein, JA (2001) Evaluation of electromyographic monitoring during insertion of thoracic pedicle screws. J. Bone Joint Surg. Br., 83: 1009–1014. Reinhold, M, Magerl, F, Rieger, M and Blauth, M (2007) Cervical pedicle screw placement: feasibility and accuracy of two new insertion techniques based on morphometric data. Eur. Spine J., 16: 47–56. Rose, RD, Balzer, JR, Welch, WC and Jacobs, GB (1997) Persistently electrified pedicle stimulation instruments in spinal instrumentation. Technique and protocol development. Spine, 22: 334–343. Rosen, CD (1991) Letter to the editor. Spine, 16: 599. Roy-Camille, R, Saillant, G and Mazel, C (1996) Plating of thoracic, thoracolumbar and lumbar injuries with pedicle screw plates. Orthop. Clin. North Am., 17: 147–159. Sakaura, H, Hosono, N, Mukai, Y, Ishii, T and Yoshikawa, H (2003) C5 palsy after decompression surgery for cervical myelopathy. Spine, 28: 2447–2451. Schwarzenbach, O, Berlemann, U, Jost, B, Visarius, H, Arm, E, Langlotz, F, Nolte, LP and Ozdoba, C (1997) Accuracy of computer-assisted pedicle screw placement. Spine, 22: 452–458. Shi, YB, Binette, M, Martin, WH, Pearson, JM and Hart, RA (2003) Electrical stimulation for intraoperative evaluation of thoracic pedicle screw placement. Spine, 28: 595–601. Skelly, JP, Toleikis, JR and Carlvin, AO (1999) Pedicle screw stimulation in a fluid environment. In: Proceeding of the 10th Annual Meeting of the American Society of Neurophysiological Monitoring, Denver, CO. Steinmann, JC, Herkowitz, HN, El-Kommos, H and Wesolowski, DP (1993) Spinal pedicle fixation: confirmation of an image-based technique for screw placement. Spine, 18: 1856–1861. Toleikis, JR, Carlvin, AO, Shapiro, DE and Schafer, MF (1993) The use of dermatomal evoked responses during surgical procedures that use intrapedicular fixation of the lumbosacral spine. Spine, 18: 2401–2407. Toleikis, JR, Skelly, JP, Carlvin, AO, Toleikis, SC, Bernard, TN, Burkus, JK, Burr, ME, Dorchak, JD, Goldman, MS and

J.R. BALZER ET AL. Walsh, TR (2000) The usefulness of electrical stimulation for assessing pedicle screw placements. J. Spinal Disord., 13: 283–289. Tsai, RY, Yang, RS, Nuwer, MR, Kanim, LE, Delamarter, RB and Dawson, EG (1997) Intraoperative dermatomal evoked potential monitoring fails to predict outcome from lumbar decompression surgery. Spine, 22: 1970–1975. Tsai, TM, Tsai, CL, Lin, TS, Lin, CC and Jou, IM (2005) Value of dermatomal somatosensory evoked potentials in detecting acute nerve root injury: an experimental study with special emphasis on stimulus intensity. Spine, 30: E540–E546. Tsirikos, AI, Aderinto, J, Tucker, SK and Noordeen, HH (2004) Spinal cord monitoring using intraoperative somatosensory evoked potentials for spinal trauma. J. Spinal Disord. Tech., 17: 385–394. Vaccaro, A, Rizzolo, S, Balderston, R, Allardyce, TJ, Garfin, SR, Dolinskas, C and An, HS (1995a) Placement of thoracic screws in the thoracic spine: II. An anatomical and radiographic assessment. J. Bone Joint Surg. Am., 77: 1200–1206. Vaccaro, A, Rizzolo, S, Allardyce, TJ, Ramsey, M, Salvo, J, Balderston, R and Cotler, JM (1995b) Placement of thoracic screws in the thoracic spine: I. Morphometric analysis of the thoracic vertebrae. J. Bone Joint Surg. Am., 77: 1193–1199. Weinstein, JN, Spratt, KF, Spengler, D, Brick, C and Reid, S (1988) Spinal pedicle fixation: reliability and validity of roentgenogram-based assessment and surgical factors on successful screw placement. Spine, 13: 1012–1018. Weiss, DS (2001) Spinal cord and nerve root monitoring during surgical treatment of lumbar stenosis. Clin. Orthop. Relat. Res., 384: 82–100. Welch, WC, Rose, RD, Balzer, JR and Jacobs, GB (1997) Evaluation with evoked and spontaneous electromyography during lumbar instrumentation: a prospective study. J. Neurosurg., 87: 397–402. West, J, Oglivie, J and Bradford, D (1991) Complications of the variable screw plate pedicle screw fixation. Spine, 16: 576–579. Whitecloud, T, III, Butler, J, Cohen, J and Candelora, PD (1989) Complications of the variable spinal plating system. Spine, 14: 472–476. Xu, R, Ebraheim, NA, Ou, Y and Yeasting, RA (1998) Anatomic considerations of pedicle screw placement in the thoracic spine. Roy-Camille technique versus open-lamina technique. Spine, 23: 1065–1068. Xu, R, Ebraheim, NA, Shepard, MD and Yeasting, RA (1999) Thoracic pedicle screw placement guided by computed tomographic measurements. J. Spinal Disord., 12: 222–226. Young, WF, Morledge, DE, Martin, WH and Park, KB (1995) Intraoperative stimulation of pedicle screws: a new method for verification of screw placement. Surg. Neurol., 44: 544–547.