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Intraoperative spinal cord and nerve root monitoring: a hospital survey and review.

As spinal surgeries have become longer, more complex, multistage, and extensive, the need for accurate and reliable neuromonitoring during surgery has become essential. (1,2) The incidence of new, postoperative motor deficit following all types of spine surgery was found to be 0.18%, with an incidence of 0.5% for thoracic procedures. (3) There is an increased risk of postoperative neurological deficit for surgeries in conditions, such as Chiari malformations, tethered cord, and intramedullary tumors. The aim of intraoperative neuromonitoring (IOM) is to inform the surgeon of impending neurological deficit, thus allowing for change in operative strategy to reverse or, at least, limit neurological insult using corrective measures, such as adjusting operative position, wound exploration, releasing distraction, or removing instrumentation or bone graft. (4-8) It has been evidenced that deformity operations utilizing IOM with an experienced neurophysiological team have as much as 50% lower neurological deficit rates than similar operations performed without IOM. (9)

The commonly used neuromonitoring modalities include:

* Stagnara wake-up test,

* Somatosensory evoked potentials (SSEP),

* Transcranial motor evoked potentials (TcMEP),

* Spontaneous electromyography (sEMG), and

* Triggered electromyography (tEMG).

A combination of SSEP and TcMEP is considered to give an adequate feedback for spinal cord integrity, while sEMG monitors the nerve root integrity, especially in lumbosacral surgeries where the risk of nerve injury is higher. tEMG is used to assess for cortical breach in pedicle screw placement during instrumentation. IOM has shown useful in a variety of spinal procedures. (3,10-14) Procedure specific-IOM modality use is guided mainly by availability and physician or institutional preferences. (5,9) There is general consensus that multi-modal IOM is superior to any single IOM modality. (1,15-19) Because differing modalities have varying sensitivities and specificities and monitor different aspects of neural elements, developing consistent and reliable guidelines for IOM use could help optimize prevention of neurological insult during spinal surgeries.

This article serves as a review of commonly used IOM modalities with an emphasis on the technique, alarm criteria, and limitations. The study also aims to document procedure-specific practice patterns among spine surgeons.

Stagnara Wake-Up Test

The Stagnara wake-up test was one of the first modalities of IOM used to evaluate spinal cord integrity. The test is performed by gradually lightening anesthesia and asking the patient to move his or her limbs. This provides a gross assessment of primary motor cortex, anterior motor pathway, nerve root, and peripheral nerve function intraoperatively. (5) The test is negative if the patient moves his limbs on command and is positive if the patient does not. If a patient can move his or her facial muscles but not the lower extremities, the presence of global ischemic injury or cervical spine injury is possible. (5)

Because the Stagnara wake-up test requires direct patient involvement, a preoperative discussion and planning, including the patient, anesthesiologist, and surgeon, is essential. Because of the risk of self-extubation, air embolism, patient recall, self-contamination, and positional changes that result in neural compression, multiple wake-up tests are usually not performed. The test evaluates gross motor integrity at one time point during surgery, and there have been reports of postoperative motor deficits even in the presence of a negative Stagnara wake-up test. As newer IOM modalities, such as SSEP and MEP can monitor nerve root function more precisely, the Stagnara wake-up test is rarely used alone, but may be used as an adjunctive monitoring modality to SSEP or TcMEP. (5)

Somatosensory Evoked Potentials (SSEP)

Somatosensory evoked potentials were introduced in the early 1970s for scoliosis surgeries and is currently the most commonly used IOM modality. (1,15,20,21) SSEPs monitor the sensory dorsal column-medial lemniscus pathway but do not directly evaluate the motor pathway, anterior blood supply, or nerve root function. (22) Use of SSEP has been estimated to reduce the incidence of paraplegia in spinal surgeries by 60%. (23)

SSEP monitoring involves electrical stimulation caudal to the region of the spinal cord that is being assessed and the recording of these signals rostrally, usually in the scalp, to evaluate the integrity of the sensory pathway. (9) The stimulating electrodes are commonly placed over the median nerve and ulnar nerve for the upper limbs and the posterior tibial nerve and the peroneal nerve for the lower limbs (Fig. 1). The afferent impulse travels through the dorsal nerve root, dorsal column of the spinal cord, and thalamus to reach the primary somatosensory cortex. Peripheral responses can be recorded at the brachial plexus or the popliteal fossa to ascertain the threshold of stimulation for the upper and lower extremities, respectively. This technique may also be used to identify nerve compression and limb ischemia. Also, as peripheral responses are less sensitive to anesthesia, they may be used to distinguish these changes from surgical manipulation. (1)

Erb's point potential is generated when impulses are transmitted through the peripheral nerve and brachial plexus and reach the spinal cord and the upper extremity is stimulated. The N13 potential is recorded at the fiber synapse in the dorsal column nuclei. The stimuli then pass through the medial lemniscus to the thalamus and reach the primary sensory cortex generating the N20/P22 potential.

[FIGURE 1 OMITTED]

In the lower extremity, impulses are transmitted through the peripheral nerves, and the popliteal potential is generated before reaching the lumbosacral plexus and then the cauda equina N21 lumbar potential. (24) The action potentials travel rostrally through the spinal cord sensory pathway to produce the P37/P40 potentials in the cortex. It is to be noted that there is evidence that some impulses may travel in the dorsal spinocerebellar pathway in addition to the dorsal medial leminiscal pathway.

Waveform characteristics monitored intraoperatively include latency, peak-to-peak amplitude, and signal morphology. The most commonly accepted alarm criteria are 10% increase in latency or 50% decrease in amplitude. (25,26) In a recent systematic review of IOM efficacy in spinal surgeries, Fehlings and coworkers reported SSEP sensitivity and specificity ranges from 0 to 100% and 27 to 100%, respectively, from 15 included studies. (4) A positive predictive value (PPV) range of 15 to 100 and negative predictive value (NPV) range of 95 to 100 were reported from the outcomes of 9 included studies. (4) A chart review published in 2005 of 871 patients undergoing elective anterior spinal deformity surgery that found a higher PPV for patients characterized as "at risk" for spinal cord injury--23.5%--versus patients with a "normal" cord, where the PPV was found to be 11.1%. (27) While SSEPs are generally thought to have a low sensitivity, the specificity range reported by Fehlings and coworkers is larger than expected. A review in 1995 by Nuwer and colleagues included the findings of a 51,263-case survey of SSEP monitoring given to members of the Scoliosis Research Society (SRS); they found a specificity of 98%. (28) Additionally, a review by Lall and associates published in 2012 reported a specificity range of 95% to 100% and a sensitivity range of 0% to 100% from a total of seven SSEP studies. (9) Given the low sensitivity to new neurological deficits of SSEP monitoring, it is rarely recommended as a stand-alone method of IOM.

Depending on the number of sweeps, an SSEP might take seconds to several minutes to obtain. SSEP signals may be affected by halogenated agents, nitrous oxide, temperature changes, hypotension, and electrical interference. SSEP is of limited use in patients having underlying neurological disorders, including cerebral palsy and neuromuscular scoliosis.

Transcranial Motor Evoked Potentials (TcMEP)

TcMEPs involves stimulating electrodes, usually ranging from 75 to 900 V in amplitude and currents up to 0.9 A depending the electrode type used, which is placed into the scalp over the motor cortex. (29) This stimulates the corticospinal pathway, and the muscle MEP is recorded distally using needle electrodes placed over the skin of the target muscles groups (Fig. 2). (30-32) TcMEPs are useful because they evaluate integrity at the level of the cortex, corticospinal tract, and nerve roots. (9) TcMEP monitoring received FDA approval for use during spine surgeries in 2003.

Currently, four methods of interpreting muscle MEP recordings are used: (1)

1. All or nothing: most widely used; complete loss of MEP signal from a preliminary baseline is indicative of a clinically significant event. It has been suggested that this method is not sensitive enough, as subtle deficits in the corticospinal tract, which may result in postoperative motor deficits, may not be detected.

2. Amplitude criterion: 80% decrease in amplitude from baseline in one of six recordings signifying a clinically significant event.

3. Threshold criterion: Threshold stimulation increase of 100 V or more to elicit muscle MEP responses that are persistent for one or more hours and not due to systemic factors are highly correlated with postoperative motor deficits

4. Morphology criterion: Impaired motor conduction of the corticospinal tracts by tracking changes in the pattern and duration of MEP waveform morphology.

A recent prospective study of 959 patients undergoing deformity correction, ossification of the posterior longitudinal element, and spinal cord tumor resection surgeries used a more conservative TcMEP alarm threshold than is typical--70% decrease in amplitude--and found sensitivity and specificity to be 95% and 91%, respectively. (33)

[FIGURE 2 OMITTED]

The signal can also be recorded at the epidural space, which forms the direct D-wave and an indirect I-wave. The I-wave is generated from the indirect activation of the corticospinal tract via synaptic activity. Activity in the horn cells decreases under anesthesia, limiting activity in the interneurons of the spinal cord and reducing the excitatory stimuli reaching the spinal cord. I-waves are also diminished upon reaching the anterior horn cell, necessitating a sequential time-locked train of activity rather than a single stimulus. TcMEPs cannot be used in conjunction with neuromuscular blockade and are sensitive to the effects of anesthesia; therefore, concurrent use of D-wave monitoring is recommended, as D-waves are not affected by anesthesia. (34-39) A complete loss of D-wave amplitude during surgery is likely to result in permanent motor deficit, while a complete loss of MEP with 50% preservation of D-wave amplitude generally results in transient paraplegia.

TcMEPs do not allow for continuous monitoring but can measure corticospinal integrity immediately after a high-risk maneuver has been performed. (9) TcMEPs might result in lip or tongue biting and intraoperative awareness because of the minimal anesthesia requirements. (40) TcMEPs also have a theoretical risk of seizures, although none have been reported presently. (9) TcMEPs are contraindicated in patients with deep brain stimulators or cochlear implants. Success rates for obtaining MEPs are about 94.8% and 66.6% in the upper and lower extremities, respectively; however, in the face of existing motor deficits, the ability to obtain MEPs in the lower extremities decreases significantly to 39%. (41-42)

Analyzing the results of six studies of TcMEP use in spine surgeries, Fehlings and coworkers reported a range from 81 to 100% for both sensitivity and specificity. (4) Similarly, Lall and colleagues reported a sensitivity range of 75% to 100% and a specificity range of 84% to 100% from the results of seven studies. (9) From three studies reporting the PPV and NPV of TcMEP recording, Fehlings and coworkers found ranges of 17 to 96 and 97 to 100 for PPV and NPV, respectively. (4)

Spontaneous Electromyography (sEMG)

Spontaneous Electromyography provides a real-time intraoperative assessment of nerve root and cauda equina integrity. sEMG constantly monitors the peripheral nerves of specific muscles via electrodes placed in the muscles of interest. (43,44) While specific muscles are theoretically paired with one nerve root, it is recommended that at least two muscles be monitored in assessment of any particular nerve root as differences in myotome distribution may occur. (1) While sEMGs are highly sensitive for nerve root injury, they lack specificity and have a relatively high false-positive rate. The presence of sEMG activity indicates manipulation of nerve root and risk of injury, while its absence is indicative of no injury. However, in the circumstance of complete transection, further sEMG would obviously be absent.

While specific warning criteria for sEMG recordings have not been defined, spikes, bursts, and trains (continuous and repetitive EMG firing while force is being applied to the nerve root) of activity are considered significant (Fig. 3).i-45'49 Bursts of activity often correspond to a nerve root being over manipulated and impinged upon, while trains of activity usually indicate a more severe manipulation or stretching of a nerve root; loss of sEMG signal generally represents a severed nerve. (5) Because sEMG recording requires prolonged signal averaging, it is possible that by the time nerve root damage becomes apparent, irreversible damage may already have occurred. (5)

Paralytic agents in anesthesia cannot be used with sEMG monitoring. sEMG signals may be similarly affected by cautery devices, electrocardiography leads, high-speed drills, neurological disorders, myasthenia gravis, botulinum toxin treatment for dystonia, muscular dystrophy, and temperature changes. (1,50,51)

sEMG recording is typically found to have a high sensitivity for nerve root injuries but a low specificity overall. (9) In a prospective study of 427 patients undergoing cervical spine surgery, sEMG was found to have a sensitivity of 46%, a specificity of 73%, a PPV of 3%, and an NPV of 97%. (52) These results are similar to the findings of a retrospective study from 2009 by Quraishi and associates, where sEMG recordings were analyzed from 89 adult deformity surgeries. (53) Quraishi and associates reported a sensitivity of 66%, a specificity of 65%, a PPV of 6%, and an NPV of 98%. (53) The results of another retrospective study of 213 various thoracolumbar surgeries were slightly better: sEMG sensitivities and specificities of 100% and 23.7%, respectively, a PPV of 8.5%, and an NPV of 100% were reported. (54)

[FIGURE 3 OMITTED]

Triggered Electromyography (tEMG)

Triggered electromyography monitoring is used during pedicle screw placement procedures and other minimally invasive procedures with ambiguous anatomical landmarks. (9) tEMG monitoring checks for medial pedicle wall breaches and has high sensitivity but low specificity.

A monopolar electrode is often used to stimulate the top of the inserted pedicle screw rather than at the tulip at increasing currents with needle electrodes measuring action potentials in the corresponding muscle groups (Fig. 4). An initial stimulus greater than 2 mA is given to ensure a muscle response. If a screw is accurately placed in the pedicle, the surrounding bone will act as an insulator to the applied stimulus. A stimuli of lesser current would be adequate in cases of medial wall breach. When the threshold stimulus is greater than 20 mA, there is strong probability that there is no breach. (55,56) A response between 10 to 20 mA gives reasonable probability of no breach, with threshold >15 mA indicating a 98% likelihood of accurate screw positioning. (57)

A threshold of >10 mA for screw stimulation and 7 mA for probe stimulation suggest a medial wall breach in lumbar pedicles. (58,59) Alarm criteria for different regions are different. (1,29,60) False-negatives may occur due to use of muscle relaxants, current spread, and pre-existing nerve injury. Neuromuscular blockades may be used during tEMG recording, with an increase in stimulation thresholds occurring. (61,62)

[FIGURE 4 OMITTED]

The high-false negative rate of tEMGs is attributed to screw stimulation, which can occur during surgery in a variety of ways. Fluid in the wound (such as from excessive bleeding), something other than the screw being stimulated, or the patient being pharmacologically paralyzed all increase the likelihood of a false-negative reading. (14) While tEMG is highly sensitive for medial pedicle breaches, the modality cannot differentiate between medial cortical breach and complete invasion of the canal. (61)

In a recent retrospective study of 2,450 consecutive lumbar pedicle screw placements in 418 patients, tEMG was found to have a specificity of 99.9% and sensitivity of 43.4% when a stimulation threshold of 5.0 mA was used. (63) When the stimulation threshold was increased to 10 mA, the resulting specificity and sensitivity were 95.9% and 69.9%, respectively. (63) Thus, the investigators concluded that tEMG is of limited use as a screening tool for pedicle screw breaches, but may be useful as a warning tool for screw malpositioning when used in conjunction with other IOM modalities. (63) Additionally, a prospective study of 17 patients undergoing thoracic pedicle screw fixation surgeries reported a sensitivity of 50% and a specificity of 83% when tEMGs were recorded from intercostal (upper thoracic track) and rectus abdominus (lower thoracic track) muscles. (64)

F-Wave and H-Reflex

Monitoring F-wave and H-reflexes intraoperatively provides a means of assessing complex spinal cord function and activity in ascending, descending, and spinal interneurons, and the dorsal and ventral nerve roots. (65) While several investigators have noted the clinical usefulness of F-wave and H-reflex monitoring, the technique is not yet common clinical practice. (34,65)

When a peripheral nerve is stimulated, an F-wave is produced with action potentials travelling both toward the periphery--forming the compound muscle action potential (CMAP)--and towards the spinal cord and anterior horn cell. When impulses reach the anterior horn cell, an impulse is sometimes fired back towards the periphery, producing the F-wave. (34) The H-reflex is produced when action potentials that travel proximally along sensory fibers enter the dorsal horn of the spinal cord and stimulate the monosynaptic reflex arc (Fig. 5). (34) While H-reflex and F-wave monitoring offers a highly sensitive means of assessing the function of the spinal cord and proximal nerve roots in real-time with little to no patient movement, the technique is highly sensitive to anesthetic effects (Fig. 6). (65)

Multimodal Intraoperative Monitoring

In order to improve the sensitivity, specificity, and overall efficacy of monitoring, surgeons frequently use more than one IOM modality for a single procedure. Multimodal intraoperative monitoring (MIOM) allows sensitivity and specificity to approach 100%, and for the simultaneous assessment of ascending and descending pathways, thus increasing the likelihood that neurological injury will be detected. (5,9) There are no studies comparing unimodal to multimodal IOM techniques.

Combining SSEp and TcMEP has been the standard of care in adult deformity surgeries, as combining increases the sensitivity, specificity, and NPV of monitoring. (4,66-70) Using both SSEP and MEP monitoring is thought to reduce the incidence of false-negatives that would occur when using SSEP, since SSEP monitors only the dorsal column sensory pathway, leaving the anterior and posterior spinal cord susceptible to injury. (71-74) There is also evidence that the anterior spinal cord may be especially vulnerable in the setting of ischemic injury, further necessitating adjunctive monitoring. (75-77) In a review of 11 retrospective studies assessing the efficacy of MIOM during spine surgery--all of which included MEP and nine of which combined SSEP and MEP--an overall sensitivity of 70% to 100%, specificity of 52.7% to 100%, PPV of 5.2% to 100%, and NPV of 96% to 100% were found. (4) It should be noted that the investigators believe the wide specificity range is due to the presence of an outlier, as 9 of the 11 studies reported specificity at or above 90%. (4)

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

While combined SSEP and MEP monitoring is considered effective, there are multiple case reports of false-negatives occurring in the literature; these false-negatives have been attributed to human and equipment failure, delayed-onset injury, injury to unmonitored neural elements, and minor spinal cord injury. (71,78-80) However, a case of a true-positive loss of SSEP signal with preservation of MEP signals during a scoliosis surgery was just reported for the first time, recently published ahead of print in Spine. (66) This case further highlights the importance of MIOM, as it documents the occurrence of selective posterior spinal cord dysfunction without injury to the anterior columns, which would have gone unnoticed with MEP monitoring alone. (66)

While MIOM is regularly used by spine surgeons to improve patient safety, there are currently no procedure-specific MIOM guidelines. At our hospital, 22 spine surgeons were queried on practice patterns of use of intraoperative monitoring for three deformity procedures and 21 non-deformity procedures, after institutional review board approval. The surgeons operated at two hospitals and had access to all four modalities: SSEP, TcMEP, sEMG, and tEMG performed by an experienced neurophysiologist. All surgeons were fellowship-trained in spine surgery with experience varying from 3 to 29 years, with an average of 14.4 years of surgical experience. Of the 18 (81%) responses received, 15 were from orthopaedic surgeons and three from neurosurgeons.

Deformity Surgery

For both cervical and thoracic deformity surgeries, all surgeons used at least SSEP and TcMEP. For cervical surgeries, 47% of surgeons additionally used sEMG, while for thoracic 71% of surgeons additionally used sEMG and tEMG. Most surgeons (44%) used all four modalities for lumbar deformity surgery (Table 1).

Non-Deformity Surgery

For patients having radiculopathy undergoing ACDF, SSEP alone was utilized by 29%. However, in patients undergoing ACDF with symptoms of myelopathy, most surgeons (31%) used SSEP and TcMEP with only 13% using SSEP only. Forty-six percent of surgeons utilized SSEP, TcMEP, and sEMG for cervical arthroplasty procedures. SSEP, TcMEP, and sEMG were most commonly used for posterior cervical laminoforaminotomy, posterior cervical laminectomy, and posterior cervical laminectomy with fusion and instrumentation, and posterior cervical laminoplasty procedures (Table 2).

For both anterior and posterior thoracic decompression, most surgeons preferred to use SSEP and TcMEP. If these procedures involved fusion with or without instrumentation, all four modalities were used most frequently (Table 3).

None of the surgeons used any IOM technique for lumbar laminectomy and laminotomy or microdiscectomies. Thirty-five percent surgeons used SSEP and tEMG when doing posterior spinal fusions with instrumentation. SSEP alone was used by 45% and 36% for lumbar arthroplasty and anterior lumbar interbody fusion (ALIF) procedures, respectively. For transforaminal interbody fusion (TLIF), posterior lumbar interbody fusions, and thoracolumbar interbody fusions four-modality combination were most commonly used. Most surgeons (60%) did not use IOM for kypoplasty procedures (Table 4).

The practice patterns demonstrate that the majority of surgeons surveyed use multi-modality monitoring during most surgeries; however, there is no consensus between the surgeons surveyed on MIOM modality combination to be used for specific procedures. This is the first study to document practice patterns on MIOM combination use in for various spine surgery procedures. A study by Margit and colleagues surveyed 180 surgeons who attended the Disorders of the Spine meeting in Whistler, BC, in 2004 and concluded that SSEP was the most preferred modality for both anterior and posterior cervical surgery. Only 70% preferred using some IOM for anterior thoracic or thoracolumbar procedure, while 55% preferred IOM for posterior thoracic or thoracolumbar cases. However, MIOM combination was not reported in the study. Though the investigators did not find any differences in IOM use between orthopaedic surgeons and neurosurgeons, they did find that surgeons who were fellowship trained were more likely to use IOM than those who were not. (81) The results differ from the results of a similar survey study conducted in Canada in 2010, which found that only 4.3% of 95 surgeons believed that monitoring should be the standard of care, with 20% claiming that IOM is never necessary. (23) All surgeons surveyed at our hospital reported IOM use for all spinal procedures except lumbar laminectomy and laminotomy or microdiscectomies. However, the differences between our study and the Canadian study may be attributable to different availabilities of IOM between the two groups.

While not suggesting specific guidelines, Lall and colleagues recommend using the following preoperative checklist when deciding which IOM modalities to use for a certain procedure:

1. What monitoring modalities are most appropriate for the case at hand? What types of neurological deficits are most likely?

2. What anesthesia protocol will optimize acquisition of neuromonitoring signals? Is total intravenous anesthesia indicated? Can paralytics be used?

3. What alarm criteria will be used for each monitoring modality?

4. What actions will be taken in the setting of a positive alarm?

5. Are new techniques involved? How will they be implemented? (9)

Cost-Effectiveness

As MIOM use has yet to be well supported by high-level prospective studies, there is controversy over whether IOM use has actually affected neurological complication rates during spinal surgeries. (9-82-83) However, many surgeons still feel that IOM is essential, especially during high risk cases, such as cervical procedures, revision cases, and instrumentation and fusion surgeries, which may benefit from sEMG or tEMG monitoring of nerve root function. (84-88) In 1997, the results of Nuwer and colleagues' survey of the SRS were compared with SRS surgical data prior to the routine use of SSEP monitoring. (28,89,90) In their comparison, the investigators estimated that the cost-per-deficit-averted is $120,000--assuming that IOM prevents one deficit per 200 surgeries--significantly less than is spent on healthcare on average during the first year for a newly paraplegic patient. (90)

In a 2012 retrospective analysis of 720 consecutive surgical decompression surgeries performed without the use of IONM, Traynelis and colleagues reported no new postoperative neurological deficits. (91) Traynelis and associates then estimated that per 4-hour surgery, the cost of SSEP monitoring is $941.82, of TcMEP monitoring $1,114.77, and of combined SSEP and TcMEP monitoring $1,423.27, thus by not monitoring the previous 720 cases, over one-million dollars were saved. (91) Regardless, the use of combined SSEP and TcMEP monitoring during cervical decompressions and fusions is still widely supported in the literature. (92)

While MIOM is currently assumed to be cost-effective for thoracic and cervical procedures, IOM use during lumbar surgeries is more controversial, as lumbar surgeries tend to be at lower risk. (9)

Although there is evidence that IOM is useful in preventing neurological deficits following spine surgeries, there are currently no set guidelines for its use. This article aims to examine the IOM practice habits of a sample of both orthopaedic and neurosurgeons at a single-center tertiary hospital.

Conclusion

While IOM is regularly used by spine surgeons to improve patient safety and to allow for more aggressive maneuvers, IOM use is currently dictated by surgeon preferences and technical availability. (5) In order to further improve patient safety, additional procedure-specific research on the efficacy of individual IOM modalities and MIOM is required, with the subsequent establishment of IOM guidelines.

Caption: Figure 1 Schematic diagram representing somatosensory evoked potential (SSEP). The stimulating electrodes are commonly placed over the median nerve and ulnar nerve for the upper limbs and the posterior tibial nerve and the peroneal nerve for the lower limbs. The afferent impulse travels through the dorsal nerve root, dorsal column of the spinal cord, and thalamus to reach the primary somatosensory cortex. Peripheral responses can be recorded at the brachial plexus or the popliteal fossa to ascertain the threshold of stimulation for the upper and lower extremities, respectively.

Caption: Figure 2 TcMEPs involves stimulating electrodes placed into the scalp over the motor cortex and recording the muscle MEP distally using needle electrodes placed over the skin of the target muscles groups.

Caption: Figure 3 Spikes (A), bursts (B), and trains (C) represent continuous and repetitive EMG firing while force is being applied to the nerve root and signify impending injury.

Caption: Figure 4 Schematic representation of triggered EMG. A monopolar electrode is used to stimulate the top of the inserted pedicle screw with needle electrodes measuring action potentials in the corresponding muscle groups.

Caption: Figure 5 Schematic representation of F-wave recording. Peripheral nerve stimulation produces potentials traveling towards the periphery (M-wave) and towards the spinal cord. When impulses reach the anterior horn cell, an impulse is sometimes fired back toward the periphery, forming the F-wave.

Caption: Figure 6 Schematic representation of H-wave recording. The H-reflex is produced when action potentials traveling proximally along sensory fibers enter the dorsal horn of the spinal cord, stimulating the monosynaptic reflex arc.

Disclosure Statement

None of the authors have a financial or proprietary interest in the subject matter or materials discussed, including, but not limited to, employment, consultancies, stock ownership, honoraria, and paid expert testimony.

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Rachel N. Rattenni, B.A., Thomas Cheriyan, M.D., Alexandra Lee, R.N., John A. Bendo, M.D., Thomas J. Errico, M.D., and Jeffrey A. Goldstein, M.D.

Rachel N. Rattenni, B.A., Thomas Cheriyan, M.D., Alexandra Lee, R.N., John A. Bendo, M.D., Thomas J. Errico, M.D., and Jeffrey A. Goldstein, M.D., Division of Spine Surgery, NYU Hospital for Joint Diseases, New York University, Langone Medical Center, New York, New York.

Correspondence: Thomas Cheriyan, M.D., 306 East 15th Street, New York, New York 10003; thomascheriyan@gmail.com.
Table 1 Distribution of IOM Modalities
Used for Deformity Surgery *

Surgery             SSEP
                    TcMEP   SSEP    SSEP    SSEP
                    sEMG    TcMEP   TcMEP   sEMG
                    tEMG    sEMG    tEMG    tEMG

Cervical fusion/     38      50      --      --
  instrumentation
Thoracic fusion/     71      12      12      --
  instrumentation
Lumbar fusion/       47       0      12      12
  instrumentation

Surgery

                    SSEP    SSEP   SSEP   sEMG
                    TcMEP   sEMG   tEMG   tEMG

Cervical fusion/     13      --     --     --
  instrumentation
Thoracic fusion/      6      --     --     --
  instrumentation
Lumbar fusion/        6      6      12     6
  instrumentation

* Values represent percentage of surgeons.

Table 2 Distribution of IOM Modalities Used
for Cervical Non-Deformity Surgery *

Surgery                               SSEP
                                      TcMEP   SSEP
                                      sEMG    TcMEP   SSEP    SSEP
                        None   SSEP   tEMG    sEMG    TcMEP   sEMG

ACDF (radiculopathy)     6      25     19      25      13      13
ACDF (myelopathy)        --     13     20      27      27      13
TDR                      --     23     15      46      --      15
Posterior cervical       27     13     20      27      --      13
  laminoforaminotomy
Posterior cervical       7      20     27      27      13      7
  laminectomy
Posterior cervical       --     --     20      47      13      20
  laminectomy/fusion/
  instrumentation
Posterior cervical       --     17     17      33      17      17
  laminoplasty

* Values represent percentage of surgeons.

Table 3 Distribution of IOM Modalities Used
for Thoracic Non-Deformity Surgery *

Surgery                     SSEP
                            TcMEP   SSEP    SSEP
                            sEMG    TcMEP   TcMEP
                     SSEP   tEMG    sEMG    tEMG

Anterior thoracic     --     27      27      --
  decompression
Anterior thoracic
  decompression/
  fusion
with or without       --     33      20      --
  instrumentation
Posterior thoracic    12     24      24      --
  decompression
Posterior thoracic
  decompression/
  fusion/             --     53      --      24
  instrumentation

Surgery

                     SSEP    SSEP   SSE
                     TcMEP   sEMG   tEMG

Anterior thoracic     40      7      --
  decompression
Anterior thoracic
  decompression/
  fusion
with or without       40      7      --
  instrumentation
Posterior thoracic    41      --     --
  decompression
Posterior thoracic
  decompression/
  fusion/             18      --     6
  instrumentation

* Values represent percentage of surgeons.

Table 4 Distribution of IOM Modalities Used
for Lumbar Non-Deformity Surgery *

Surgery                                            SSEP
                                                   TcMEP   SSEP
                                                   sEMG    TcMEP
                       None   SSEP   sEMG   tEMG   tEMG    sEMG

Lumbar laminotomy/     100     --     --     --     --      --
  microdiscectomy
Lumbar laminectomy     100     --     --     --     --      --
Posterior spinal        6      --     --     12     12      --
  fusion/
  instrumentation
TDR                     18     45     9      --      9      --
ALIF                    21     36     14     --      7      --
TLIF/PLIF               6      --     --     6      25       6
XLIF                    --     8      --     --      8      25
Thoracolumbar fusion    --     13     --     --     47      13
Posterior thoracic/     62     --     --     --     15       8
  lumbar kyphoplasty

Surgery
                       SSEP    SSEP
                       TcMEP   sEMG   SSEP    SSEP   SSEP   sEMG
                       tEMG    tEMG   TcMEP   sEMG   tEMG   tEMG

Lumbar laminotomy/      --      --     --      --     --     --
  microdiscectomy
Lumbar laminectomy      --      --     --      --     --     --
Posterior spinal        --      18     --      6      35     12
  fusion/
  instrumentation
TDR                     --      --     --      18     --     --
ALIF                    --      7      --      14     --     --
TLIF/PLIF               --      13     --      --     25     19
XLIF                    --      33     --      8      8      8
Thoracolumbar fusion     7      13      7      --     --     --
Posterior thoracic/     --      --      8      8      --     --
  lumbar kyphoplasty

* Values represent percentage of surgeons.
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Author:Rattenni, Rachel N.; Cheriyan, Thomas; Lee, Alexandra; Bendo, John A.; Errico, Thomas J.; Goldstein,
Publication:Bulletin of the NYU Hospital for Joint Diseases
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Geographic Code:1USA
Date:Jan 1, 2015
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