Conventional Monitoring Modalities and Evidence for Monitoring

A subset of electrophysiologic monitoring techniques is commonly used in routine clinical practice, such as SSEPs, EMG, and MEPs. Each technique has its own advantages and disadvantages, and the choice of one or a combination of several should be carefully considered in individual patient scenarios as part of the overall surgical management plan. Monitoring plans are made in consultation with the electrophysiologist and anesthetist and should always take into consideration preoperative information such as the clinical examination, previous electrophysiologic testing, structural anatomic imaging, comorbid systemic diseases, and tolerance of the planned anesthesia. All these factors have implications on the methodology and reliability of intraoperative monitoring. The surgical site, positioning, and draping may also interfere with electrode placement and sometimes necessitate the need for combined sterile and nonsterile leads as part of the surgical planning. Increased use of intraoperative imaging such as fluoroscopy, optical tracking–based spinal navigation, and intraoperative C-arm computed tomography increases the complexity of access around the patient. The temporal flow of the preincisional period in modern complex spine surgery is therefore an integral part of the planning of intraoperative monitoring and involves consideration of the presence of the surgical team, the electrophysiologist, the anesthetist, medical imaging technicians, and other support staff. All these individuals must be organized to ensure proper electrical integrity of the monitoring modality used and to obtain appropriate postinduction baseline measurements with reproducibility and redundancy. Confidence in the electrical integrity and baseline measurements and anticipation of movement or repositioning of equipment around the patient during the operation provide the necessary environment to proceed with surgery and maximize the utility of intraoperative monitoring. With these considerations in mind, the next sections provide a focused overview of commonly used individual intraoperative monitoring techniques.

Somatosensory Evoked Potentials

Monitoring of dorsal column integrity with SSEPs is the most commonly used technique in spine surgery.¹¹ Early monitoring solely with SSEPs resulted in false-negative results in multiple reports and thus necessitated the development of multimodality monitoring.^12–14 Nevertheless, SSEPs remain a standard modality with demonstrated improved neurological outcomes in corrective procedures for spinal deformity,^4,6,7 and it serves as a building block for more complex monitoring arrangements. Multiple reports of the use of SSEPs in spine surgery, ranging from the early 1980s to current prospective studies involving larger patient populations, have demonstrated an improvement in outcome.^15–17

In SSEP monitoring, stimulation electrodes excite controlled repetitive action potentials propagating from peripheral nerves to dorsal roots, the dorsomedial tracts of the spinal cord, and eventually the contralateral sensory cortex, along which multiple recording sites may be anatomically accessible. Typically, platinum subdermal needle electrodes are used for both purposes. Stimulation sites for the upper extremity include the median and ulnar nerves. For the lower extremity, the posterior tibial nerve is a standard location. Bipolar stimulation (requiring both cathode and anode placement) is typically used. Electrode impedances are key parameters to ensure contact and reduce artifacts. Impedance values below 1 to 5 kΩ are clinically used thresholds for interelectrode or individual electrode measurements, respectively. Typical stimulation waveforms are 250-µsec-duration square-wave pulse trains at 4.7 Hz, with stimulation amplitudes ranging from 20 to 40 mA, depending on patient variables. Recording electrode positioning follows set standards, such as the international 10-20 system (Fig. 269-1).¹⁸ For example, Cv2-Fpz for the N31 subcortical response at the cervicomedullary junction and Cpz-Fpz for P37/N45 responses at the cerebral sensory cortex are commonly used. Other recording sites such as Erb’s point are also used. It is worth noting that after neural stimulation of the lower extremities (e.g., at the posterior tibial nerve), SSEP monitoring also reflects the physiologic integrity of the spinocerebellar tracts, which involve the dorsal nucleus of Clarke’s column between T1 and L2.

FIGURE 269-1 The international 10-20 system seen from the left (A) and above the head (B). A, ear lobe; C, central; F, frontal; F_p, frontal polar; P, parietal; P_g, nasopharyngeal; O, occipital. C, Location and nomenclature of the intermediate 10% electrodes, as standardized by the American Electroencephalographic Society.

(From Jasper HH. The ten-twenty electrode system of the international federation. Electroencephalogr Clin Neurophysiol. 1958;10:371-375. Reprinted with permission.)

Recorded responses require bandpass filters to remove artifacts and continuous averaging over multiple stimulation pulse trains to improve the signal-to-noise ratio. Comparison is made with the postinduction baseline measurements. The resultant changes, such as a 50% decrease in amplitude with an associated 10% increase in latency in comparison to the patient’s baseline values, prompt electrophysiologists to alert the surgical and anesthetic teams. The morphology of the recorded SSEPs may also be associated with intraoperatively detected changes in amplitude and latency, thus requiring a comprehensive assessment and evaluation by the electrophysiologist. Nonsurgical variables such as depth of anesthesia, patient temperature, and MAP have been shown to cause changes in SSEPs, and these variables must be ruled out when changes occur, or corrective actions must be taken. If surgical manipulation is determined to be the cause, timely cessation of additional manipulation, coordinated management to improve spinal cord perfusion, or initiation of intraoperative medical/steroid treatment would be required. Administration of a wake-up test may be necessary in select clinical scenarios.

Subcortical signals have lower signal amplitude and a reduced signal-to-noise ratio within a given amplifier gain and averaging setting but are more resistant to changes in depth of anesthesia. Cortical signals have larger amplitude and a higher signal-to-noise ratio and therefore tend to be more reliable electrically but are sensitive to anesthetic-induced changes. This tradeoff is illustrated in the improvement in reliability, approaching 93% to 98% in selected studies,^4,19–26 when using multiple cortical and subcortical recording sites. Recent advances in digitization equipment, filtering algorithms, and signal processing have resulted in the continued reduction of system complexity, which may potentially translate into a reduction in cost or an increase in the number of concurrently monitored channels.

The continuous recording nature and ease of setup are the main advantages of SSEPs, thus explaining its widespread use. The main disadvantage of the SSEP technique lies in the fact that only somatosensory pathways are monitored and, therefore, although SSEPs can be highly sensitive to surgically induced sensory deficits, motor deficits can be missed in a significant proportion of patients. This issue leads to the combination of additional modalities such as MEPs.

Motor Evoked Potentials

Transcranial electrical or magnetic stimulation of the cerebral motor cortex causes muscle activation, which can be detected peripherally to provide additional monitoring coverage specifically designed to detect motor deficits.^27,29 This is particularly important with intramedullary resections and in procedures with a risk for vascular compromise of the anterior spinal artery and its associated vascular territory, which can affect motor function. Initial work^28,29 paved the way for current single-pulse stimulation techniques, in which corticospinal tract recordings of direct waves (D waves) are detected, and multipulse stimulation techniques, in which short pulse trains are used to elicit CMAPs.³⁰

Typically, in patients under general anesthesia, MEP monitoring involves an electrode montage over the scalp based on the international 10-20 electroencephalography system. The standard setup uses C3-4 for upper limb MEPs and C1-2 for the lower limbs, with selection of corkscrew, needle, or cup electrodes being based on the clinical scenario and the patient’s age. Skull thickness, electrode type and impedance, and the need for depth of stimulation determine the required stimulation amplitude. Typically, selective stimulation is preferred, and therefore superficial coverage of a small cortical area is used to provide activation of a single limb. With few exceptions, D waves directly generated by the electrical activation of axons of cortical motor neurons are recorded in patients under general anesthesia. This is due, in part, to the fact that vertically oriented excitatory chains of neurons terminating on the cortical motor neuron are blocked by anesthetics, thus abolishing indirect activation of the motor neuron and the associated indirect (I) waves.¹ For D-wave recording, one or more sterile intraoperative epidural electrodes are required and placed caudally and cranially relative to the surgical lesion. Signal amplification typically requires a gain setting of 10,000× and bandpass filtering between 1.5 and 1700 Hz. Only a few traces are required for averaging, and the stimulation can be repeated at 0.5 to 2 Hz to provide real-time feedback.¹ A reduction in D-wave amplitude of greater than 50% has been shown to correlate with new postoperative motor deficits.³¹ Myogenic MEPs use a short (5 to 7) pulse train with a 4-msec interpulse interval at the transcranial electrodes. Recordings of the resultant CMAPs at sites such as the thenar muscles or tibialis anterior are amplified at 10,000× and bandpass-filtered between 1.5 and 853 Hz. Again, multitrace signal averaging is not frequently required, and real-time feedback is feasible.

Multipulse stimulation and CMAP detection provide the benefit of monitoring the corticospinal tract, in addition to the distal muscle functional units, including the neuromuscular junction. Although the muscle units provide a physiologic nonlinear amplifier for the MEPs, which improves the electrical signal-to-noise ratio, the nonlinear relationship and preoperative muscle atrophy may cloud the interpretation. Thus, myogenic MEPs are typically interpreted as an all-or-none phenomenon with regard to preoperative clinical assessment and electrophysiologic findings. When MEP amplitude is lost, immediate intraoperative intervention can result in complete reversal, which can be sensitive and specific in predicting new postsurgical motor deficits.³² In conditions in which reliable transcranially stimulated MEPs are difficult to obtain, neurogenic MEPs, obtained through percutaneous stimulation, have been investigated.^33,34 When combined with SSEP monitoring of the dorsal column, MEPs provide additional coverage for the ventral corticospinal tract and improve the efficacy of intraoperative monitoring (Fig. 269-2). The intermittent nature of MEP monitoring requires the surgeon to actively request interval measurements. These measurements are generally first taken as a baseline before surgery and then at surgical opening, during exposures approaching the spinal cord, on manipulation of the spinal cord, during different phases of intramedullary resection, and at closure. Close interaction between the neurophysiologist and the surgical team also allows periodic pauses intraoperatively to ensure that reliable recordings are being obtained throughout the procedure.

FIGURE 269-2 Schematic diagram of the functional tracts monitored by motor evoked potential (MEP) and somatosensory evoked potential (SSEP) recordings illustrating the complementary coverage of these two modalities. The anterior and lateral corticospinal tracts are primarily supplied by the anterior spinal and sulcal arteries. The dorsal columns are primarily supplied by the posterior spinal artery. The spinocerebellar tracts are supplied by penetrating branches of the arterial vasocorona. 1, Spinocerebellar tract; 2, anterior cortical spinal tract; C, cervical; L, lumbar; S, sacral; T, thoracic.

Electromyography

A third complementary modality is EMG, both in its continuous free-running form and as triggered EMG. EMG can provide additional information sensitive to surgical manipulation or compression, such as during cervical spine surgery with a risk for radicular injury, lumbosacral spine surgery, or any other procedure in which peripheral nerves. may be damaged The value of intraoperative EMG is thus particularly recognized in scenarios such as pedicle screw placement and tethered cord release. In comparison, the need for thoracic myotome monitoring has a lesser impact on clinical outcome. Myotomal selection of muscles accessible to percutaneous needle or wire electrodes or to a surface electrode in the sterile surgical field can be determined from the anatomic level and the nerve roots at risk during the operation.

Typically, free-running or spontaneous EMG activities can be recorded with paired stainless steel needle electrodes insulated to within 5 mm of the tip and transdermally inserted into the target muscle. For example, lumbosacral surgeries may use bilateral quadriceps femoris, tibialis anterior, and gastrocnemius recordings to achieve myotomal coverage from L2 to S2. Electrode impedance below 5 kΩ and interelectrode impedance below 1 kΩ ensure electrical contact. EMG recordings are obtained with a 200-µV/division gain, bandpass filtering between 10 and 5000 Hz, and analysis time of 1.5 to 2.0 seconds. When the nerve or nerve root is compressed, irritated, or injured, spontaneous firing occurs and activation of the corresponding motor unit is monitored by EMG, with different manifestations noted that may correlate with increasing degrees of injury. Morphologic descriptions such as spikes, bursts, train activity, and neurotonic discharges correspond to individual discharges, brief bundles of discharges, persistent regularly repeated discharge patterns, and persistent prolonged bursting, respectively. Intraoperative neuromonitoring units typically contain loudspeakers that convert the electrical activity into audio feedback to the surgical team in real time. A single burst of activity synchronized to surgical manipulations is useful for functional identification and correlation of specific roots and typically does not result in adverse outcomes. Sustained activity (>2 seconds) prompts the electrophysiologist to alert the surgeon to potential neurological deficits. The overall level of EMG activity may also correlate with outcome; however, further research is required to provide definitive sensitivity and specificity. It is also possible that during EMG-monitored procedures, false-positive results may be the direct result of intraoperative monitoring, with excessive EMG activity usually resulting in alteration or cessation of surgical manipulation and irritation of the nerve roots in question. Figure 269-3 shows typical intraoperative free-running EMG monitoring.

FIGURE 269-3 Free-running electromyographic recordings of the left lower limb and anal sphincter during intradural tumor resection demonstrating activity in multiple nerve roots as a result of irritation.

(Courtesy of Dr. David Houlden, Department of Surgery, University of Toronto, Canada.)

Triggered or evoked EMG activity can be obtained from suspicious tissues involving scar, tumor, filum terminale, or any tissue structure that may contain functional neural elements. Evoked EMG can assess the integrity between the interrogated tissue and the muscle units being recorded. Typical stimulation uses a bipolar probe intraoperatively that delivers monophasic square-wave pulses of 3 Hz with a duration of 100 µsec and a constant current source of less than 10 mA (generally 1 to 5 mA to elicit muscle responses). Figure 269-4 shows a disposable bipolar stimulator probe (Axon Systems, Hauppauge, NY.) designed for evoked EMG monitoring.

FIGURE 269-4 Bipolar disposable stimulator probe (Axon Systems, Hauppauge, NY) for evoked electromyographic monitoring.

Because all EMG recordings are performed simultaneously, multichannel recording units are required. Historically, for technical reasons, only limited anatomic areas can be recorded by a given set of equipment. With the advent of new low-cost digital equipment, new intraoperative monitoring units with 32, 64, or more channels are available, thus increasing the anatomic areas that can be recorded in a given surgery. Continuous EMG has high sensitivity but low specificity in predicting postoperative neurological deficits,^10,35,36 EMG is therefore typically used as part of a comprehensive monitoring regimen in most spine surgeries.

Additional Options for Comprehensive Multimodality Monitoring

Combining the aforementioned modalities may help improve the overall sensitivity and specificity of intraoperative monitoring.³⁷ A prospective analysis of a consecutive series of cases demonstrated reciprocal sensitivity and specificity of SSEPs and EMG in thoracolumbar spine surgery, where SSEPs had a sensitivity of 28.6% and a specificity of 94.7% and EMG had a sensitivity of 100% and a specificity of 23.7% for the detection of new postoperative neurological deficits.³⁸ Similarly, the combination of MEPs and SSEPs has also been investigated to address the relatively low sensitivity of SSEPs in a variety of spinal procedures.^32,39

Additional options for comprehensive monitoring can include coverage for anal or bladder sphincter function (or both), especially during microsurgical procedures involving the lumbosacral spinal canal for resection of tumor or untethering of the spinal cord. The associated risk for loss of urinary bladder or anal sphincter function, or both, can be assessed by monitoring the S2-4 spinal nerve roots. Individual^7,40 or simultaneous⁴¹ monitoring of the EAS and EUS by EMG has been investigated. Figure 269-5 shows some urinary bladder and anal sphincter electrodes (Medtronic Dantec Skovlunde, Denmark) used for EUS and EAS EMG monitoring.

FIGURE 269-5 Photographs showing urinary bladder (A) and anal (B) sphincter electrodes (Medtronic Dantec Skovlunde, Denmark). The recording electrode for bladder sphincter electromyography (EMG) is available in two sizes (outer diameters of 4.6 × 3.6 and 5.7 × 4.7 mm). The urethral electrode has two platinum leads and is mounted on a Foley catheter (no. 10 or 14). The recording electrode for anal sphincter EMG has two platinum leads separated by a sponge that is inserted into the anal canal.

(From Krassioukov AV, Sarjeant R, Arkia H, et al. Multimodality intraoperative monitoring during complex lumbosacral procedures: Indications, techniques, and long-term follow-up review of 61 consecutive cases. J Neurosurg Spine. 2004;1:243-253. Reproduced with permission.)

Insertion of a urethral ring electrode is facilitated with a two-way Foley catheter (10 or 14 French); the electrode is placed over the catheter tip in sterile fashion and positioned approximately 1 to 2 cm proximal to the inflated balloon. Insertion of an anal sphincter electrode is performed manually, similar to rectal temperature probes, except that care must be taken to ensure proper electrical contact between the electrodes and the sphincter muscle with the aid of conductive gels. EMG recording parameters for the EAS and EUS are similar to those for other continuous EMG settings.⁴¹ Figure 269-6 shows a typical intraoperative monitoring record with combined SSEP, EMG, EUS, and EAS measurements during resection of an L3 schwannoma.

FIGURE 269-6 A, Intraoperative recordings of somatosensory evoked potentials (SSEPs) and electromyographic (EMG) activity during resection of an L3 intradural/extradural schwannoma. (LL, lower limb) Well-defined baseline SSEPs were obtained, and there were no significant changes during surgery. Arrows mark the symmetrical and well-defined N31 components of SSEPs in both channels. A new postoperative L3 motor deficit, however, was observed (mild weakness of the left quadriceps muscle). Trace 1 (left) and trace 2 (right) SSEPs were recorded in the Cv2-Fpz region. Two consecutive means of 1000 stimulations are present in both channels. One of these means, obtained at the beginning of surgery (baseline recording), is saved on the screen and compared with the subsequent responses. Traces 3 to 8 represent simultaneous EMG recordings from the following six muscles: trace 3, external urethral sphincter (EUS); trace 4, external anal sphincter (EAS); trace 5, left rectus femoris (RF); trace 6, right RF; trace 7, left gastrocnemius (G); and trace 8, right gastrocnemius. B and C, Two representative recordings of intraoperative EMG activity in the left rectus femoris. Prolonged high-frequency and high-amplitude EMG activity was observed during tumor resection and nerve root manipulation.

(From Krassioukov AV, Sarjeant R, Arkia H, et al. Multimodality intraoperative monitoring during complex lumbosacral procedures: Indications, techniques, and long-term follow-up review of 61 consecutive cases. J Neurosurg Spine. 2004;1:243-253. Reproduced with permission.)

Krassioukov and colleagues demonstrated that in approximately 90% of patients, individual intraoperative EAS or EUS activities were recorded.⁴¹ In 11% of the cases (3/27 patients), simultaneous activities in both the EAS and EUS were observed. Therefore, continuous and simultaneous recordings for both the EAS and EUS are advocated to ensure proper and adequate coverage of the pudendal nerve branches (S2-4) and provide sufficient information to avoid adverse outcomes given the significant impact of sphincter dysfunction on patients.

Anesthetic Considerations

In general, intraoperative monitoring requires an increased level of finesse in anesthesia so that reliable recording and comparisons can be made. SSEP amplitude and latency are especially affected by halogenated or nitrous oxide–based agents. Myogenic MEPs require total intravenous anesthesia without neuromuscular blockade to allow CMAP monitoring. Under such conditions, close communication between the surgical and anesthetic teams during all phases of the operation becomes paramount for ensuring safety during surgical exposure and the efficacy of monitoring during peridural or intradural microsurgical maneuvers. Typically, MEP monitoring requires induction with short-lived muscle relaxants, continuous infusion of propofol and fentanyl, and nitrous oxide use kept to a low level (not exceeding 50% by volume). A single intravenous injection of rocuronium at the time of induction offers a recovery index of approximately 30 minutes less than other anesthetic agents and wears off at the time of EMG recordings. Intermittent train-of-four testing provides confirmation that a low level of neuromuscular blockade is present to allow consistent EMG monitoring. Significant changes in SSEPs can be the result of changes in body temperature and MAP, which are constantly monitored.⁴² Halogenated agents are avoided if possible.³⁰ Bolus injections of anesthetics can cause variations in the recorded waveforms, and their use needs to be minimized or coordinated with the neurophysiologist.¹

Illustrative Cases

Case Example 1
Intramedullary Tumor

A 66-year-old woman with a complex past medical history, including breast cancer and lower limb osteosarcoma, was evaluated for progressive gait difficulty over a 9-month period. Her clinical examination confirmed myelopathy. In addition, she had diffuse bilateral sensory impairment in the legs with the right side worse than the left. Serial magnetic resonance imaging suggested a T8-9 intramedullary mass measuring 1.2 cm anteroposteriorly, 1.0 cm in width, and 4.7 cm craniocaudally (Fig. 269-7). There was extensive edema within the cord above and below the lesion. On a magnetic resonance angiogram, the mass demonstrated very early arterial enhancement, homogeneous enhancement in the venous phase, and very large dilated vessels surrounding the lesion. The patient was electively booked for T7 to T9 laminectomy and microsurgical resection of this intramedullary mass. After induction and intubation, baseline intraoperative recordings of SSEPs and MEPs were obtained. As shown in Figure 269-8A to D, there was a marked difference in the SSEPs recorded from the upper and the lower extremities. Because the upper extremity SSEPs demonstrated good amplitude and the expected waveform, they provided the evidence to exclude anesthetics or other effects as the cause of the low signal amplitude in the lower extremity SSEPs. It was concluded that the intramedullary lesion had caused significant cord compression at the thoracic level and thus SSEP monitoring would be insufficient for this case. Fortunately, multichannel MEP recordings (Fig. 269-8E to H) demonstrated adequate reproducibility and redundancy for intraoperative monitoring. With MEPs for monitoring and intraoperative ultrasound for localization, complete microsurgical resection of the lesion, including both the extramedullary and intramedullary components, was carried out. There were no changes in MEPs during the operation. There were no immediate postoperative mortor deficits and no worsening of her sensory deficits. The final pathology was consistent with capillary hemangioma.

FIGURE 269-7 Preoperative magnetic resonance imaging (MRI) scans. A, T2-weighted MRI scan showing the T8-9 lesion with marked signal changes in the cord above and below the lesion. Insert, Flow voids demonstrating multiple dilated vessels in the vicinity of the mass. B, T1-weighted MRI scan with contrast enhancement showing the detailed morphology of the lesion, with extramedullary and intramedullary components causing marked cord compression. C and D, T2-weighted MRI scans with contrast enhancement illustrating the extramedullary and intramedullary extension of the lesion in the axial plane at two different cuts.

FIGURE 269-8 Intraoperative somatosensory evoked potential (SSEP) and motor evoked potential (MEP) recordings. SSEPs were recorded at Erb’s point (Erb), the cervical-medullary junction (Cv2), and the cerebral cortex (Cort). MEPs were recorded at the first dorsal interosseous (FDI) muscle and tibialis anterior (Tib Ant). A and B, Left and right upper limb (UL) SSEPs demonstrating reliable recordings. BL, baseline. C and D, Low SSEP signal amplitudes from the lower limbs (LL) bilaterally demonstrating impaired dorsal column conductivity, probably secondary to the thoracic spinal mass. E and F, Left and right UL MEPs. G and H, Left and right LL MEPs. The lack of MEP response of the left tibialis anterior was attributed to electrode placement, and the left plantar lead provided redundancy for the recording. The excellent MEP amplitude and reproducibility allowed intraoperative monitoring in this case.

Case Example 2
Adult Tethered Cord Syndrome

A 49-year-old woman with a childhood diagnosis of spinal bifida and tethered conus previously underwent L3-4 laminectomy, untethering, and duraplasty. She had symmetrical 4/5 lower extremity power, progressive spastic gait difficulties, patchy sensory loss below the level of the umbilicus, and episodic urinary and fecal incontinence. Over a 4-month period, her neurological condition worsened from using two canes to being wheelchair-bound. Magnetic resonance imaging of the thoracic and lumbar spine at 3 T demonstrated a complex intradural mass adherent to the posterior aspect of the cord at L1-2, with the cord tethering down to the L4 level. A large posterior defect in the spinal canal from T12 to L4 was present, along with a large sac at the level of the dysraphism that was herniating through the bony defect. There was also spondylolisthesis at the L5-S1 level, anomalous segmentation of the lumbosacral junction, anomalous formation of pedicles in the lower lumbar region, and a thoracolumbar scoliotic deformity (Fig. 269-9). The patient underwent L3 to S1 laminectomy, bilateral foraminotomy, durotomy, intradural lysis of adhesions, detethering of the spinal cord, and thoracolumbosacral reconstruction with instrumented fusion from T10 to the sacrum. The patient was set up for SSEP, segmental EMG, and EAS monitoring.

FIGURE 269-9 Three-tesla preoperative magnetic resonance imaging (MRI) scans with a dedicated tethered cord protocol. A-C, Representative T2-weighted MRI sagittal slices showing the large thecal sac herniating through the bony defect from T12 to L3, with the spinal cord tethered down to the L4 level. At the L1-2 level there is an intradural complex mass adherent to the posterior aspect of the cord with signal characteristics consistent with fat. The dashed lines indicate two representative axial slices shown in D and E. The spinal cord is tethered to the posterior aspect at the inferior end of the bony defect (arrow). Extensive scarring from previous surgery is seen throughout the lumbar region.

As shown in Figure 269-10A to D, there were marked differences in the SSEPs recorded from the upper and the lower extremities. The low signal amplitudes in the SSEPs from the lower limbs and the random fluctuations suggested severe sensory impairment, in accordance with this patient’s condition. The upper limb SSEPs had good amplitude and provided confidence in the monitoring technique and state of anesthesia. Segmental EMG and EAS recordings were then used for monitoring throughout the duration of this procedure. There were no changes in the upper limb SSEPs. The activity in the EMG and EAS recordings provided real-time warnings to the surgical team during the detethering and instrumentation portions of the operation. There were no perioperative neurological deficits, and the patient reported improvement in the extent of her lower limb neuropathic pain.

FIGURE 269-10 Intraoperative somatosensory evoked potentials (SSEP), electromyography (EMG), and external anal sphincter (EAS) monitoring. SSEPs were recorded at Erb’s point (Erb), the cervical-medullary junction (Cspine), and the cerebral cortex (Cort). Free-running EMGs were recorded at the bilateral hip adductors (Hip Add), quadriceps femoris (Quad), tibialis anterior (Tib Ant), gastrocnemius (Gastroc), and EAS (Bowel). A and B, Left and right (bilateral [BL]) upper limb (UL) SSEPs demonstrating reliable recordings. C and D, Low SSEP signal amplitudes and random fluctuations from the lower limbs (LL) bilaterally demonstrating impaired dorsal column conductivity. E, EMG and EAS recordings demonstrating activities in the tibialis anterior, gastrocnemius, and EAS.

Future Developments for Combining Surgical Microscopy with Intraoperative Monitoring

Modern complex spine surgeries are typically assisted with surgical microscopes. Although current intraoperative monitoring strategies rely on electrode-based stimulation and recordings, it may be possible to directly visualize the myelinated axonal structures in vivo with improved microscopy techniques, especially with the recent advances in optics and laser technology. For example, a novel imaging modality based on coherent anti-Stokes Raman scattering (CARS) has been demonstrated to be sensitive to lipid-rich cells such as adipocytes, Schwann cells, and oligodendrocytes without the use of exogenous labeling. This technique uses the C-H Raman vibration that is present in lipids by exciting it with two lasers tuned to different wavelengths such that the difference in their frequency corresponds to the frequency of the vibration. Through a vibrationally resonant third-order nonlinear process (coherent anti–Stokes Raman scattering), a photon at a higher frequency is emitted to provide lipid-based contrast. By virtue of being a nonlinear process, it benefits from three-dimensional sectioning and high spatial resolution similar to two-photon fluorescence excitation. When combined with in vivo video rate microscopy, CARS imaging (Fig. 269-11) is ideally positioned to directly visualize and quantify myelinated fibers intraoperatively. It may also be possible to directly visualize membrane depolarization and action potentials through the fibers by using advanced microscopy techniques such as second-harmonics generation (SHG) imaging. In the future, a hybrid system consisting of electrode-based stimulation and microscopy imaging–based recordings may be able to provide spatially localized functional information of the spinal cord and nerve roots to the surgical team.

FIGURE 269-11 Ex vivo myelin image of a spinal cord obtained with endogenous contrast material based on lipid Raman vibrations using a coherent anti–Stokes Raman scattering (CARS) microscope. The oligodendrocytes and the nodes of Ranvier are clearly visible. Insert, Cross-sectional image (2× zoom), which will allow direct assessment of the G ratio of myelinated fibers.

(Courtesy of Drs. Érik Bélanger and Daniel Côté, Université Laval, Centre de Recherche Université Laval Robert-Giffard, Centre for Optics, Photonics, and Lasers, Canada.)

Conclusion

Although there is extensive level II evidence to support the use of intraoperative monitoring in neurosurgery (especially spine and spinal cord surgery), level I evidence to support this technique is lacking. Moreover, it is doubtful that large prospective, randomized, blinded, and controlled trials of intraoperative monitoring will be undertaken because of logistic, ethical, and potential medicolegal concerns. Despite the lack of level I evidence to support intraoperative monitoring, we advocate multimodality neurophysiologic recordings as an adjunct to assist the surgical team during spine and spinal cord surgery. The number and types of monitoring modalities used depend on the individual patient and the experience and decision making of the electrophysiologist and the anesthesia and surgical teams.

Table 269-1 provides a summary of the respective advantages and disadvantages of the most commonly used intraoperative monitoring techniques; it is intended to serve as a quick reference on what should be monitored during a particular procedure. Multimodality intraoperative monitoring with an increasing number of simultaneously recorded channels is becoming economically and technically feasible and thus may provide the optimal monitoring environment for complex spine surgery, especially for patients with preexisting spinal cord deficits. As with any technique that involves multiple disciplines, constant communication, regular use, and attempts to fully integrate it into daily clinical practice will make intraoperative monitoring a valuable asset to spine surgery centers.