Intraoperative Neurophysiologic Monitoring of the Spine

CHAPTER 14 Intraoperative Neurophysiologic Monitoring of the Spine

The primary objective in intraoperative neurophysiologic monitoring is to identify and prevent the development of a new neurologic deficit or worsening of a preexisting neurologic injury in a patient who is undergoing surgery. The aim of most spinal cord monitoring is to prevent intraoperative injury that results in irreversible paraplegia or quadriplegia. Because a neurologic examination cannot be performed in an anesthetized patient, intraoperative neurophysiologic monitoring is used to determine the patient’s neurologic status during surgery. By evaluating the responses that are produced by the patient’s nervous system to various stimulations, the integrity of that neural pathway can be monitored.

These recordings are started before surgery, referred to as baseline recordings, and continued throughout the surgery. Any significant changes or fluctuations from these baseline values are used to determine whether significant neurologic injury has occurred. In using this strategy, the patient’s own response serves as the control for the detection of any abnormalities that may occur during the surgery. The term significant change is used to refer to the degree of changes seen in the neurophysiologic recordings. Changes that are termed significant have been shown to correlate well with intraoperative injury to the nervous system.

It is also possible that some of these significant changes may arise from other changes in physiologic parameters, anesthetic parameters, or technical issues. It is the responsibility of the intraoperative neurophysiologic monitoring team to determine whether or not the significant changes noted in the neurophysiologic responses are truly related to the surgical procedure at hand. The challenge to the intraoperative neurophysiologist and the monitoring team is to alert the surgeon of these changes as early as possible so that changes can be instituted to reverse the electrophysiologic changes and to avert neurologic catastrophe.

Key to the success of intraoperative neurophysiologic monitoring is a good understanding of the capabilities and limitations of the neurophysiologic tests being monitored. These limitations should be understood not only by the intraoperative neurophysiologist, but also by the anesthesiologist and surgeon. For seamless integration of intraoperative neurophysiologic monitoring into the intraoperative team, it is imperative that a good established working relationship eventually develops between the intraoperative neurophysiology team, anesthesiologist, and surgeon. This relationship allows for rapid communication between teams and a quick resolution of issues, optimizing the benefits of intraoperative neurophysiologic monitoring for the patient.

One of the first issues to address when planning for intraoperative neurophysiologic monitoring is to determine the types of neurophysiologic tests to perform on a particular patient undergoing surgery. This decision is made based on an understanding of the type of surgery the patient is to undergo, the types of intraoperative injuries that may occur, and the mechanisms of how these injuries occur in surgery. By planning ahead with these issues in mind, the team can also attempt to anticipate the type of changes that might be expected to occur and the risky periods during surgery when these changes would likely occur. Ideally, the team would prospectively plan for interventions to reduce intraoperative neurologic injury.

Intraoperative Monitoring of the Spinal Cord

Somatosensory-evoked potential (SEP) monitoring has been used for many years to monitor spinal function intraoperatively during various surgeries involving the spine, such as corrective surgery for scoliosis or other congenital deformities and removal of intraspinal tumors or arteriovenous malformations. SEP monitoring has been shown to reduce the incidence of neurologic damage in large studies of experienced monitoring teams.1 SEPs monitor only sensory transmission through the dorsal column pathways, however; SEPs do not provide a direct measure of motor function. In addition, the dorsal columns receive their blood supply from the posterior spinal arteries, whereas the anterior spinal arteries supply the motor pathways. Ischemic damage to the spinal cord from the anterior spinal artery may be undetectable with SEP monitoring.2,3

A significant change in SEP monitoring might mandate further assessment of the patient’s motor function by waking the patient up during surgery to evaluate leg and arm motor function (this has been called the “wake-up” test). The disadvantages of this strategy include the lack of real-time intraoperative motor function assessment and the anesthesia risks associated with performing the “wake-up” test. An alternative is monitoring of the motor pathway through the recording of motor-evoked potentials (MEPs).

MEPs are a more direct technique that evaluates the motor pathway. Monitoring has been previously performed by relying on stimulation of the spinal cord directly.4 Spinal cord stimulation can be done with the use of epidural electrodes inserted after a laminectomy has been performed or by percutaneous intraspinous needle electrodes. The epidural electrodes are invasive and often require placement by a skilled anesthesiologist. Percutaneous intraspinous needles are difficult to place accurately and may not achieve adequate or consistent stimulation of the spinal cord. In addition, there is the question of whether MEPs generated through spinal cord stimulation arise solely from propagation through the motor pathway or if multiple pathways are involved in their generation.5,6 There are reports of MEP monitoring in which spinal cord stimulation resulted in no significant intraoperative changes despite postoperative neurologic motor deficits (so-called false-negative result).7 It is now believed that motor cortex stimulation with transcranial electrical stimulation provides a more reliable method for monitoring of the motor pathways. This technique is now routinely used in spinal cord monitoring together with SEPs.

Somatosensory-Evoked Potential Monitoring

The use of SEPs in intraoperative monitoring of complex spine surgeries began in the early 1970s.8 Although SEP monitoring evaluates primarily the integrity of the posterior columns, it is often used to give an overall assessment of the spinal cord based on the assumption that many intraoperative mechanisms of injury would affect the spinal cord diffusely. An example of such an injury is spine distraction during scoliosis surgery. In addition, ischemic injury may initially result in a more diffuse dysfunction of the spinal cord that could be detected by SEPs (Fig. 14–1). SEP responses are thought to pass through large fiber somatosensory pathways of the dorsal column and possibly through the anterior spinothalamic tract, and this may be another reason why anterior spinal artery ischemia could be detected using this technique.

Generators of Somatosensory-Evoked Potential Responses

The cortical response for the lower extremity is called the P37 potential. The generator of this response arises from the primary somatosensory cortex of the leg, which is located in the mesial parietal cortex. The cortical response for the upper extremity, which is generated from the primary somatosensory cortex of the hand, is called the N20 potential (Fig. 14–2). Two important characteristics of these waveforms are (1) amplitude, which is recorded in microvolts and determined by either a baseline-to-peak or a peak-to-trough measure of the waveform, and (2) latency, which is recorded in milliseconds and is the time interval from the stimulus to the occurrence of the potential. An amplitude change from the initial baseline measure to a decrease of more than 50% is often considered a significant change in SEP amplitudes.9

Significant latency changes in SEP monitoring consist of a 10% prolongation beyond the baseline latency value (Table 14–1).10 Although these deviations from the baseline measures are thought to be significant, they should be interpreted with caution, taking into account various factors, including the evolution of the changes (e.g., a trend toward worsening is an ominous sign) and other intraoperative factors such as length of the surgery, type of anesthetic agent, and temperature effects. Also, significant latency and amplitude changes can occur in isolation. It is quite common to see a significant amplitude change without any associated latency changes. The most significant change is a complete loss of the cortical potential.

TABLE 14–1 Significant Changes in Different Monitoring Modalities

Type of Study Significant Changes Highly Significant Changes
Somatosensory-evoked potentials Amplitude <50%; latency >10% Complete loss of amplitude
Motor-evoked potentials Increase threshold voltage >50-100 V Complete loss of amplitude
Pedicle screw stimulation Current intensity <7-10 mA  

Another measurement made in posterior tibial or peroneal nerve SEP monitoring is the popliteal fossa potential. This is a nerve action potential that is recorded as the impulses pass within the popliteal fossa in the peripheral nervous system. The reason for this measurement is to ensure that an adequate stimulus has been applied. If there is an absence of the popliteal fossa response in addition to an absence of the leg cortical (P37) response, it suggests that the changes seen are not a result of a lesion at the level of the spinal cord. In this case, the change may be technical (e.g., the stimulating needles may have dislodged) or may be seen when the leg is ischemic (e.g., with femoral artery catheterization during thoracoabdominal aneurysm surgery or with direct compression of the peripheral nerve) (Fig. 14–3).

The other posterior tibial stimulation SEP response that can be monitored (besides the P37 response) is the P31/N34 complex, often termed the subcortical response because the generator for this response is at the level of medulla and midbrain. The benefit in recording these responses is that they are relatively more resistant to the effects of anesthesia compared with the cortical P37 response (see Fig. 14–3). The same is true for the subcortical potentials from median nerve stimulation (P14/N18) potential. In pediatric cases, the subcortical potentials may also be better formed and more easily monitored than cortical responses. Some of this effect may be the result of the variation of myelination in the younger age groups and the more significant effects of anesthetics on these patients. These differences from the adult morphology can persist into the early teenage years. Other factors affecting the responses include core body temperature changes. The core body temperature commonly decreases more than 1° C. The cooling affects the limbs more than the core body temperature, which can result in slowing of conduction.

Motor-Evoked Potential Monitoring

As mentioned earlier, various methods have been used to monitor spinal motor pathways during surgery. Most of these methods involve recording of electromyography (EMG) readings from appropriate muscles in response to stimulation of a motor pathway rostral to the operative site. The difference between the various methods is the nature of the stimulation. There are three basic categories of stimulation: rostral spinal stimulation, transcranial magnetic stimulation, and transcranial electrical stimulation (TCES). Magnetic stimulation is effective in nonanesthetized patients for evaluation of motor pathways, but the suppression of cortical responsiveness under anesthesia (mainly inhalational anesthetics) renders this method less effective for surgical use. In addition, the equipment used for magnetic stimulation is expensive, is bulky, and has a tendency to overheat.

Noninvasive stimulation of the brain using TCES was first reported in 1980.11 Single-pulse TCES was subsequently used in monitoring motor pathways.1217 Because of effects of general anesthesia, single-pulse stimulation was found to be less effective in reliably recording MEPs.1822 With the introduction of the multipulse technique for motor pathway monitoring, reliable and robust MEP recording can now be obtained in most patients using some specific general anesthesia protocols.2327 The use of the multipulse technique requires that neuromuscular blockade not be used during this part of the monitoring. Occasionally, the use of partial neuromuscular blockade may still allow for TCES monitoring.28 This method reportedly achieves more reliable stimulation of the motor cortex intraoperatively and is more resilient to the effects of general anesthesia.

The method of MEP monitoring has been revolutionized by the use of multipulse TCES. Previous methods for MEP recording used a variety of stimulation and recording techniques. Spinally elicited neurogenic responses were used in the past and were putatively stated to be a result of activation of the motor pathways in the spinal cord. More recent evidence has suggested that these spinally elicited neurogenic responses are generated through activation of the sensory pathways and retrograde activation of alpha motor neurons. In a collision experiment using stimulation of the spinal cord followed by stimulation of the posterior tibial nerve at various interstimulus intervals, the neurogenic responses were abolished, suggesting that the potentials were colliding in the spinal cord.6

At the beginning of TCES-MEP monitoring, threshold voltages for each side of the body and MEP amplitudes are calculated.23 The motor cortex on the side of the brain receiving the anodal stimulus is typically the first region to activate at the lowest stimulus threshold. The initial current used is typically 100 V, with a train of stimuli delivered to the cortex. After the stimulation, an MEP response is monitored in the muscles contralateral to the side receiving the anodal stimulus. If no response is seen, the voltage is increased typically by 50-V increments, and the process is repeated until an MEP response is seen in all the muscles contralateral to the anodal stimulus. This voltage is called the threshold voltage for that side.

The highest amplitude of the myogenic response below the level of surgery is also noted. Typically, amplitude measures for myogenic responses are best recorded as the area-under-the-curve measurements or simply by just documenting either presence or absence of the myogenic response. This procedure is repeated after reversing the anodal-cathodal configuration using a switch box. The voltage used for TCES-MEP recordings typically does not exceed 500 V. The anticipated latency of the EMG responses ranges from 20 to 40 msec or more, depending on the patient’s height, owing to the conduction time in the descending motor pathways. Latency values have not always been found to be reliable indicators of significant change in TCES monitoring in clinical practice. Another advantage of the multipulse technique is that it elicits a train of pulses that is of sufficient amplitude that it does not require averaging.

Two different methods of recording MEPs can be used. In myogenic MEPs, responses can be recorded directly from the muscle (either a surface electrode or needle electrodes placed within the muscle). In spinal cord MEPs, responses may be recorded directly from the spinal cord with use of an epidural catheter electrode that records a direct “D” wave and a volley of indirect “I” waves. Using single-pulse TCES, recording “D” and “I” waves is frequently required, meaning a “D” wave could be recorded when a myogenic MEP is not yet seen. This is because a series of “D” and “I” waves is required for alpha motor neurons to generate a myogenic response. The spinal recorded responses can also be recorded with full muscle relaxation, whereas myogenic responses require either no or very little muscle relaxation, even with the multipulse technique.

Determining significant changes during the course of surgery typically is most reliable if there is an absolute loss of myogenic responses to stimulation (Fig. 14–4). Some authors have also suggested that reduction of MEP amplitude to 25% of baseline amplitude values is predictive of motor pathway impairments.29 Significant changes can also be determined by the change of voltage required to obtain MEPs of greater than 50 V beyond baseline thresholds used in obtaining MEPs at the beginning of monitoring (see Table 14–1).30

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Jul 28, 2016 | Posted by in ORTHOPEDIC | Comments Off on Intraoperative Neurophysiologic Monitoring of the Spine

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