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Somatosensory evoked potential monitoring with scalp and cervical recording
Intraoperative Monitoring of Neural Function Handbook of Clinical Neurophysiology, Vol. 8 M.R. Nuwer (Ed.) # 2008 Elsevier B.V. All rights reserved
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CHAPTER 11
Somatosensory evoked potential monitoring with scalp and cervical recording Marc R. Nuwer* and James W. Packwood Department of Clinical Neurophysiology, UCLA School of Medicine and UCLA Medical Center, Los Angeles, CA 90095, USA
The most common technique for spinal cord monitoring uses somatosensory evoked potentials (SEPs) with stimulation at the ankles and recording over the neck and scalp. A similar technique is used for monitoring the intracranial lemniscal sensory system as it traverses the brainstem and cerebral hemispheres. For cervical or intracranial procedures, wrist stimulation often substitutes for or supplements the ankle stimulation techniques. Monitoring provides services beyond simply warning of complications. With monitoring, a surgeon can feel reassured about spinal cord or lemniscal sensory pathway integrity, and therefore, extend the surgical procedure to a greater degree than would have been done without monitoring. Some patients are eligible to undergo procedures with monitoring when they may have been denied surgery in the past because of a high risk of neurologic complications. Patients and families can be reassured that certain feared complications are screened for during surgery. As such, monitoring provides an added dimension to surgical cases even when the SEP itself is unchanged throughout the procedure. 11.1. Techniques Ankle–wrist to scalp–neck SEP monitoring techniques are adapted from SEPs as used commonly in the outpatient testing. These techniques should be reasonably familiar to technologists and neurophysiologists who conduct routine outpatient evoked potential (EP) testing. *
Correspondence to: Marc R. Nuwer, M.D., Ph.D., Department of Clinical Neurophysiology, UCLA School of Medicine, Reed Research Building, Room 1-194, 710 Westwood Plaza, Los Angeles, CA 90095-6987, USA. Tel.: +1-310-206-3093; fax: +1-310-267-1157. E-mail: [email protected] (M.R. Nuwer).
Usually, these techniques require about 300 trials (individual stimulations) to produce a well-defined EP tracing suitable for measurement and comparison to baseline. That could be accomplished ideally in 1 min. In the more realistic setting, recording is interrupted by electrocautery artifact, movement, muscle, and other artifacts, resulting in many rejected trials or the need to start over for a heavily contaminated tracing. In this more realistic setting, a new EP can be generated in several minutes except when the surgeon uses electrocautery or produces other artifacts frequently or continuously. Table 1 summarizes common simple techniques for SEP monitoring.
Table 1 Summary of techniques for SEP monitoring in surgery Stimulation Lower extremity: posterior tibial nerve at the ankle Upper extremity: median or ulnar nerve at the wrist Intensity to cause a 1–2 cm movement Stimulate at 5.1/s/nerve, adjust rate as needed Recording Thoracic monitoring with lower extremity stimulations CSp5–forehead Cz0 –forehead C30 –C40 Cervical or intracranial monitoring with upper extremity stimulations Erb’s ipsilateral–forehead Ear–forehead Cz0 –forehead C30 –C40 Filters 30 and 3,000 Hz, notch filter off 300 or more trials per EP Criteria for alarm 50% drop in amplitude 5–10% increase in latency
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11.2. Stimulation 11.2.1. Sites Lower extremity stimulation is delivered to the posterior tibial nerve at the ankle. The nerve is found easily at the ankle where it passes superficially just posterior to the medial malleolus. That site has the advantages that it can be accessed easily if needed during the operation. Stimulation there causes a relatively small movement when neuromuscular blockade is incomplete. The desired movement is flexion of the foot and toes. Its scalp EPs are produced reasonably well in most patients. The ideal stimulation site is posterior to or slightly above the level of the malleolus, so as to stimulate both the medial and lateral plantar terminal nerve branches. Stimulation too distally may catch only one terminal branch. The alternate lower extremity site is the common peroneal nerve. Like the posterior tibial nerve, this is a mixed motor nerve, that is, with both sensory and motor components. Its sensory components project up the lemniscal system to the cerebral hemispheres. The peroneal nerve is stimulated where it runs superficially over the fibula immediately distal to the fibular head, which is just distal to the lateral knee. Stimulation there produces dorsiflexion at the ankle. The peroneal nerve’s sensory distribution — the lateral lower leg and dorsal foot — has a smaller cortical representation than the posterior tibial nerve’s plantar sensory distribution, because the plantar foot is a more sensitive location. Therefore, peroneal stimulation may produce on average a smaller cortical EP. But patients with a peripheral neuropathy may have a much smaller posterior tibial cortical SEP because of the damage to distal sensory fibers. The peroneal nerve may be more normal in many such patients, because it is more proximal. It should be considered a suitable alternative lower extremity site among patients in whom the posterior tibial site produces unsatisfactory EPs. The best upper extremity stimulation site often is the median nerve at the wrist. This nerve is found in the middle of the volar wrist. This serves a sensory distribution that includes at least the thumb, index, and middle fingers, and the lateral two-thirds of the palm. Stimulation there produces thumb abduction. Its sensory areas are very well represented at the cerebral cortex because of the importance of human hand function. This produces an excellent, easily obtained cortical EP in addition to well-defined peaks at cervical, subcortical, and plexus levels. It
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is well suited as a control channel in thoracolumbar spine cases. It is used often as the primary SEP monitoring channel for upper cervical and intracranial lemniscal sensory monitoring. However, it enters the spinal cord too rostrally for detection of some distal cervical impairment. For middle to lower cervical spinal cord monitoring, the ulnar nerve is a stimulation site more suitable than the median nerve. The ulnar nerve is stimulated at the medial volar wrist just above the boney ulnar head. Stimulation there produces fifth finger and lateral hand movement. Its sensory distribution includes the lateral hand and fifth finger, a sensory region much smaller than that of the median nerve. As a result, its cortical peak often is smaller than for the median nerve technique. Stimulation usually is delivered to both left and right side nerves. Those stimuli are alternated rather than delivered simultaneously. This allows simultaneous measuring of each lemniscal pathway separately. The latter is helpful in the rare situation in which a significant change occurs only with impairment of one half of the spinal cord (Lesser et al., 1986; Molaie, 1986). Most modern operative monitoring equipment allows for separate averaging from each stimulation site. For thoracic and lumbar cases, the posterior tibial nerve technique usually is used. Median nerve stimulation also is used. The latter may detect systemic or anesthetic-related changes in the cortical EPs, so they are used as a kind of control. In those cases, the median or ulnar nerve channels rarely detect an incidental brachial plexus impairment, for example, from arm positioning. For cervical cases, median or ulnar nerve pathways are monitored along with posterior tibial nerve channels. This gives a good coverage of the many different possible areas of impairment. The lower extremity channels also cover in case of a high thoracic or very low cervical level injury. 11.2.2. Rate and intensity Stimuli are delivered above the motor threshold sufficient to cause a 1–2-cm movement. This stimulation intensity can be determined prior to initiation of neuromuscular blockade. Otherwise, a default stimulus intensity value such as 20 mA is chosen. The presence of the stimulus can be checked during the case, even in the presence of neuromuscular
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blockade, by noting the continuing presence of the stimulus artifact in the recording channels. Needle electrodes can be used for nerve stimulation. Two are placed along the course of the nerve. This allows for secure location, and avoids the problems of slippage or drying of the conductive paste during long cases. It also avoids changes in skin resistance that can occur over many hours of long cases. Some users prefer using traditional surface disc electrodes or bar electrodes with two disc or rectangular contacts. These would need to be secured in place, but without pressure enough to cause skin damage from prolonged pressure in long cases. For surface electrodes, paste should be calcium-free to avoid chemical skin burns from iontophoresis of those ions into the skin. Stimulation is delivered generally at approximately 5 stimuli/s (Nuwer and Dawson, 1984a). Faster stimulation would have been helpful to produce EPs quicker in the operating room. However, faster stimulation generally produces lower amplitude EPs, making monitoring more difficult. Slower stimulation can produce larger EPs, but slows the speed of creating new EP tracings. As illustrated in Fig. 1, the best trade-off of these two factors is about 5 stimuli/s/nerve. For patients with particularly large EPs, a EFFECTS OF STIMULUS RATE Rate ⫻ Amplitude
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11.3. Recording Recording at multiple sites above the level of surgery is helpful. In individual patients, it is difficult to predict in advance which specific sites will be most useful for monitoring. Some degree of flexibility of technique is helpful. Availability of many recording channels also helps greatly — many modern systems can track 16–32 channels simultaneously. Once monitoring under anesthesia is under way, the monitoring neurophysiologist or technologist can choose among the best recordable potentials available for that particular patient. These optimal recording sites and potentials can then be used for the further monitoring procedure itself. Near-field scalp recording channels are less affected by background muscle and movement artifact. Far-field potentials recorded from the cervical channel are less affected by changes in anesthetic concentrations. A combination of recording channels is usually optimal, rather than monitoring few channels. 11.3.1. Lower extremity recording channels
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slightly faster stimulation rate can be employed. For patients with particularly small EPs, a slower stimulation rate is helpful. Stimulation rates avoid exact multiples of 60 Hz (or 50 Hz), such as 5.00/s. That helps to average out any residual 60 Hz (or 50 Hz) artifact in the recording.
11.1
STIMULI PER (S)
Fig. 1. Effects of increasing the rate of stimulus presentation in one patient, for posterior tibial stimulation and scalp recordings. As rate is increased, EP amplitude decreases. The product rate amplitude helps compare the advantageous increase in speed of testing and the disadvantageous loss of amplitude. In the patient, the rate 5.1/s appeared to be the best compromise between speed and attenuation. (Reprinted from Nuwer and Dawson, 1984a, with permission from the International Federation of Clinical Neurophysiology.)
With lower extremity stimulation, recordings are taken from skin over the cervical spine and scalp vertex. These are designed to monitor far-field cervical and brainstem activity, as well as near-field somatosensory cortical potentials. The primary scalp active recording is made from site Cz0 (Cz-prime). The “prime” mark indicates a modified site located 2 cm posterior to the named International 10–20 system scalp site. This midline site Cz0 often is the optimal placement for recording for each of the left and right lower extremities. Secondary scalp active recording sites are offcenter at C10 and C20 , or at C30 and C40 . These are better recording sites than Cz0 in some patients. Orientation of the active somatosensory cortical generator varies among patients, a point illustrated in Fig. 2. Note that the dipole crosses the midline before appearing at the scalp. This results in a “paradoxical localization” of the primary cortical peak, which is best seen over the scalp ipsilateral to the leg stimulated.
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is often over the second cervical spine (CSp2). These are referenced to the forehead or shoulder. These channels are desirable because the recorded cervical and brainstem peaks are less affected by anesthesia depth. For cervical procedures, electrodes cannot be placed over the cervical spine. Instead, subcortical peaks are recorded from mastoid or ear electrodes. A lumbar channel is used occasionally to ascertain that the stimulus induced transmission arrived at the lumbar spinal cord. For older or obese patients, these lumbar potentials are difficult to record.
+ CZ C4
C3
11.3.2. Upper extremity recording channels
−
A
CZ
+
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B Fig. 2. Hypothesized variations in the electrical field distribution of the early cortical potentials upon stimulation of the posterior tibial nerve. These dipole orientation variations are based on the known variability of the location of the leg area in the cortex in the interhemispheric fissure. Notice how the positive peak generated in one hemisphere can show up best at the scalp vertex as in (A), or on the scalp overlying the other hemisphere as in (B). The latter phenomenon has been called “paradoxical localization.” (Reprinted from Seyal et al., 1983, with permission from the International Federation of Clinical Neurophysiology.)
Several reference electrodes sites are in common use. Many monitorists measure from Cz0 using a forehead, ear, or mastoid reference site. For the offcenter scalp active sites, many users reference the contralateral electrodes, that is, C10 –C20 and C30 – C40 . Short distances to the reference reduce noisiness in channels due to distant sources. But short distances also may reduce some peak amplitudes. The optimal recording technique varies among patients. The subcortical peaks are recorded from electrodes placed at the neck, ear, or mastoid. The neck site
With upper extremity stimulation, recordings are taken from skin over the shoulder, cervical spine, and scalp vertex. These are designed to monitor farfield cervical and brainstem activity, as well as near-field somatosensory cortical potentials. The primary scalp active recording sites are at C30 and C40 . Several reference electrode sites are in common use, including forehead, ear, or mastoid reference sites. Some users reference the contralateral electrodes, that is, C30 –C40 . For some intracranial surgery, those sites may be in the surgical field. In those cases, modified scalp sites are needed. In some cases, subdural or direct cortical electrodes are placed in the surgical field. Keeping the latter electrodes in place throughout a procedure requires tethering of the electrode’s cable with sutures or other ad hoc techniques. The subcortical brainstem or cervical peaks are recorded from electrodes placed at the neck, ear, or mastoid. The neck site is often over the fifth cervical spine (CSp5), directly over the median nerve’s cervical peak generator located in the spinal cord at the C5 level. These sites are referenced to the forehead or contralateral shoulder. These channels are desirable because the recorded cervical and brainstem peaks are less affected by anesthesia depth, and to assure monitoring of the peripheral portion of the pathway. An Erb’s point channel is sometimes used to ascertain that the stimulus-induced transmission arrived at the proximal brachial plexus. Erb’s point is located above the clavicle, 2 cm lateral to the insertion of the sternocleidomastoid muscle. 11.3.3. Filters Recording filters are generally chosen at 30 Hz and 3 kHz. These filter settings are chosen to optimize
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noise rejection while retaining the principal EP characteristics in a typical surgical setting. They also minimize the ordinary background fluctuations in EP due to minor changes in anesthetic depth (Nuwer and Dawson, 1984a). Open filters (e.g., a low filter of 1 Hz) are more susceptible to variability due to background noise. The individual techniques used are open to clinical choice by the neurophysiology monitoring team. Figure 3 illustrates the effects of various filter settings. In this case, the optimal trade-off occurred for a lower filter set at 30 Hz. Lower filter settings led to a greater amount of background variability. Higher settings of the low filter resulted in attenuation of the desired EP. Figure 4 shows tracings from scalp recordings during a routine monitoring during surgery for scoliosis. This shows an example of stable recordings from upper and lower extremity stimulation. Such stable, low-noise, easily reproducible potentials are desirable for spinal cord monitoring. The 60 Hz (or 50 Hz) notch filter usually is kept off. That filter can interact with the stimulus signal itself, resulting in a sinusoidally decaying artifact. The artifact has a 60 Hz (or 50 Hz) frequency, resulting in peaks at 16.6, 33, and 50 ms. Some users have mistaken these artifacts for stable EPs, which do not change during the monitoring period.
11.4. Peaks, generators, and alarm criteria 11.4.1. Lower extremity peaks For posterior tibial nerve techniques, an N22 peak appears as a negative polarity near-field potential over the lumbar spinal cord at about 22 ms after the stimulation. The lumbar spinal cord anatomically lies around the T12 or L1 spine, so the potential is seen best over those spines. See Fig. 5 for an illustration of typical routine lower extremity SEP peaks and their nomenclature. A cervical peak is seen using far-field recording from the neck region to a distant reference. The peak often occurs around 30 ms. The generator is thought to be the gracilis nucleus, one of the posterior column nuclei at the cervicomedullary junction. The lemniscal pathways synapse at that level. That peak may be followed by an opposite polarity trough that may represent conduction up the brainstem’s medial lemniscus. The P37 is generated at the primary somatosensory cortex. As seen in Fig. 2, it may show up at the midline or off-center, paradoxically, on the side of the leg stimulated. It may have a doubled negative peak, which sometimes leads to peak-picking errors by inexperienced users. When encountering the doubled peak morphology, a user might sometimes choose the first, and at other times choose the second
EFFECTS OF VARIOUS FILTER SETTINGS
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Fig. 3. Effects of four different filter settings during intraoperative recordings in one patient, taken from a single scalp channel (Cz channel). The four recordings were made simultaneously from the same data. Variability is greatest in the 1 Hz channel and is reduced by higher filter settings. Each pair of EPs is a typical set of two consecutive recordings. The amplitude scale is doubled for the lower two EPs. (Reprinted from Nuwer and Dawson, 1984b, with permission from Lippincott Williams and Wilkins.)
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Normal Stable Somatosensory Evoked Potentials Cervical-Fz
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Fig. 4. Normal stable SEPs. Note the good reproducibility for the tracings in each channel. The montage includes upper and lower extremity tracings, collected separately for each limb. Cortical and subcortical (cervical) channels are used. (From UCLA Department of Clinical Neurophysiology, with permission.)
peak, leading to an erroneous impression that the latency has shifted. Watching the peak shape (morphology) can help to avoid peak-picking errors. The primary measurements are the P37 peak’s amplitude and latency. Secondary measurements are the amplitude and latency for the cervical peak. Latencies should stay within about 5–10% of baseline values, for example, 3–4 ms of normal variation for a 37-ms P37 peak. Amplitudes should stay within 50% of baseline values. Some users raise alarms at 30% decrease of the P37, since such a degree of amplitude drop is uncommon among well-defined P37 peaks. These latencies and amplitudes usually are chosen after the patient has been under anesthesia for 20 min, since there is a period of gradual amplitude loss and minor latency increase due solely to the anesthetic itself. That effect takes 20–30 min to take effect after induction. Even during that time period, though, monitoring needs to watch for early deterioration due to positioning on the table or problems related to intubation. Anesthetic effect is distinguished from clinical pathway impairment by
observing the cervical peaks, since they are relatively steady despite anesthesia. It is the P37 cortical peak that is affected more greatly by the onset of anesthetic. This trick applies to spine surgery, but not to intracranial surgery. The effects of anesthetics are described in detail in another chapter. 11.4.2. Upper extremity peaks For median or ulnar nerve techniques, an N9 peak appears as a negative polarity near-field potential over the proximal brachial plexus at about 9 ms after the stimulation. The electrode best used to measure this is at a place referred to as Erb’s point, and the peak itself sometimes is referred to as the Erb’s point peak. Fig. 6 illustrates typical routine upper extremity SEP peaks and their nomenclature. A cervical peak is seen using far-field recording from the neck region to a distant reference. The negative polarity peak often occurs around 13 ms. The generator of this N13 is thought to be the cervical spinal cord gray matter at the level where the nerve
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PF-K N22
1.0 µV/division
T12-Ic C5Sp-Fz
Ci-Cc
C⬘z-Fz P37 6 ms/division
Fig. 5. SEPs from posterior tibial nerve stimulation are shown. Typical peaks N8, N22, and P37 are noted. A cervical peak was also found. These peaks have normal latencies and amplitudes. (Reprinted from Nuwer et al., 1994, with permission from the International Federation of Clinical Neurophysiology.)
enters, synapses onto reflex arcs, and send axons rostrally into the posterior column. For the median nerve, this is in the C5 spinal cord. Shortly thereafter, a positive polarity peak is generated probably from the cuneate nucleus, one of the posterior column nuclei at the cervicomedullary junction. This peak is referred to as the P14. The lemniscal pathways synapse at that level. That peak may be followed by a negative polarity N18 that may represent conduction up the brainstem’s medial lemniscus and into the thalamus. The N18 is a broad potential that gradually decreases over about 10 ms. Both the P14 and N18 are best seen as a far-field potential seen broadly over the scalp using a non-cephalic reference. The N18 may be obscured by the N20 when recording near that peak’s active site. The N20 is generated at the primary somatosensory cortex. This usually is a large well-defined peak seen over the C30 or C40 scalp site. It is a near-field potential, that is, its amplitude drops off when the electrode is relocated even a short distance away from the optimal recording site. The primary measurements are the N20 peak’s amplitude and latency. Latencies should stay within about 5–10% of baseline values, for example, 1–2 ms
Fig. 6. SEPs from median nerve stimulation are shown. Typical peaks are shown in each of four recording channels. The test is normal. (Reprinted from Nuwer et al., 1994, with permission from the International Federation of Clinical Neurophysiology.)
of normal variation for a 20 ms N20 peak. Amplitudes should stay within 50% of baseline values. Some users raise alarms at 30% decrease of the N20, since such a degree of amplitude drop is uncommon among welldefined N20 peaks. As with the lower extremity P37 peak, these latencies and amplitudes usually are chosen after the patient has been under anesthesia for 20 min. That effect takes 20–30 min to take effect after induction. The effects of anesthetics are described in detail in another chapter. 11.5. Interpretation of changes: alarms and considerations Generally, monitoring teams use both latency and amplitude criteria for raising an alarm. A 50% drop in recorded potentials is generally considered to be sufficient for raising an alarm (Nuwer, 1986; Nuwer et al., 1995). Latency increases greater than 5–10% also are cause for alarm. Latency measures need to take into account temperature effects. Amplitude measures need to take into account anesthetic effects especially for cortical potentials. The initial period
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Systemic factors can cause EP changes. Examples are hypotension and hypoxia. Occasionally, such systemic changes are first detected by EP changes. A patient with preexisting neurologic problems, such as spinal cord compression, may be particularly sensitive to otherwise minor degrees of hypotension. Surgical problems causing EP attenuation include direct cord trauma, excessive traction, blunt trauma, excessive compression, stretching of the cord from spinal distraction, vascular insufficiency from compression, embolus, thrombus, or occasionally other clinical problems. Anesthetic effects are common. Monitoring often assesses not only the lower extremity SEPs, but also the upper extremity SEPs even in a thoracic or lumbar procedure. This allows for an assessment of anesthetic effect. Usually, that affects all cortical channels but causes little change in subcortical channels. See Fig. 7 for an example of an asymmetric
after anesthetic induction is accompanied by latency and amplitude changes as the nervous system equilibrates over 20–30 min. Experienced monitoring teams learn how to factor out these extraneous causes of variability and so minimize false alarms due to such effects as boluses of medication, raised levels of inhalation anesthetics, and decreasing temperature. When changes occur, the monitoring team needs to assess quickly whether these have a technical, systemic, or surgical cause. Technical and systemic issues also can cause change and need to be investigated. Technical problems can occur from the electrodes themselves, for example, if they become disconnected. Equipment can malfunction. A good stimulus artifact or motor signs of acceptable stimulus help assure the adequacy of a stimulus. Impedance checks and review of raw input data can assure the recording system’s integrity.
UNILATERAL CORTICAL EVOKED POTENTIAL LOSS DUE TO ANESTHETIC DEPTH CHANGE
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Fig. 7. The baseline testing shows a relatively attenuated left lower extremity cortical peak (left tracings). After an increase in anesthetic depth (right tracings), that channel no longer shows a reliable EP. (The baseline is superimposed on the newly acquired tracings at the right.) An anesthetic effect is the likely cause of the change, as suggested by both the preserved subcortical peaks for the affected pathway and somewhat attenuated cortical peaks in all other pathways. (From UCLA Department of Clinical Neurophysiology, with permission.)
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anesthetic effect seen in the setting of a partial preexisting unilateral impairment in one pathway. Occasional transient significant diminishing of EPs can occur without placing the patient at significant risk for postoperative neurologic problems. Even transient EP loss for a few minutes can occur without substantially raising the risk of postoperative problems, especially if the patient’s EPs return shortly thereafter to baseline levels of amplitude and latency. If EPs abruptly or slowly diminish to beyond the criterion levels for raising an alarm, the patient is at risk for postoperative new neurologic complications, especially if the EPs remain attenuated and prolonged through the end of the case. The gravest situation is the complete loss of previously easily detected EPs. Even in that circumstance, the risk of new neurologic postoperative impairment may only be about 50–75% (Nuwer et al., 1995).
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for many hour or days. In each delayed onset case, the patient awoke from surgery initially with good motor strength in the lower extremities. Motor function was lost several hours to several days later. The scoliosis surgery multicenter outcome study assessed the likelihood of false results. Those results are shown in Table 2. False negative studies were rare, and many of those were delayed onset. False positive outcomes were more common. Equivocal results are those with minor or transient changes. Table 3 shows the sensitivity and specificity results as determined in the same large study. 11.7. Conclusions SEP monitoring can be conducted with ankle or wrist stimulation and recording over the scalp and neck. These techniques are familiar to neurophysiologists and technologists who conduct routine outpatient
11.6. False results Occasional false positive monitoring events are seen. In a false positive event, SEPs show significant changes but the patient develops no postoperative deficits. Many false positive events may be true physiologic clinical changes detected by monitoring. In those cases, monitoring raises an alarm, the clinical team makes changes, and the patient has no neurological deficit because the interventions were successful. Others may be false alarms due to technical or anesthetic issues. False negative cases have been reported despite stable EPs in some studies (Tamaki et al., 1984; Wilber et al., 1984; Ginsburg et al., 1985; Johnston et al., 1986; Ben-David et al., 1987; Harper et al., 1988; More et al., 1988). Some such patients had patches of dysesthesia for several days or weeks postoperatively, possibly due to segmental spinal wiring. Rare cases have had serious, long-lasting neurological complications. Lesser and colleagues (Lesser et al., 1986) reported six cases from four institutions. Each suffered new postoperative neurologic deficits despite stable intraoperative SEPs. In three of these cases, the new impairment probably developed postoperatively, referred to as “delayed onset” deficits. Such delayed onset cases may be due to latent effects of vascular compromise or mechanical compression. Some may have been exacerbated by patient movement after awakening. Alternately, some compression or ischemia may gradually result in permanent impairment only if the condition remains in place
Table 2 Neurologic outcome prediction rates for SEP monitoring in spinal surgery (Total Procedures: 51,263 (100%)) False-negative (FN) rate: neurologic postoperative deficits despite stable SEPs: Definite 34 (0.063%) Equivocal 13 (0.025%) Delayed onset 18 (0.035%) Total 65 (0.127%) False-positive rate: no neurologic deficits despite SEP changes: Definite 504 (0.983%) Equivocal 270 (0.527%) Total 774 (1.510%) True-positive (TP) rate: neurologic deficits predicted by SEP changes: Definite 150 (0.293%) Equivocal 67 (0.131%) Total 217 (0.423%) Neurologic deficits (FN plus TP): Definite 184 (0.356%) Equivocal 80 (0.156%) Delayed onset 18 (0.035%) Total 282 (0.550%) True-negative rate: no neurologic deficit and stable SEPs: Total 50,207 (97.94%) These data are from the multicenter outcome study of SEP spinal cord monitoring in scoliosis organized through the Scoliosis Research Society (Nuwer et al., 1995). They were obtained from 153 US surgeon respondents. Note the very low rate of definite false negative cases (0.063%).
SOMATOSENSORY EVOKED POTENTIALS Table 3 SEP monitoring validity measures Sensitivity 417/451 Specificity 50,207/50,781 Positive predictive value 417/991 Negative predictive value 50,207/50,241
92% 98.9% 42% 99.93%
These outcome measures are from the multicenter outcome survey of 153 US surgeons organized through the Scoliosis Research Society (Nuwer et al., 1995). The table shows a breakout of validity measures from that survey. The very high negative predictive value here indicates the high reliability of the monitoring when the SEP remains normal and stable. The outcome survey report (Nuwer et al., 1995) discusses in detail these data and related assumptions.
EP testing. In the nearly 30 years since these techniques were developed, they have spread into common use in surgery. They are appropriate for orthopedic, neurosurgical, and cardiovascular procedures. False negative cases are rare. False positive results occur in a small portion of cases. With appropriate choices of reference sites and filter settings, and with avoidance of excess inhalation anesthetic, suitable peaks can be found in most patients. References Ben-David, B, Haller, G and Taylor, P (1987) Anterior spinal fusion complicated by paraplegia. A case report of a false-negative somatosensory evoked potential. Spine, 12: 536–539. Ginsburg, HH, Shetter, AG and Raudzens, PA (1985) Postoperative paraplegia with preserved intraoperative somatosensory evoked potentials. J. Neurosurg., 6: 296–300. Harper, CM, Daube, JR, Lichy, WJ and Klassen, RA (1988) Lumbar radiculopathy after spinal fusion for scoliosis. Muscle Nerve, 11: 386–391. Johnston, CE II, Happel, LT, Jr., Norris, R, Burke, S, King, AG and Roberts, JM (1986) Delayed paraplegia complicating sublaminar segmental spinal instrumentation. J. Bone Joint Surg. Am., 68: 556–563.
189 Lesser, RP, Raudzens, P, Lueders, H, Nuwer, MR, Goldie, WD, Morris, HH, III, Dinner, DS, Klem, G, Hahn, JF, Shetter, AG, Ginsburg, HH and Gurd, AR (1986) Postoperative neurological deficits may occur despite unchanged intraoperative somatosensory evoked potentials. Ann. Neurol., 19: 22–25. Molaie, M (1986) False negative intraoperative somatosensory evoked potentials with simultaneous bilateral stimulation. Clin. Electroencephalogr., 17: 6–19. More, RC, Nuwer, MR and Dawson, EG (1988) Cortical evoked potential monitoring during spinal surgery: sensitivity, specificity, reliability, and criteria for alarm. J. Spinal Disord., 1: 75–80. Nuwer, MR (1986) Evoked Potential Monitoring in the Operating Room, Raven Press, New York, 246 pp. Nuwer, MR and Dawson, E (1984a) Intraoperative evoked potential monitoring of the spinal cord: enhanced stability of cortical recordings. Electroencephalogr. Clin. Neurophysiol., 59: 318–327. Nuwer, MR and Dawson, E (1984b) Intraoperative evoked potential monitoring on the spinal cord: a restricted filter, scalp method during Harrington instrumentation for scoliosis. Clin. Orthop., 183: 42–50. Nuwer, MR, Aminoff, M, Demedt, J, Eisen, AA, Goodin, D, Matsuoka, S, Mauguie`re, F, Shibasaki, H, Sutherling, W and Vibert, JF (1994) IFCN recommended standards for short latency somatosensory evoked potentials: report of an IFCN committee. Electroencephalogr. Clin. Neurophysiol., 91: 6–11. Nuwer, MR, Dawson, EG, Carlson, LG, Kanim, LEA and Sherman, JE (1995) Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey. Electroencephalogr. Clin. Neurophysiol., 96: 6–11. Seyal, M, Emerson, RG and Pedley, TA (1983) Spinal and early scalp-recorded components of the somatosensory evoked potential following stimulation of the posterior tibial nerve. Electroencephalogr. Clin. Neurophysiol., 55: 320–330. Tamaki, T, Noguchi, T, Takano, H, Tsuji, H, Nakagawa, T, Imai, K and Inoue, S (1984) Spinal cord monitoring as a clinical utilization of the spinal evoked potentials. Clin. Orthop., 184: 58–64. Wilber, RG, Thompson, GH, Shaffer, JW, Brown, RH and Nash, CL, Jr. (1984) Postoperative neurological deficits in segmental spinal instrumentation. J. Bone Joint Surg. [Am.], 66A: 1178–1187.
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CHAPTER 11
Somatosensory evoked potential monitoring with scalp and cervical recording Marc R. Nuwer* and James W. Packwood Department of Clinical Neurophysiology, UCLA School of Medicine and UCLA Medical Center, Los Angeles, CA 90095, USA
The most common technique for spinal cord monitoring uses somatosensory evoked potentials (SEPs) with stimulation at the ankles and recording over the neck and scalp. A similar technique is used for monitoring the intracranial lemniscal sensory system as it traverses the brainstem and cerebral hemispheres. For cervical or intracranial procedures, wrist stimulation often substitutes for or supplements the ankle stimulation techniques. Monitoring provides services beyond simply warning of complications. With monitoring, a surgeon can feel reassured about spinal cord or lemniscal sensory pathway integrity, and therefore, extend the surgical procedure to a greater degree than would have been done without monitoring. Some patients are eligible to undergo procedures with monitoring when they may have been denied surgery in the past because of a high risk of neurologic complications. Patients and families can be reassured that certain feared complications are screened for during surgery. As such, monitoring provides an added dimension to surgical cases even when the SEP itself is unchanged throughout the procedure. 11.1. Techniques Ankle–wrist to scalp–neck SEP monitoring techniques are adapted from SEPs as used commonly in the outpatient testing. These techniques should be reasonably familiar to technologists and neurophysiologists who conduct routine outpatient evoked potential (EP) testing. *
Correspondence to: Marc R. Nuwer, M.D., Ph.D., Department of Clinical Neurophysiology, UCLA School of Medicine, Reed Research Building, Room 1-194, 710 Westwood Plaza, Los Angeles, CA 90095-6987, USA. Tel.: +1-310-206-3093; fax: +1-310-267-1157. E-mail: [email protected] (M.R. Nuwer).
Usually, these techniques require about 300 trials (individual stimulations) to produce a well-defined EP tracing suitable for measurement and comparison to baseline. That could be accomplished ideally in 1 min. In the more realistic setting, recording is interrupted by electrocautery artifact, movement, muscle, and other artifacts, resulting in many rejected trials or the need to start over for a heavily contaminated tracing. In this more realistic setting, a new EP can be generated in several minutes except when the surgeon uses electrocautery or produces other artifacts frequently or continuously. Table 1 summarizes common simple techniques for SEP monitoring.
Table 1 Summary of techniques for SEP monitoring in surgery Stimulation Lower extremity: posterior tibial nerve at the ankle Upper extremity: median or ulnar nerve at the wrist Intensity to cause a 1–2 cm movement Stimulate at 5.1/s/nerve, adjust rate as needed Recording Thoracic monitoring with lower extremity stimulations CSp5–forehead Cz0 –forehead C30 –C40 Cervical or intracranial monitoring with upper extremity stimulations Erb’s ipsilateral–forehead Ear–forehead Cz0 –forehead C30 –C40 Filters 30 and 3,000 Hz, notch filter off 300 or more trials per EP Criteria for alarm 50% drop in amplitude 5–10% increase in latency
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11.2. Stimulation 11.2.1. Sites Lower extremity stimulation is delivered to the posterior tibial nerve at the ankle. The nerve is found easily at the ankle where it passes superficially just posterior to the medial malleolus. That site has the advantages that it can be accessed easily if needed during the operation. Stimulation there causes a relatively small movement when neuromuscular blockade is incomplete. The desired movement is flexion of the foot and toes. Its scalp EPs are produced reasonably well in most patients. The ideal stimulation site is posterior to or slightly above the level of the malleolus, so as to stimulate both the medial and lateral plantar terminal nerve branches. Stimulation too distally may catch only one terminal branch. The alternate lower extremity site is the common peroneal nerve. Like the posterior tibial nerve, this is a mixed motor nerve, that is, with both sensory and motor components. Its sensory components project up the lemniscal system to the cerebral hemispheres. The peroneal nerve is stimulated where it runs superficially over the fibula immediately distal to the fibular head, which is just distal to the lateral knee. Stimulation there produces dorsiflexion at the ankle. The peroneal nerve’s sensory distribution — the lateral lower leg and dorsal foot — has a smaller cortical representation than the posterior tibial nerve’s plantar sensory distribution, because the plantar foot is a more sensitive location. Therefore, peroneal stimulation may produce on average a smaller cortical EP. But patients with a peripheral neuropathy may have a much smaller posterior tibial cortical SEP because of the damage to distal sensory fibers. The peroneal nerve may be more normal in many such patients, because it is more proximal. It should be considered a suitable alternative lower extremity site among patients in whom the posterior tibial site produces unsatisfactory EPs. The best upper extremity stimulation site often is the median nerve at the wrist. This nerve is found in the middle of the volar wrist. This serves a sensory distribution that includes at least the thumb, index, and middle fingers, and the lateral two-thirds of the palm. Stimulation there produces thumb abduction. Its sensory areas are very well represented at the cerebral cortex because of the importance of human hand function. This produces an excellent, easily obtained cortical EP in addition to well-defined peaks at cervical, subcortical, and plexus levels. It
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is well suited as a control channel in thoracolumbar spine cases. It is used often as the primary SEP monitoring channel for upper cervical and intracranial lemniscal sensory monitoring. However, it enters the spinal cord too rostrally for detection of some distal cervical impairment. For middle to lower cervical spinal cord monitoring, the ulnar nerve is a stimulation site more suitable than the median nerve. The ulnar nerve is stimulated at the medial volar wrist just above the boney ulnar head. Stimulation there produces fifth finger and lateral hand movement. Its sensory distribution includes the lateral hand and fifth finger, a sensory region much smaller than that of the median nerve. As a result, its cortical peak often is smaller than for the median nerve technique. Stimulation usually is delivered to both left and right side nerves. Those stimuli are alternated rather than delivered simultaneously. This allows simultaneous measuring of each lemniscal pathway separately. The latter is helpful in the rare situation in which a significant change occurs only with impairment of one half of the spinal cord (Lesser et al., 1986; Molaie, 1986). Most modern operative monitoring equipment allows for separate averaging from each stimulation site. For thoracic and lumbar cases, the posterior tibial nerve technique usually is used. Median nerve stimulation also is used. The latter may detect systemic or anesthetic-related changes in the cortical EPs, so they are used as a kind of control. In those cases, the median or ulnar nerve channels rarely detect an incidental brachial plexus impairment, for example, from arm positioning. For cervical cases, median or ulnar nerve pathways are monitored along with posterior tibial nerve channels. This gives a good coverage of the many different possible areas of impairment. The lower extremity channels also cover in case of a high thoracic or very low cervical level injury. 11.2.2. Rate and intensity Stimuli are delivered above the motor threshold sufficient to cause a 1–2-cm movement. This stimulation intensity can be determined prior to initiation of neuromuscular blockade. Otherwise, a default stimulus intensity value such as 20 mA is chosen. The presence of the stimulus can be checked during the case, even in the presence of neuromuscular
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blockade, by noting the continuing presence of the stimulus artifact in the recording channels. Needle electrodes can be used for nerve stimulation. Two are placed along the course of the nerve. This allows for secure location, and avoids the problems of slippage or drying of the conductive paste during long cases. It also avoids changes in skin resistance that can occur over many hours of long cases. Some users prefer using traditional surface disc electrodes or bar electrodes with two disc or rectangular contacts. These would need to be secured in place, but without pressure enough to cause skin damage from prolonged pressure in long cases. For surface electrodes, paste should be calcium-free to avoid chemical skin burns from iontophoresis of those ions into the skin. Stimulation is delivered generally at approximately 5 stimuli/s (Nuwer and Dawson, 1984a). Faster stimulation would have been helpful to produce EPs quicker in the operating room. However, faster stimulation generally produces lower amplitude EPs, making monitoring more difficult. Slower stimulation can produce larger EPs, but slows the speed of creating new EP tracings. As illustrated in Fig. 1, the best trade-off of these two factors is about 5 stimuli/s/nerve. For patients with particularly large EPs, a EFFECTS OF STIMULUS RATE Rate ⫻ Amplitude
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11.3. Recording Recording at multiple sites above the level of surgery is helpful. In individual patients, it is difficult to predict in advance which specific sites will be most useful for monitoring. Some degree of flexibility of technique is helpful. Availability of many recording channels also helps greatly — many modern systems can track 16–32 channels simultaneously. Once monitoring under anesthesia is under way, the monitoring neurophysiologist or technologist can choose among the best recordable potentials available for that particular patient. These optimal recording sites and potentials can then be used for the further monitoring procedure itself. Near-field scalp recording channels are less affected by background muscle and movement artifact. Far-field potentials recorded from the cervical channel are less affected by changes in anesthetic concentrations. A combination of recording channels is usually optimal, rather than monitoring few channels. 11.3.1. Lower extremity recording channels
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slightly faster stimulation rate can be employed. For patients with particularly small EPs, a slower stimulation rate is helpful. Stimulation rates avoid exact multiples of 60 Hz (or 50 Hz), such as 5.00/s. That helps to average out any residual 60 Hz (or 50 Hz) artifact in the recording.
11.1
STIMULI PER (S)
Fig. 1. Effects of increasing the rate of stimulus presentation in one patient, for posterior tibial stimulation and scalp recordings. As rate is increased, EP amplitude decreases. The product rate amplitude helps compare the advantageous increase in speed of testing and the disadvantageous loss of amplitude. In the patient, the rate 5.1/s appeared to be the best compromise between speed and attenuation. (Reprinted from Nuwer and Dawson, 1984a, with permission from the International Federation of Clinical Neurophysiology.)
With lower extremity stimulation, recordings are taken from skin over the cervical spine and scalp vertex. These are designed to monitor far-field cervical and brainstem activity, as well as near-field somatosensory cortical potentials. The primary scalp active recording is made from site Cz0 (Cz-prime). The “prime” mark indicates a modified site located 2 cm posterior to the named International 10–20 system scalp site. This midline site Cz0 often is the optimal placement for recording for each of the left and right lower extremities. Secondary scalp active recording sites are offcenter at C10 and C20 , or at C30 and C40 . These are better recording sites than Cz0 in some patients. Orientation of the active somatosensory cortical generator varies among patients, a point illustrated in Fig. 2. Note that the dipole crosses the midline before appearing at the scalp. This results in a “paradoxical localization” of the primary cortical peak, which is best seen over the scalp ipsilateral to the leg stimulated.
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is often over the second cervical spine (CSp2). These are referenced to the forehead or shoulder. These channels are desirable because the recorded cervical and brainstem peaks are less affected by anesthesia depth. For cervical procedures, electrodes cannot be placed over the cervical spine. Instead, subcortical peaks are recorded from mastoid or ear electrodes. A lumbar channel is used occasionally to ascertain that the stimulus induced transmission arrived at the lumbar spinal cord. For older or obese patients, these lumbar potentials are difficult to record.
+ CZ C4
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11.3.2. Upper extremity recording channels
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B Fig. 2. Hypothesized variations in the electrical field distribution of the early cortical potentials upon stimulation of the posterior tibial nerve. These dipole orientation variations are based on the known variability of the location of the leg area in the cortex in the interhemispheric fissure. Notice how the positive peak generated in one hemisphere can show up best at the scalp vertex as in (A), or on the scalp overlying the other hemisphere as in (B). The latter phenomenon has been called “paradoxical localization.” (Reprinted from Seyal et al., 1983, with permission from the International Federation of Clinical Neurophysiology.)
Several reference electrodes sites are in common use. Many monitorists measure from Cz0 using a forehead, ear, or mastoid reference site. For the offcenter scalp active sites, many users reference the contralateral electrodes, that is, C10 –C20 and C30 – C40 . Short distances to the reference reduce noisiness in channels due to distant sources. But short distances also may reduce some peak amplitudes. The optimal recording technique varies among patients. The subcortical peaks are recorded from electrodes placed at the neck, ear, or mastoid. The neck site
With upper extremity stimulation, recordings are taken from skin over the shoulder, cervical spine, and scalp vertex. These are designed to monitor farfield cervical and brainstem activity, as well as near-field somatosensory cortical potentials. The primary scalp active recording sites are at C30 and C40 . Several reference electrode sites are in common use, including forehead, ear, or mastoid reference sites. Some users reference the contralateral electrodes, that is, C30 –C40 . For some intracranial surgery, those sites may be in the surgical field. In those cases, modified scalp sites are needed. In some cases, subdural or direct cortical electrodes are placed in the surgical field. Keeping the latter electrodes in place throughout a procedure requires tethering of the electrode’s cable with sutures or other ad hoc techniques. The subcortical brainstem or cervical peaks are recorded from electrodes placed at the neck, ear, or mastoid. The neck site is often over the fifth cervical spine (CSp5), directly over the median nerve’s cervical peak generator located in the spinal cord at the C5 level. These sites are referenced to the forehead or contralateral shoulder. These channels are desirable because the recorded cervical and brainstem peaks are less affected by anesthesia depth, and to assure monitoring of the peripheral portion of the pathway. An Erb’s point channel is sometimes used to ascertain that the stimulus-induced transmission arrived at the proximal brachial plexus. Erb’s point is located above the clavicle, 2 cm lateral to the insertion of the sternocleidomastoid muscle. 11.3.3. Filters Recording filters are generally chosen at 30 Hz and 3 kHz. These filter settings are chosen to optimize
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noise rejection while retaining the principal EP characteristics in a typical surgical setting. They also minimize the ordinary background fluctuations in EP due to minor changes in anesthetic depth (Nuwer and Dawson, 1984a). Open filters (e.g., a low filter of 1 Hz) are more susceptible to variability due to background noise. The individual techniques used are open to clinical choice by the neurophysiology monitoring team. Figure 3 illustrates the effects of various filter settings. In this case, the optimal trade-off occurred for a lower filter set at 30 Hz. Lower filter settings led to a greater amount of background variability. Higher settings of the low filter resulted in attenuation of the desired EP. Figure 4 shows tracings from scalp recordings during a routine monitoring during surgery for scoliosis. This shows an example of stable recordings from upper and lower extremity stimulation. Such stable, low-noise, easily reproducible potentials are desirable for spinal cord monitoring. The 60 Hz (or 50 Hz) notch filter usually is kept off. That filter can interact with the stimulus signal itself, resulting in a sinusoidally decaying artifact. The artifact has a 60 Hz (or 50 Hz) frequency, resulting in peaks at 16.6, 33, and 50 ms. Some users have mistaken these artifacts for stable EPs, which do not change during the monitoring period.
11.4. Peaks, generators, and alarm criteria 11.4.1. Lower extremity peaks For posterior tibial nerve techniques, an N22 peak appears as a negative polarity near-field potential over the lumbar spinal cord at about 22 ms after the stimulation. The lumbar spinal cord anatomically lies around the T12 or L1 spine, so the potential is seen best over those spines. See Fig. 5 for an illustration of typical routine lower extremity SEP peaks and their nomenclature. A cervical peak is seen using far-field recording from the neck region to a distant reference. The peak often occurs around 30 ms. The generator is thought to be the gracilis nucleus, one of the posterior column nuclei at the cervicomedullary junction. The lemniscal pathways synapse at that level. That peak may be followed by an opposite polarity trough that may represent conduction up the brainstem’s medial lemniscus. The P37 is generated at the primary somatosensory cortex. As seen in Fig. 2, it may show up at the midline or off-center, paradoxically, on the side of the leg stimulated. It may have a doubled negative peak, which sometimes leads to peak-picking errors by inexperienced users. When encountering the doubled peak morphology, a user might sometimes choose the first, and at other times choose the second
EFFECTS OF VARIOUS FILTER SETTINGS
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Fig. 3. Effects of four different filter settings during intraoperative recordings in one patient, taken from a single scalp channel (Cz channel). The four recordings were made simultaneously from the same data. Variability is greatest in the 1 Hz channel and is reduced by higher filter settings. Each pair of EPs is a typical set of two consecutive recordings. The amplitude scale is doubled for the lower two EPs. (Reprinted from Nuwer and Dawson, 1984b, with permission from Lippincott Williams and Wilkins.)
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Normal Stable Somatosensory Evoked Potentials Cervical-Fz
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Fig. 4. Normal stable SEPs. Note the good reproducibility for the tracings in each channel. The montage includes upper and lower extremity tracings, collected separately for each limb. Cortical and subcortical (cervical) channels are used. (From UCLA Department of Clinical Neurophysiology, with permission.)
peak, leading to an erroneous impression that the latency has shifted. Watching the peak shape (morphology) can help to avoid peak-picking errors. The primary measurements are the P37 peak’s amplitude and latency. Secondary measurements are the amplitude and latency for the cervical peak. Latencies should stay within about 5–10% of baseline values, for example, 3–4 ms of normal variation for a 37-ms P37 peak. Amplitudes should stay within 50% of baseline values. Some users raise alarms at 30% decrease of the P37, since such a degree of amplitude drop is uncommon among well-defined P37 peaks. These latencies and amplitudes usually are chosen after the patient has been under anesthesia for 20 min, since there is a period of gradual amplitude loss and minor latency increase due solely to the anesthetic itself. That effect takes 20–30 min to take effect after induction. Even during that time period, though, monitoring needs to watch for early deterioration due to positioning on the table or problems related to intubation. Anesthetic effect is distinguished from clinical pathway impairment by
observing the cervical peaks, since they are relatively steady despite anesthesia. It is the P37 cortical peak that is affected more greatly by the onset of anesthetic. This trick applies to spine surgery, but not to intracranial surgery. The effects of anesthetics are described in detail in another chapter. 11.4.2. Upper extremity peaks For median or ulnar nerve techniques, an N9 peak appears as a negative polarity near-field potential over the proximal brachial plexus at about 9 ms after the stimulation. The electrode best used to measure this is at a place referred to as Erb’s point, and the peak itself sometimes is referred to as the Erb’s point peak. Fig. 6 illustrates typical routine upper extremity SEP peaks and their nomenclature. A cervical peak is seen using far-field recording from the neck region to a distant reference. The negative polarity peak often occurs around 13 ms. The generator of this N13 is thought to be the cervical spinal cord gray matter at the level where the nerve
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Fig. 5. SEPs from posterior tibial nerve stimulation are shown. Typical peaks N8, N22, and P37 are noted. A cervical peak was also found. These peaks have normal latencies and amplitudes. (Reprinted from Nuwer et al., 1994, with permission from the International Federation of Clinical Neurophysiology.)
enters, synapses onto reflex arcs, and send axons rostrally into the posterior column. For the median nerve, this is in the C5 spinal cord. Shortly thereafter, a positive polarity peak is generated probably from the cuneate nucleus, one of the posterior column nuclei at the cervicomedullary junction. This peak is referred to as the P14. The lemniscal pathways synapse at that level. That peak may be followed by a negative polarity N18 that may represent conduction up the brainstem’s medial lemniscus and into the thalamus. The N18 is a broad potential that gradually decreases over about 10 ms. Both the P14 and N18 are best seen as a far-field potential seen broadly over the scalp using a non-cephalic reference. The N18 may be obscured by the N20 when recording near that peak’s active site. The N20 is generated at the primary somatosensory cortex. This usually is a large well-defined peak seen over the C30 or C40 scalp site. It is a near-field potential, that is, its amplitude drops off when the electrode is relocated even a short distance away from the optimal recording site. The primary measurements are the N20 peak’s amplitude and latency. Latencies should stay within about 5–10% of baseline values, for example, 1–2 ms
Fig. 6. SEPs from median nerve stimulation are shown. Typical peaks are shown in each of four recording channels. The test is normal. (Reprinted from Nuwer et al., 1994, with permission from the International Federation of Clinical Neurophysiology.)
of normal variation for a 20 ms N20 peak. Amplitudes should stay within 50% of baseline values. Some users raise alarms at 30% decrease of the N20, since such a degree of amplitude drop is uncommon among welldefined N20 peaks. As with the lower extremity P37 peak, these latencies and amplitudes usually are chosen after the patient has been under anesthesia for 20 min. That effect takes 20–30 min to take effect after induction. The effects of anesthetics are described in detail in another chapter. 11.5. Interpretation of changes: alarms and considerations Generally, monitoring teams use both latency and amplitude criteria for raising an alarm. A 50% drop in recorded potentials is generally considered to be sufficient for raising an alarm (Nuwer, 1986; Nuwer et al., 1995). Latency increases greater than 5–10% also are cause for alarm. Latency measures need to take into account temperature effects. Amplitude measures need to take into account anesthetic effects especially for cortical potentials. The initial period
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Systemic factors can cause EP changes. Examples are hypotension and hypoxia. Occasionally, such systemic changes are first detected by EP changes. A patient with preexisting neurologic problems, such as spinal cord compression, may be particularly sensitive to otherwise minor degrees of hypotension. Surgical problems causing EP attenuation include direct cord trauma, excessive traction, blunt trauma, excessive compression, stretching of the cord from spinal distraction, vascular insufficiency from compression, embolus, thrombus, or occasionally other clinical problems. Anesthetic effects are common. Monitoring often assesses not only the lower extremity SEPs, but also the upper extremity SEPs even in a thoracic or lumbar procedure. This allows for an assessment of anesthetic effect. Usually, that affects all cortical channels but causes little change in subcortical channels. See Fig. 7 for an example of an asymmetric
after anesthetic induction is accompanied by latency and amplitude changes as the nervous system equilibrates over 20–30 min. Experienced monitoring teams learn how to factor out these extraneous causes of variability and so minimize false alarms due to such effects as boluses of medication, raised levels of inhalation anesthetics, and decreasing temperature. When changes occur, the monitoring team needs to assess quickly whether these have a technical, systemic, or surgical cause. Technical and systemic issues also can cause change and need to be investigated. Technical problems can occur from the electrodes themselves, for example, if they become disconnected. Equipment can malfunction. A good stimulus artifact or motor signs of acceptable stimulus help assure the adequacy of a stimulus. Impedance checks and review of raw input data can assure the recording system’s integrity.
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Fig. 7. The baseline testing shows a relatively attenuated left lower extremity cortical peak (left tracings). After an increase in anesthetic depth (right tracings), that channel no longer shows a reliable EP. (The baseline is superimposed on the newly acquired tracings at the right.) An anesthetic effect is the likely cause of the change, as suggested by both the preserved subcortical peaks for the affected pathway and somewhat attenuated cortical peaks in all other pathways. (From UCLA Department of Clinical Neurophysiology, with permission.)
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anesthetic effect seen in the setting of a partial preexisting unilateral impairment in one pathway. Occasional transient significant diminishing of EPs can occur without placing the patient at significant risk for postoperative neurologic problems. Even transient EP loss for a few minutes can occur without substantially raising the risk of postoperative problems, especially if the patient’s EPs return shortly thereafter to baseline levels of amplitude and latency. If EPs abruptly or slowly diminish to beyond the criterion levels for raising an alarm, the patient is at risk for postoperative new neurologic complications, especially if the EPs remain attenuated and prolonged through the end of the case. The gravest situation is the complete loss of previously easily detected EPs. Even in that circumstance, the risk of new neurologic postoperative impairment may only be about 50–75% (Nuwer et al., 1995).
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for many hour or days. In each delayed onset case, the patient awoke from surgery initially with good motor strength in the lower extremities. Motor function was lost several hours to several days later. The scoliosis surgery multicenter outcome study assessed the likelihood of false results. Those results are shown in Table 2. False negative studies were rare, and many of those were delayed onset. False positive outcomes were more common. Equivocal results are those with minor or transient changes. Table 3 shows the sensitivity and specificity results as determined in the same large study. 11.7. Conclusions SEP monitoring can be conducted with ankle or wrist stimulation and recording over the scalp and neck. These techniques are familiar to neurophysiologists and technologists who conduct routine outpatient
11.6. False results Occasional false positive monitoring events are seen. In a false positive event, SEPs show significant changes but the patient develops no postoperative deficits. Many false positive events may be true physiologic clinical changes detected by monitoring. In those cases, monitoring raises an alarm, the clinical team makes changes, and the patient has no neurological deficit because the interventions were successful. Others may be false alarms due to technical or anesthetic issues. False negative cases have been reported despite stable EPs in some studies (Tamaki et al., 1984; Wilber et al., 1984; Ginsburg et al., 1985; Johnston et al., 1986; Ben-David et al., 1987; Harper et al., 1988; More et al., 1988). Some such patients had patches of dysesthesia for several days or weeks postoperatively, possibly due to segmental spinal wiring. Rare cases have had serious, long-lasting neurological complications. Lesser and colleagues (Lesser et al., 1986) reported six cases from four institutions. Each suffered new postoperative neurologic deficits despite stable intraoperative SEPs. In three of these cases, the new impairment probably developed postoperatively, referred to as “delayed onset” deficits. Such delayed onset cases may be due to latent effects of vascular compromise or mechanical compression. Some may have been exacerbated by patient movement after awakening. Alternately, some compression or ischemia may gradually result in permanent impairment only if the condition remains in place
Table 2 Neurologic outcome prediction rates for SEP monitoring in spinal surgery (Total Procedures: 51,263 (100%)) False-negative (FN) rate: neurologic postoperative deficits despite stable SEPs: Definite 34 (0.063%) Equivocal 13 (0.025%) Delayed onset 18 (0.035%) Total 65 (0.127%) False-positive rate: no neurologic deficits despite SEP changes: Definite 504 (0.983%) Equivocal 270 (0.527%) Total 774 (1.510%) True-positive (TP) rate: neurologic deficits predicted by SEP changes: Definite 150 (0.293%) Equivocal 67 (0.131%) Total 217 (0.423%) Neurologic deficits (FN plus TP): Definite 184 (0.356%) Equivocal 80 (0.156%) Delayed onset 18 (0.035%) Total 282 (0.550%) True-negative rate: no neurologic deficit and stable SEPs: Total 50,207 (97.94%) These data are from the multicenter outcome study of SEP spinal cord monitoring in scoliosis organized through the Scoliosis Research Society (Nuwer et al., 1995). They were obtained from 153 US surgeon respondents. Note the very low rate of definite false negative cases (0.063%).
SOMATOSENSORY EVOKED POTENTIALS Table 3 SEP monitoring validity measures Sensitivity 417/451 Specificity 50,207/50,781 Positive predictive value 417/991 Negative predictive value 50,207/50,241
92% 98.9% 42% 99.93%
These outcome measures are from the multicenter outcome survey of 153 US surgeons organized through the Scoliosis Research Society (Nuwer et al., 1995). The table shows a breakout of validity measures from that survey. The very high negative predictive value here indicates the high reliability of the monitoring when the SEP remains normal and stable. The outcome survey report (Nuwer et al., 1995) discusses in detail these data and related assumptions.
EP testing. In the nearly 30 years since these techniques were developed, they have spread into common use in surgery. They are appropriate for orthopedic, neurosurgical, and cardiovascular procedures. False negative cases are rare. False positive results occur in a small portion of cases. With appropriate choices of reference sites and filter settings, and with avoidance of excess inhalation anesthetic, suitable peaks can be found in most patients. References Ben-David, B, Haller, G and Taylor, P (1987) Anterior spinal fusion complicated by paraplegia. A case report of a false-negative somatosensory evoked potential. Spine, 12: 536–539. Ginsburg, HH, Shetter, AG and Raudzens, PA (1985) Postoperative paraplegia with preserved intraoperative somatosensory evoked potentials. J. Neurosurg., 6: 296–300. Harper, CM, Daube, JR, Lichy, WJ and Klassen, RA (1988) Lumbar radiculopathy after spinal fusion for scoliosis. Muscle Nerve, 11: 386–391. Johnston, CE II, Happel, LT, Jr., Norris, R, Burke, S, King, AG and Roberts, JM (1986) Delayed paraplegia complicating sublaminar segmental spinal instrumentation. J. Bone Joint Surg. Am., 68: 556–563.
189 Lesser, RP, Raudzens, P, Lueders, H, Nuwer, MR, Goldie, WD, Morris, HH, III, Dinner, DS, Klem, G, Hahn, JF, Shetter, AG, Ginsburg, HH and Gurd, AR (1986) Postoperative neurological deficits may occur despite unchanged intraoperative somatosensory evoked potentials. Ann. Neurol., 19: 22–25. Molaie, M (1986) False negative intraoperative somatosensory evoked potentials with simultaneous bilateral stimulation. Clin. Electroencephalogr., 17: 6–19. More, RC, Nuwer, MR and Dawson, EG (1988) Cortical evoked potential monitoring during spinal surgery: sensitivity, specificity, reliability, and criteria for alarm. J. Spinal Disord., 1: 75–80. Nuwer, MR (1986) Evoked Potential Monitoring in the Operating Room, Raven Press, New York, 246 pp. Nuwer, MR and Dawson, E (1984a) Intraoperative evoked potential monitoring of the spinal cord: enhanced stability of cortical recordings. Electroencephalogr. Clin. Neurophysiol., 59: 318–327. Nuwer, MR and Dawson, E (1984b) Intraoperative evoked potential monitoring on the spinal cord: a restricted filter, scalp method during Harrington instrumentation for scoliosis. Clin. Orthop., 183: 42–50. Nuwer, MR, Aminoff, M, Demedt, J, Eisen, AA, Goodin, D, Matsuoka, S, Mauguie`re, F, Shibasaki, H, Sutherling, W and Vibert, JF (1994) IFCN recommended standards for short latency somatosensory evoked potentials: report of an IFCN committee. Electroencephalogr. Clin. Neurophysiol., 91: 6–11. Nuwer, MR, Dawson, EG, Carlson, LG, Kanim, LEA and Sherman, JE (1995) Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey. Electroencephalogr. Clin. Neurophysiol., 96: 6–11. Seyal, M, Emerson, RG and Pedley, TA (1983) Spinal and early scalp-recorded components of the somatosensory evoked potential following stimulation of the posterior tibial nerve. Electroencephalogr. Clin. Neurophysiol., 55: 320–330. Tamaki, T, Noguchi, T, Takano, H, Tsuji, H, Nakagawa, T, Imai, K and Inoue, S (1984) Spinal cord monitoring as a clinical utilization of the spinal evoked potentials. Clin. Orthop., 184: 58–64. Wilber, RG, Thompson, GH, Shaffer, JW, Brown, RH and Nash, CL, Jr. (1984) Postoperative neurological deficits in segmental spinal instrumentation. J. Bone Joint Surg. [Am.], 66A: 1178–1187.