(BQ) Part 2 book A concise guide to intraoperative monitoring presents the following contents: Evoked activity, spine surgery, cranial surgery, artifacts and troubleshooting, closing remarks.
chapter Evoked Activity 7.1 Introduction A clinically important tool in assessing the integrity of cortical and subcortical neuronal relays is the study of evoked responses (ERs) which result from external stimulation of a neural pathway The rationale for using ERs intraoperatively is very simple: all naturally occurring external stimuli detected by the sense organs, such as sounds and lights, are transmitted to the brain in the form of electrical signals through various sensory neural pathways If these pathways are structurally and functionally intact, the signals reaching the brain give rise to certain patterns of activity Thus, like the natural stimuli, the delivery of experimental stimuli, such as tones or electrical pulses, and the simultaneous observation of the resulting patterns of activity provide an instantaneous display of the status of the sensory neural structures intervening between the stimulation and recording sites ERs can be subdivided further into averaged and nonaveraged responses, examples of which are the familiar evoked potentials (EPs) and the electrically triggered EMG, respectively In this chapter we present details on the use, features, stimulation, and recording procedures, as well as interpretation criteria of the various kinds of averaged and nonaveraged ERs 7.2 Evoked Potentials An EP is the electrical response of the nervous system to external stimulation There are two major types of EPs, sensory and motor In the former category, a stimulus is delivered peripherally (e.g., at a leg nerve) and the resulting response is recorded centrally (e.g., the cortex) In the latter category, a stimulus is delivered centrally (e.g., at the cortex) and the resulting response is recorded peripherally (e.g., at a leg nerve or muscle) Depending on the stimulus modality, sensory EPs are divided into somatosensory, auditory, and visual, indicated as SEPs, AEPs, and VEPs, respectively Early AEPs are referred to as brainstem auditory evoked responses (BAERs) Motor EPs can be 89 90 chapter 7: Evoked Activity further divided into neurogenic and myogenic, depending on whether the response is recorded at a nerve or at a muscle Single-trial evoked responses are not readily apparent in the background activity and, to detect them, averaging of several trials is necessary (see Section 4.4.10) The averaged EPs consist of an ordered series of negative or positive components (waves or peaks) of particular morphology, amplitude, and latency These three characteristics are the variables to be monitored intraoperatively For averaged responses, regardless of the stimulus modality, the stimulation rate should be relatively high, so that data are collected fast enough and average responses are updated sufficiently often to allow early detection of possible response changes However, this rate should not exceed a certain critical value, to avoid degradation of response amplitude and morphology Moreover, the interval between successive stimuli should not match the period of any oscillatory signals, such as the well-known 60 Hz power-line cycle, otherwise the averaged responses will contain a periodic artifact To avoid this synchronization problem, a noninteger stimulation rate should be used, such as, for instance, 4.7 Hz Also, in all modalities the analysis time or time base, that is, the length (in msec) of the segment of signal collected following each stimulus, is another factor to consider in selecting the stimulation rate If the interval between successive stimuli is shorter than the analysis time, a stimulus artifact will be present in the averaged response The analysis time is selected so that all peaks of interest fall within the analysis window During the course of surgery, ongoing responses (the last set of EPs) are compared against a set of baselines which are obtained after induction of anesthesia and final positioning of the patient and before any surgical manipulation However, if after the incision and before any surgical maneuvering, the responses have changed excessively due, for example, to drastic changes in anesthesia regime, such as use of different anesthetic agents or induction of hypotension, then the baselines should be reestablished Baseline recordings should be of familiar morphology, should contain clear and reliable components, and should also be consistent with the clinical picture of the patient However, one should keep in mind that the purpose of intraoperative monitoring is to detect response changes due to surgery, not to make a clinical diagnosis Baselines should remain on the screen for comparison with the current responses throughout the case Like the ongoing activity presented in Chapter 6, evoked responses are affected by anesthetic agents, blood pressure, and body temperature, since all these factors can alter blood perfusion and metabolic rate in neural cells In the following sections we concentrate on different types of evoked activity typically recorded during the course of neurological, orthopedic, or vascular surgery and we give details regarding the generation, information content, recommended electrode locations, and typical acquisition parameters A quick summary of the various factors that affect the recorded neurophysiological signals, such as pharmacological agents and induced neuroprotective conditions, is also presented, along with information to assist with the interpretation of the results 7.3 Somatosensory Evoked Potentials 7.3 7.3.1 91 Somatosensory Evoked Potentials Generation Somatosensory evoked potentials (SEPs) can be elicited by electrical stimulation of a peripheral nerve, such as the median nerve at the wrist or the posterior tibial nerve at the ankle The location of these nerves is schematically shown in Figure 7.1 Radial nerve Common Peronial nerve Ulnar nerve Median nerve Tibial nerve (a) (b) Figure 7.1 Schematic diagram of (a) the median nerve at the wrist and (b) the posterior tibial nerve at the ankle These nerves are part of the somatosensory system, a schematic diagram of which is shown in Figure 7.2 Evoked activity travels along the stimulated nerve and enters the spinal cord through the dorsal roots From there, ascending pathways take the impulses first to the brainstem, then to the thalamus and, finally, to the primary sensory cortex Ascending volleys of SEPs can be recorded at any point along this pathway More specifically, activity within the spinal cord is conveyed by the dorsomedial tracts, and remains ipsilateral to its side of entry A first synapse is formed in the medulla, the inferior portion of the brainstem, in the nucleus gracilis for fibers from the lower portion of the body and in the nucleus cuneatus for fibers from the upper portion of the body Fibers leaving the medulla decussate to form the contralateral medial lemniscus and terminate in the thalamus, where a second synapse is formed Fibers leaving the thalamus terminate in the sensory cortex in a somatotopic arrangement Legs are represented close to the midline, whereas arms and hands are represented more laterally A diagram of the somatotopic arrangement of the primary sensory cortex is shown in Figure 7.3 chapter 7: Evoked Activity 92 Cerebral Cortex Thalamus Medulla C7 Brachial Plexus Median Nerve S1 Sacral Plexus Posterior Tibial Figure 7.2 Schematic diagram of the somatosensory system 7.3.2 Use Somatosensory evoked potentials are used intraoperatively to: • Monitor blood perfusion of the cortex or the spinal cord (e.g., during an aneurysm clipping) • Monitor the structural and functional integrity of the spinal cord during orthopedic or neurological surgery (e.g., for scoliosis or a spinal tumor) • Monitor structural and functional integrity of peripheral nerves (e.g., sciatic nerve during ascetabular fixation), spinal nerve roots (e.g., during decompression in radiculopathy), and peripheral nerve structures (e.g., the brachial plexus) 7.3.3 SEP Features Figure 7.3 lus.” 93 Somatotopic arrangement of the primary sensory cortex showing the “homuncu- • Determine functional identity of cortical tissue (e.g., one can separate the sensory from motor cortex by identifying the central sulcus) 7.3.3 SEP Features In general, monitoring protocols require stimulation of the left and right sides of the body independently, resulting in two sets of responses, one from each side Typical recordings include a peripheral, a subcortical, and a cortical response The peripheral response is typically recorded from the Erb’s point for arm stimulation, or the popliteal fossa for leg stimulation The two central responses are obtained from a cervical and a cortical location, respectively The locations of the stimulating and recording electrodes are schematically shown in Figures 7.4 and 7.5 for arm and leg stimulation, respectively Normal SEPs consist of clear, reliable, and bilaterally symmetric components That is, the waveforms obtained have standard, known morphology, and the individual peaks are clearly identifiable against the background (noise-free recordings) Additionally, repeated recordings from the same limb result in similar (within 10%) amplitudes and latencies Similarly, the difference in amplitude and latency between the two limbs is minimal (typically, less than 10%) 7.3.4 Recording Procedure The choice for the peripheral nerve to stimulate depends on the site of surgery [54] Typically, if the site of surgery is (1) above the level of the seventh cervical vertebra (Cvii), one should stimulate the median nerve; (2) above and including the level of chapter 7: Evoked Activity 94 C4' Fpz Erb's point Cii Ground Stimulation Figure 7.4 Location of the stimulating and recording electrodes to record median nerve SEPs Cvii, the ulnar nerve; and (3) below Cvii, the posterior tibial nerve It is recommended, however, to always monitor brachial plexus function, through ulnar nerve stimulation, to avoid a possible plexopathy from improper positioning of a patient’s shoulders Stimulation Parameters Electrical stimulation of a peripheral nerve is commonly used to elicit somatosensory responses which can be recorded from the spinal cord or the brain The number of fibers excited by an electrical stimulus is determined by the amount of current delivered Constant current stimulation results in more stable responses, because the number of fibers excited with each stimulus remains the same The intensity and duration of the stimuli are adjusted so that the stimulation achieved is supramaximal [54, 66], that is, all neuronal axons are forced to fire However, care should be taken to avoid skin damage and local burns from stimuli of excessively high intensity or long duration Typical intensity values are 25 mA for arm stimulation and 50 mA for leg stimulation The stimulus duration is set at 0.3 msec in both cases [57] As explained in Section 7.2, a noninteger stimulation rate, such as 4.7 Hz, is used to avoid synchronization with power line interference A time base of 100 msec is sufficient to produce reliable responses with all peaks falling within the analysis window [20] Recording Parameters When recording cortical responses, which represent mostly activity on neuronal dendrites, a bandwidth between 10 and 300 Hz is required, whereas for subcortical activity, which is primarily due to axonal sources, a frequency range between 10 7.3.4 Recording Procedure 95 Cz' Fpz Cii Ground Popliteal Fossa Stimulation Figure 7.5 Location of the stimulating and recording electrodes to record posterior tibial nerve SEPs and 3000 Hz is necessary Reliable SEPs can be obtained with 300 trials for arm stimulation and 500 trials for leg stimulation [54] Recording Sites The somatotopic arrangement of the sensory cortex should be kept in mind when recording SEPs The electrodes are placed on the scalp on specific locations in order to obtain maximum responses to sensory stimuli, according to the 10–20 international placement system used in clinical applications The typical electrode locations for recording median nerve SEPs are shown in Figure 7.6(a), where arrows indicate the recording montage For simplicity, only the channels corresponding to right-hand stimulation are shown Similarly, typical electrode locations for recording posterior tibial nerve SEPs are shown in Figure 7.6(b) In this case, an additional channel, not shown in Figure 7.6(b), is used for the recording from the popliteal fossa Electrode C3 , C4 , and Cz are placed cm behind C3 , C4 , and Cz , respectively Table 7.1 summarizes the acquisition parameters recommended for intraoperative monitoring of median and posterior tibial nerve SEPs, respectively chapter 7: Evoked Activity 96 Ground EPL EPR Ground (a) (b) Figure 7.6 Typical electrode locations for intraoperative recordings of (a) median nerve and (b) posterior tibial nerve SEPs Table 7.1 Recommended Parameter Settings for Recording Median and Posterior Tibial Nerve SEPs Side (stim) Recording Channel Bandwidth Fpz –C4 Left Right Fpz –Cii Fpz –EPL Stimulation Intensity Rate Duration Median Nerve Time Base Sensitivity 100 msec 10 µV 100 msec 10 µV 10–300 Hz Fpz –C3 20–2000 Hz 10–300 Hz Fpz –Cii Fpz –EPR 10–2000 Hz 25 mA 4.7 Hz 0.3 msec Posterior Tibial Nerve Left Right 7.3.5 Fpz –Cz Fpz –Cii P FL Fpz –Cz Fpz –Cii P FR 10–300 Hz 10–2000 Hz 10–300 Hz 50 mA 4.7 Hz 0.3 msec 10–2000 Hz SEPs to Arm Stimulation A common technique is to stimulate the median nerve at the wrist1 while recording along the nerve pathway, initially from Erb’s point, a clavicular location shown in Figure 7.7, then from a cervical point at the level of the second vertebra (Cii ), and finally from the contralateral parietal cortex (C3 or C4 ) As explained in Section 3.5.2, the negative stimulating electrode is always placed closer to the recording side 7.3.5 SEPs to Arm Stimulation 97 Figure 7.7 Anatomic location of Erb’s point When the wrist is not accessible, as when, for example, the patient’s arm is in a cast, the median nerve can be stimulated at alternate sites, namely at the elbow or the axilla The correct locations for placing the stimulating electrodes at the wrist, elbow, and axilla are shown in Figure 7.8 Figure 7.8 Placement of stimulating electrodes along the median nerve pathway Similar responses are detected from ulnar or radial nerve stimulation, although the amplitude of individual peaks is lower, apparently due to a smaller number of fibers being activated [66] Figure 7.9 shows the correct sites for placing the stimulation electrodes along the pathway of the ulnar nerve at the wrist and at the elbow To record SEPs, the active (negative) electrodes are placed over the Erb’s point, the cervical Cii vertebra, and the C3 and C4 locations on the scalp Electrode C3 and C4 are placed cm behind C3 and C4 , respectively The inactive (positive) electrode is placed on the forehead (Fpz ) [20] with a ground on a shoulder Approximately msec after stimulation of the median nerve at the wrist the Erb’s point electrode detects a negative component (N9), which represents action potentials generated by the peripheral nerve fibers contained in the brachial plexus [9] About chapter 7: Evoked Activity 98 Figure 7.9 Placement of stimulating electrodes along the ulnar nerve pathway 13 msec following stimulation the cervical electrode detects a major negative component (N13), which is generated probably by several sources in the dorsal column of the spinal cord This component is presumably made up of both excitatory postsynaptic potentials and action potentials The most important scalp-recorded component has a negative peak at about 20 msec which is followed by a positive peak at about 25 msec, forming the N20–P25 complex The N20 probably originates from the parietal sensory cortical area contralateral to the side of stimulation [66] An example of typical components obtained along the sensory pathway after stimulation of the median nerve at the wrist is shown in Figure 7.10 Notice the symmetry of the responses obtained on the left and right sides 7.3.6 SEPs to Leg Stimulation SEPs to leg stimulation can be obtained by stimulating the posterior tibial nerve at the ankle while recording peripherally from the popliteal fossa, and from cervical and scalp electrodes When the ankle is not accessible, as when, for example, the patient’s leg is in a cast, the posterior tibial nerve can be stimulated at the popliteal fossa The correct placement of the stimulating electrodes along the pathway of the posterior tibial nerve is shown in Figure 7.11 Similar responses are detected from peroneal nerve stimulation, although the amplitude of individual peaks is lower Figure 7.12 shows the correct sites for placing the stimulation electrodes along the pathway of the peroneal nerve To record SEPs, the active (negative) electrode for the peripheral response is placed above the popliteal crease, whereas the inactive (positive) electrode is placed on the medial surface of the knee The cervical and cortical responses can be obtained by placing the active (negative) electrode over Cii and Cz , respectively, whereas the inactive (positive) electrode for both responses is placed on the forehead (Fpz ) The popliteal fossa response consists of a negative component (N9) with latency approximately msec, and it is generated by the peripheral nerve fibers [9, 66] The cervical component (N30) has a latency of approximately 30 msec and probably reflects activity of nuclei in the dorsal column of the spinal cord The most prominent cortical component has a positive peak at about 37 msec and is followed by a negative Further Reading Burke, D and Hicks, R.G., Surgical monitoring of motor pathways, J Clin Neurophysiol., 15, 194, 1998 Cioni, B., Meglio, M., and Rossi, G.F., Intraoperative motor evoked potentials monitoring in spinal neurosurgery, Arch Ital Biol., 137, 115, 1999 de Haan, P., Kalkman, C.J., and Jacobs, M.J., Spinal cord monitoring with myogenic motor evoked potentials: early detection of spinal cord ischemia as an integral part of spinal cord protective strategies during thoracoabdominal aneurysm surgery, Semin Thorac Cardiovasc Surg., 10, 19, 1998 Deutsch, H., Arginteanu, M., Manhart, K., Perin, N., Camins, M., Moore, F., Steinberger, A.A., and Weisz, D.J., Somatosensory evoked potential monitoring in anterior thoracic vertebrectomy, J Neurosurg., 92, 155, 2000 Gharagozloo, F., Neville, R.F., Jr., and Cox, J.L., Spinal cord protection during surgical procedures on the descending thoracic and thoracoabdominal aorta: a critical overview, Semin Thorac Cardiovasc Surg., 10, 73, 1998 Harper, C.M and Daube, J.R., Facial nerve electromyography and other cranial nerve monitoring, J Clin Neurophysiol., 15, 206, 1998 Holland, N.R., Intraoperative electromyography during thoracolumbar spinal surgery, Spine, 23, 1915, 1998 Kartush, J.M., Intra-operative monitoring in acoustic neuroma surgery, Neurol Res., 20, 593, 1998 Nuwer, M.R., Spinal cord monitoring, Muscle Nerve, 22, 1620, 1999 10 Padberg, A.M and Bridwell, K.H., Spinal cord monitoring: current state of the art, Orthop Clin North Am., 30, 407, 1999 11 Robertazzi, R.R and Cunningham, J.N., Jr., Monitoring of somatosensory evoked potentials: a primer on the intraoperative detection of spinal cord ischemia during aortic reconstructive surgery, Semin Thorac Cardiovasc Surg., 10, 11, 1998 189 190 Further Reading 12 Rossini, P.M and Rossi, S Clinical applications of motor evoked potentials, Electroencephalogr Clin Neurophysiol., 106, 180, 1998 13 Schlake, H.P., Goldbrunner, R., Milewski, C., Siebert, M., Behr, R., Riemann, R., Helms, J., and Roosen, K., Technical developments in intra-operative monitoring for the preservation of cranial motor nerves and hearing in skull base surgery, Neurol Res., 21, 11, 1999 14 Slimp, J.C., Intraoperative monitoring of nerve repairs, Hand Clin., 16(1), 25, 2000 15 Sloan, T.B., Anesthetic effects on electrophysiologic recordings, J Clin Neurophysiol., 5, 217, 1998 16 Stump, D.A., Jones, T.J., and Rorie, K.D., Neurophysiologic monitoring and outcomes in cardiovascular surgery, J Cardiothorac Vasc Anesth., 13, 600, 1999 17 Taylor, C.L and Selman, W.R., Temporary vascular occlusion during cerebral aneurysm surgery, Neurosurg Clin N Am., 9, 673, 1998 Index AAA, 139 abdominal aortic aneurysm, 139 ABRs, 1, ac, 22 ACA, 156, 165 acetabular fixation, 144 acetylcholine, 17 ACom, 164 acoustic neuromas, 153 acromion, 80 action potential, 13, 15, 29 active electrode, 29 activity EEG, 25, 54, 71 EMG, evoked, 89 muscle, myogenic, 116 neurogenic, 116 of muscle cells, 16 of neural cells, 12 of peripherical nerves, 16 of the brain, of the cerebral cortex, 15 of the nervous system, rhythmic, 47 spontaneous, 3, 69 age and SEPs, 102 AICA, 155, 161, 165 alternating current, 22 American Electroencephalographic Society, amnesia, 63 amplifiers, 32 balanced, 38, 39 characteristics, 40 design of, 25 differential, 33 dynamic range, 40 gain, 32 input impedance, 35 inverting, 32 need for differential, 34 performance, 35 polarity convention, 40 sensitivity, 41 single-ended, 33 amplitude, 45 analgesia, 63 analog filters, 55 analog to digital conversion, 56 analysis time, 90 anatomic references, anatomy, anesthesia balanced, 64 components of, 63 drugs in, 17 effect on EEG, 74 general, 63 management, 3, 63 regime, anesthetic agents effects, 73 anesthetic drugs, anesthetics efficacy, 64 aneurysm, 159 aneurysms clipping, 164 aneurysms, common sites of, 161 191 Index 192 anterior, ACA, 156 acromion, 80 circulation, 164 circulation aneurysms, 165 communicating artery, 164 deltoid, 80 fossa, 149 fossa tumors, 156 inferior cerebellar artery, 155 tibialis, 81 antidromic sensory component, 115 arm stimulation, 96 arteriovenous malformation, 139, 159 artifacts, 174 artifacts and troubleshooting, 173 atoms, 21 auditory brainstem responses, 1, auditory EPs, 89 auditory tones, 57 averaged responses, 3, 89 averaging, 57 AVM, 168 AVMs, 139, 162 axial, 10 axon, 12 BA, 161 backbone, 125 BAERs, 55, 89, 106, 152, 154 affecting factors, 110 and SEPs, 161 features, 108 generation, 106 intraoperative interpretation, 110 recommended parameters for, 109 recording procedure, 108 recording sites, 108 stimulation approach, 109 use, 107 balanced amplifier, 38, 39 balanced anesthesia, 64 bandwidth, 54 barbiturates, 64, 65, 75, 100, 110 bare hunger, 174 baseline, 40, 45 basic concepts, electrical, 21 benzodiazepines, 64, 75, 100, 110 biceps brachii, 80 bioelectric signals, bipolar montage, 31 blood flow, 63 blood pressure, 3, 63 body temperature, bolus injection, 66 bovie, 29, 174 brachial plexopathy, 145 brachial plexus function, 130 brain, 11 brainstem, 91, 107, 114, 149, 156 and skull base, 162 auditory evoked responses, 89, 106 compressing the, 160 function, 108 infarction, 155 inferior portion of the, 91 ischemia, 77 lesion in the, 156 parts, 149 structures, 106 burst suppression, 71 capacitors and inductors, 25 CCT, 161 cell, 10 activity of muscle, 16 activity of neural, 12 axon of the, 13 body, 12 dead, 30 membrane, 11 negative, 11 nerve, 17, 29 pyramidal, 16 saver, 174 central conduction time, 161 central sulcus, central sulcus localization, 169 cerebellar tentorium, 149 cerebellum, 149 cerebral cortex, activity of the, 15 cerebrum, 149 changes due to surgery, 90 channels, 11 circle of Willis, 163 CMAPs, 17 Index CMRR, 36 collodion, 31 common mode gain, 36 common mode rejection, 35 common mode rejection ratio, 36 components of anesthesia, 63 compound muscle action potentials, 17 compound nerve action potentials, compressed spectral array, 49, 72 computerized EEG analysis, 48 condensation, 109 conductor, 22 connecting resistors in parallel, 25 connecting resistors in series, 23 constant current, 94 conus medullaris, 127 coronal, 10 cortex, activity of the cerebral, 15 cortical ischemia, 69 cortical neurons, 15 corticosteroids, 65 cost effectiveness of intraoperative monitoring, Coulomb’s Law, 21 cranial base, 149 cranial nerve surgery, 165 cranial nerves, 157, 166 cranial surgery, 149 CSA, 49, 72 CUSA, 174 data collection, 21 display, 58 lost, 42 processing, 49 storage, 6, 59 dB, 36 dc, 22 decibels, 36 decompression of the spine, 126 dendrites, 12 dermatomal somatosensory evoked potentials, 104 diazepam, 114 diencephalon, 149 differential amplifiers, 33 differential gain, 36 193 digital filters, 55 digitization, 56 direct and alternating currents, 22 disc disease, 131 discectomy, 131 display, displayed signal sensitivity, 41 double banana, 31 drugs, anesthetic, DSEPs, 104, 128 affecting factors, 106 features, 105 generation, 104 intraoperative interpretation, 106 recording procedure, 105 recording sites, 106 stimulation parameters, 105 use, 104 duration of a wave, 47 duration of the stimuli, 94 dynamic range of an amplifier, 40 early warnings of ischemia, 167 ECG, EEG, 1, 3, 9, 35, 45, 47, 69, 152 activity, 16, 25, 54 amplifier, 36 amplitude, 71 analysis, 48 burst suppression, 65 continuous recordings of, 162 effect of anesthesia on, 74 effects of age on, 77 electrodes, 30 epochs, 49 features, 71 flat, 71 frequency, 71, 75 from multiple areas, 34 generation, 70 instrumentation, 25 intraoperative interpretation, 77 locations, 112 machine, conventional, 41 monitoring, 165 recording procedure, 72 recordings, 27, 34, 48 signal, 39, 70 Index 194 silence, 74, 77 symmetry, 71 use, 70 effects of a filter, 50 a new noise source, 55 age on EEG, 77 anesthetic agents, 73 blood pressure changes, 66 changes in anesthesia, 63 filtering, 55 imbalances, 38 induced neuroprotective conditions, 76 low and high frequency filters, 54 effects on neurophysiological signals, 66 efficacy of anesthetics, 64 efficacy of monitoring, 173 electrical concepts, 21 electrical currents, 22 electrical pulses, 57 electrocautery, 29 electrodes, 26 10-20 system, 30 active, 29 characteristics, 26 disconnected, 174 EEG, 30 impedance identical, 37 impedance of the, 26 impedance small, 36 needle, 28 placement of recording, 30 placement of stimulation, 29 reference, 29 shared by amplifiers, 39 specially designed “hook”, 28 stick-on, 28 surface, 26 types, 21 types of recording, 28 types of stimulation, 27 electroencephalogram, 1, 70 electrolyte, 26 electromotive force, 22 electromyogram, 1, 17, 78 electrons, 21 electrophysiological monitoring, 169 electrophysiological recordings, 1, 45 EMG, 1, 3, 9, 17, 69, 127, 155 abnormal responses, 142 affecting factors, 82 and brachial plexus, 144 electrically triggered, 89 features, 78 generation, 78 intraoperative interpretation, 86 recording procedure, 79 spontaneous, 166 triggered, 118 endarterectomy, 2, 167 enflurane, 74, 99, 114 EP test, epilepsy, epoch, 49, 57 EPs, 1, 9, 57 auditory, 89 motor, 89 myogenic, 90 neurogenic, 90 sensory, 89 somatosensory, 89 types of, 89 visual, 89 EPSP, 14 equipment failure, Erb’s point, 132 ERs, 89 etomidate, 64, 75, 100, 114 evoked activity, 89 evoked potentials, 1, 9, 89 evoked responses, 3, 9, 17, 50, 89 excessive noise, 177 excitatory postsynaptic potential, 14 external anal sphincter, 82 extramedullary tumors, 138 false negatives, 173 false positives, 173 fast Fourier transform, 47 fentanyl, 110 FFT, 47 filtering, 21, 50 filtering effects, 55 filters analog, 55 design, 25 Index digital, 55 frequency response, 50 high frequency, 50, 52 highpass, 50 impedance, 26 low frequency, 50, 51 lowpass, 50 notch, 50, 53 flat EEG, 71 Fourier transform, 47 frequency, 45 analysis, 47 content, 2, 27 domain, 46 low, 27 response, 50 functional groups, 10 functional integrity of the nervous system, functional unit, 11 fusion, 126 gain of an amplifier, 32 ganglia, 11 gastrocnemius muscle, 81 general anesthesia, 63 giant aneurysms, 139 ground, 29, 32 guidelines for proper intraoperative monitoring, halothane, 74, 99, 110, 114 Harrington rod, 129 head box, 29 Hermann hospital, 181 HFF, 50 high-amplitude responses, high frequency filters, 50, 52 highpass filters, 50 human body organization, hunchback, 129 hypercarbia, 76 hyperthermia, 110 hyperventilation, 66 hypervolemia, 66 hypotension, 63, 65, 76, 101 hypothermia, 63, 65, 76, 101, 110, 114 195 iatrogenic factors, 173 ICA, 159, 162, 167 identification of cranial nerves, 120 identification of neural tissue, 120 identification of root level, 120 imbalance, 39 imbalances effects, 38 impedance, 25 amplifiers input, 35 higher, 27 identical, 37 internal input, 35 of a filter, 26 of an amplifier, 26 of the electrodes, 26 small, 37 induced conditions, 65, 110 induced neuroprotective conditions, 76 inductor, 25 inhalation anesthetics, 74 inhalational agents, 63, 114 inhalational anesthetic agents, 110 inhalational anesthetics, 64 inhibitory postsynaptic potential, 14 input, 32 instrumentation, 21 insulator, 22 intensity of the stimuli, 94 internal carotid artery, 167 internal carotida artery, 162 internal input impedance, 35 intervention, 177 intracranial procedures, intramedullary tumors, 138 intraoperative monitoring, advantages, affecting factors, artifacts, 174 artifacts and troubleshooting, 173 cost effectiveness, EEG, 73 efficacy of, 173 equipment, 6, 21 factors to successful, 181 interpretation, intervention, 177 of compound nerve action potentials, personnel, Index 196 precautions in, 174 rationale of, remarks, 181 spinal cord, troubleshooting, 176 types of tests, use of, usefulness, intravenous agents, 63, 75, 100, 110 intravenous anesthetics, 64 intrinsic tumors, 153 inverse Fourier transform, 48 inverting amplifiers, 32 IOM, 1, 3, 127, 173 ions, 11 IPSP, 14 ischemia, 2, 4, 127, 150 brainstem, 77 cerebral, 65, 66 cortical, 69 early warnings of, 167 inducing local, 150 minimizing the risk of, 155 of the cerebral cortex, 77 of the internal capsule, 77 risk of brain, 161 isoelectricity, 74 isoflurane, 66, 74, 99, 110, 114 isolation device, 29 ketamine, 64, 100, 110 kyphosis, 129 leg stimulation, 98 LFF, 50 limb length, 102 localizing the somatosensory cortex, 170 low frequency filters, 50, 51 lowpass filters, 50 lumbar enlargement, 127 MAC, 64, 74 mannitol, 65 matter, structure of, 21 MCA, 156, 162, 167 mechanical injury, 2, 127, 150 mechanical insult, medial lemniscus, 91 medulla, 91 membrane cell, 11 depolarization, 13 meninges, 149 polarized, 11 potential, resting, 11 selectively permeable, 11 meningiomas, 153 MEPs, 127, 130, 134, 136 affecting factors, 118 features, 115 generation, 114 intraoperative interpretation, 118 monitoring of lower extremity, 139 myogenic, 116 neurogenic, 117 parameters, 117 recording procedure, 116 recording sites, 117 stimulation, 116 use, 115 metabolic rate, 63 microvascular decompression, 165 middle cerebral artery, 162, 167 middle fossa, 149 middle fossa tumors, 155 midsagittal, 10 minimum anesthetic concentration, 74 monitoring, intraoperative, monitoring personnel, 181 montages, 31 motor EPs, 89 motor evoked potential, 3, 114 multi-channel referential recordings, 39 multiple recording sites, 175 muscle cells activity, 16 muscle fibers, 16 muscle relaxants, 101, 110 muscle relaxation, 63, 65 myelin, 16 myogenic activity, 116 myogenic EPs, 90 N20-P25, 161 need for differential amplifiers, 34 Index needle electrodes, 28 nerves, 11 nerves activity, 16 nervous system, functional integrity of the, neural axons, 16 neural cells activity, 12 neurogenic activity, 116 neurogenic EPs, 90 neuromuscular blockers, 17 neuromuscular junction, 17 neuron, 12 cortical, 15 postsynaptic, 14 presynaptic, 14 neurophysiological background, neurophysiological signals, 11 neurophysiologist, neuroprotective agents, 65 neuroradiological procedures, 168 neurotransmitter, 14 neurotransmitter action, 63 neurovascular cases, neurovascular procedures, 158 neutrons, 21 nitroprusside, 66 nitrous oxide, 74, 99, 110, 113 noise, 38, 42, 49, 57 noise, excessive, 177 nonaveraged responses, 18, 89 notch filter, 50, 53 nucleus, 21 nucleus cuneatus, 91 nucleus gracilis, 91 Ohm’s Law, 23, 35 opiates, 64, 75, 100 optic chiasm, 111 optic nerve, 111 optic radiations, 111 optic tracts, 111 optimal recordings, 36 organ, 10 organization of the human body, origin of neurophysiological signals, 11 orthodromic motor component, 115 output, 32 197 paraplegia, 178 parasagittal, 10 patient positioning, 145 patient setup, 29 patients clinical status, 176 pedicle screw placement, 120 percutaneous stimulation, 116 perfusion, 63 peripheral nerve, 17 peripheral nerve monitoring, 143 peripherical nerves activity, 16 perisurgical factors, 3, 173 phase shift, 55 PICA, 161, 165 placement of recording electrodes, 30 placement of stimulation electrodes, 29 polarity convention of an amplifier, 40 polarized membrane, 11 popliteal fossa, 16 posterior, 10 posterior circulation, 162 posterior fossa, 149 posterior fossa aneurysms, 161 posterior fossa tumors, 153 postsynaptic neuron, 14 postsynaptic potential, 14 potential action, 13, 15, 29 compound muscle action, 17 compound nerve action, dermatomal somatosensory evoked, 104 difference, 22 evoked, 1, 9, 89 excitatory postsynaptic, 14 inhibitory postsynaptic, 14 motor evoked, 3, 114 postsynaptic, 14 resting membrane, 11 somatosensory evoked, 73, 91 visual evoked, 111 power spectrum, 48 preamplifiers, 39 precautions in monitoring, 174 preganglionic lesion, 143 presynaptic neuron, 14 propofol, 64, 75, 100, 110 protective induced conditions, 65 protons, 21 198 pyramidal cells, 16 radiculopathy, 2, 131 rarefaction, 109 rationale of intraoperative monitoring, reactance, 26 recording, recording channel, 29 recording electrodes, 28 recordings, electrophysiological, 45 rectus femoris, 81 reference electrode, 29 referential montage, 31 repair of brachial plexus, 143 resistance, 22, 26 resistors, 22 resistors in parallel, 25 resistors in series, 23 responses auditory brainstem, 1, averaged, 3, 18, 89 electrophysiological, evoked, 3, 9, 17, 50 high-amplitude, nonaveraged, 18, 89 resting membrane potential, 11 reversed patient, 86 rheumatoid arthritis, 134 rhizotomy, selective dorsal, 141 risk of infection, 28 saccular aneurysm, 160 sagittal, 10 sartorius muscle, 81 saturated amplifier’s output, 41 sciatic nerve, 144 scoliosis, 2, 4, 129 sedation, 63 segmental fixation, 129 selective dorsal rhizotomy, 141 selectively permeable membrane, 11 sella turcica, 156 sensitivity of a recording system, 24 sensitivity of an amplifier, 41 sensory EPs, 89 SEPs, 91, 127, 130, 152, 161 affecting factors, 99 Index age effects on, 102 and EEG combination, 165 baseline, 135 changes, 103 features, 93 induced conditions, 101 inhaled anesthetic agents, 99 intraoperative interpretation, 103 limb length, 102 median nerve, 163 monitoring, 132 monitoring median nerve, 132 monitoring of lower extremity, 139 recording, 136, 143 recording parameters, 94 recording procedure, 93 recording sites, 95 stimulation parameters, 94 tibial recordings, 144 to arm stimulation, 96 to leg stimulation, 98 upper and lower extremity, 165 use of, 92 set of baselines, 90 signal, 57 abrupt changes, 49 amplitude, 49 amplitude of a, 45 averaging, 57 bioelectric, bioelectrical, 69 characteristics, 45 common interference, 174 decomposed, 48 EEG, 39, 70 effects on neurophysiological, 66 electric, electrophysiological, EMG, 120, 155 enhancement, 49 frequency, 45 interference, 35, 38, 49 loss, 39 low frequency, 27 most commonly recorded, most useful, 54 neurophysiological, 9, 21, 63, 173 noise, 177 nonzero output, 41 Index origin of neurophysiological, 11 oscillatory, 47 power spectrum, 48 processing, quality of the recorded, 26 reconstruction, 57 sinusoidal, 45 spectrum of a, 46 tEMG, 166 to noise ratio, 42, 49 transfer of, 13 unrecognizable, 55 unwanted, 39 single-ended amplifier, 32 sites of aneurysms, 161 skin preparation, 31 skull base tumors, 156 smoothing operation, 49 SNR, 58 somatosensory cortex, 170 somatosensory EPs, 89 somatosensory evoked potentials, 73, 91 spasticity, 141 spatial summation, 15 spectogram, 46 spectrum of a signal, 46 spinal canal, 126 cord, 11, 126 cord damage, 130, 139 cord monitoring, deformities, 126, 128 degenerative conditions, 134 disc disease, 131 fractures and instabilities, 134 Harrington rod, 129 reflex arc, 141 segmental fixation, 129 stenosis, 131 tumors, 138 vascular abnormalities, 138 spine surgery, 125 spondylitis, 134 spondylolisthesis, 134 spondylosis, 134 spontaneous activity, 3, 9, 12, 69 stabilization of the spine, 126 stenosis, 134 stick-on electrodes, 28 199 stimulation, stimulation electrodes, 27 stimulation rate, 90 structural unit, 11 structure of matter, 21 SuCA, 161 supratentorial procedures, 162 surface electrodes, 26 surgery, 59 and BAER criteria, 110 and blood flow, 71 and DSEPs, 106 and intraoperative monitoring, and nerve injuries, 144 and tEMG, 118 and VEPs criteria, 114 below the conus medullaris, 127 blood lost in, 174 changes due to, 90 common procedures of spinal, 127 cranial, 149, 150 cranial nerve, 165 epilepsy, for acoustic tumors, 108 for disc disease, 132 for root decompression, 104 for tumor removal, 152 high cervical spine, 116 involving the optic nerves, 111 lumbosacral spinal canal, 118 neuro, 65 neurological, 63, 65 neuronal damage, neurovascular, 63 orthopedic, 63, 65 posterior fossa, 78, 118 risks, 150 scoliosis, 2, 92 spinal tumor, 92 spine, 125, 138 stenosis, 78 vascular, synapse, 14 systems, 11 tEMG, 141, 152 affecting factors, 120 features, 119 Index 200 generation, 118 intraoperative interpretation, 120 monitoring, 166 recording procedure, 119 recording sites, 120 stimulation, 119 use, 118 temporal and spatial summation, 15 test, wake-up, 178 tetanus, 78 tethered cord, 140 tibialis anterior, 81 time and frequency representation, 48 time base, 73, 90 time constant, 52 time domain, 46 tissue, 10 translaminar stimulation, 116 triceps, 80 trigeminal neuralgia, 165 triggered EMG, 3, 118, 141 tumor, 152 tumors in the spine, 138 types of amplifiers, 32 electrodes, 21 EPs, 89 evoked activity, 90 noise, 174 recording electrodes, 28 sensory stimulation, 18 stimulation electrodes, 27 surgery, tests in intraoperative monitoring, upper trapezius, 80 use of EMG, 78 use of intraoperative monitoring, vascular abnormalities, 138 ventral, VEPs, 156 affecting factors, 113 features, 112 generation, 111 intraoperative interpretation, 114 recording procedures, 112 recording sites, 112 stimulation approach, 113 use, 111 vertebrae, 125 vertebral column, 125 vertebral foramen, 126 vestibular schwannoma, 153 VII nerve, 155 VIII nerve, 154, 155, 166 visual EPs, 89 visual evoked potentials, 111 voltage, 22 voltage difference, 33 voltage divider, 24 wake-up test, 177 Willis circle, 163 ... negative peak at about 20 msec which is followed by a positive peak at about 25 msec, forming the N20–P25 complex The N20 probably originates from the parietal sensory cortical area contralateral to. .. differentiation of SEP changes due to iatrogenic factors is based on (1) evaluation of the change pattern (e.g., a sudden change vs a gradual change, or a change that affected the cortical component... cerebello-pontine angle and the posterior fossa 7.5.3 BAER Features The basic BAER features used for intraoperative analysis include measurement of peak amplitudes, as well as peak and interpeak latencies