Introduction to Evoked Potentials
Need to talk integratively about the overall function and transduction of evoked potentials and the significance to the patient. The dependency of accurate monitoring and successful techniques and the diligence required to acquire the signal. emphasize the fact that you are the last pieces of the puzzle to get between the patient and neurological injury. critical moments can last for a lifetime for a person. proper antiseptic technique and a more holistic view of what a person can do. the overall encompassing ideas before you dive right into the different tests. then, talk about the history, and how we got here. Talk about Wilder Pennfield and decerebrate rats, talk about the rich and interesting history of examining the brain and steps taken to get here. talk about what is possible now and what will be possible in the future.
Potentials measured in response to specific stimuli are called Evoked Potentials. The problem is that these potentials are often hidden among the ongoing electrical activity of cortical neurons/inherent noise of the nervous system not involved in processing the stimulus. Using a technique called signal averaging it is possible to separate out the evoked response. By taking many traces (EEG SSEP, etc) recorded after presentation of the stimulus and averaging them together, activity occurring randomly with respect to the stimulus is averaged out and only activity occurring at a constant latency from the stimulus remains. In this way evoked potentials can be visualized. Keep in mind however, that any large transient signal (e.g. due to head movement) occurring during the averaging can distort the average and any small signals occurring at a constant latency (e.g. from the stimulus delivery) can be the source of artifacts.
Intraoperative Monitoring is real-time assessment of neurophysiologic function during surgical casework to minimize neurologic trauma and injury and prevent permanent or worse damage of precious and vital neural function. Mediate the balance of anesthesia and provide live, real-time, continuous feedback to the surgeon and surgical team
EEG
The purpose of Evoked Potentials is to measure the response of the human brain to sensory stimulation in conjunction with different internal states. To monitor a living human nervous system requires a non-invasive/minimally invasive technique as opposed to the invasive techniques of neurosurgery. One such technique involves recording the electrical activity of the brain at the scalp. This recording is called the electroencephalogram. (EEG) In general, the EEG comes from synaptic currents flowing between dendrites and cell bodies of cortical neurons (however it is possible to pick up activity in deeper brain structures). The dipole electric field that these currents generate spreads to the surface of the head where it can be measured. At least three electrodes are needed; the "active," "reference," and common (ground) electrodes. The EEG is a measurement of the potential difference between the "active" and "reference" electrodes. The electrodes are connected to pre-amplifiers and the outputs of the amplifiers are fed into a data acquisition board in the computer. This board converts the analog output of the pre-amplifiers into digital form to be processed and displayed on the computer.
EEG Waveforms
The ongoing electrical activity of the cerebral cortex appears in the EEG as waveforms that vary in frequency from 1-30 Hz and in amplitude up to 100 uV. The waveforms most typically observed are designated by Greek letters.
- Alpha Waves vary in frequencey from 8-13 Hz and are of medium amplitude. They represent the synchronous activity of many cells. They are most easily recorded over the occipital and parietal lobes and are associated with a conscious but relaxed state
- Beta Waves are fast, desynchronized waves, which vary in frequency from 13-30 Hz and are of low amplitude. These waves are associated with alertness and arousal. They are most easily recorded from the frontal lobes but can be seen in other areas during intense mental activity.
- Delta and Theta Waves are slow, synchronized waves with high amplitudes and frequencies less than 8 Hz. They are associated with sleep in the normal adult.
The raw EEG record is complex and difficult to analyze because it consists of different frequency waveforms that sum together and vary over time. A common way to analyze EEG recordings is to plot the different waveforms in the EEG record according to their frequency. Analysis separates any complex waveform into its component frequencies and allows the EEG record to be visualized as the relative proportion (also called power) of the different frequency waveforms making up the raw signal. The resulting plot, which displays power vs. frequency is called a Power Spectrum.
Intraoperative Neurophysiology
Over the past 30 years, neurophysiologic intraoperative monitoring (IOM) has grown from an interesting investigational procedure into a widely used method to protect patients from neurologic injury during surgery.
IOM techniques include most neurophysiologic modalities commonly used among outpatients. These include electroencephalography (EEG), electromyography (EMG), evoked potentials (EPs), and nerve conduction velocity (NCV) testing of various types. IOM also includes some techniques not used in out- patients, such as transcranial electrical motor EPs (tceMEPs).
IOM helps in a number of ways. Most obviously, it can warn the surgeon of a serious complication in time to intervene and correct the problem before it becomes permanent. Second, it sometimes identifies a serious systemic problem that needs to be corrected. Third, the surgeon can feel comfortable about the patient’s neurologic safety to that point in the case, and therefore go forward to provide a more thorough procedure. Fourth, with IOM the surgeons can feel more confident about a procedure’s safety, allowing surgery on a high-risk patient who might otherwise be turned away. Fifth, the patient and his or her family can take comfort that the very real neurologic risks of surgery are lessened by IOM.
IOM is not a perfect procedure. False positive cases (false alarms) occur in a portion of cases. In scoliosis, that rate is around 1% of procedures. In some other types of procedures, the rate is higher. These may be due to problems with the technique itself, the difficulty of obtaining good quality tracings from some patients, or as a result of anesthetic changes. True positive cases (true predictions of postoperative deficits) also occur. Those are cases where IOM raises an alarm, any avail- able interventions are accomplished, but the patient has neurologic injury anyway. Just because an alarm is raised does not necessarily allow for prompt, complete correction of the problem. False negative cases are those in which the patient suffers from a neurologic injury that was not predicted by IOM changes. False negative cases are rare, but do occur. Some are due to immediate postoperative deterioration. Other cases are a result of injury in pathways not monitored. Occasionally, they are due to errors by the IOM team, who failed to recognize changes when they occurred.
Two fundamentally different types of procedures are used in surgery. They differ not by their techniques but by their goals. IOM aims to watch over neurologic pathways and to identify any signs of injury. Intraoperative testing aims to identify neurologic structures. Monitoring occurs over many hours, whereas testing is usually done at one or several discrete times. Despite the differences between these two different concepts, the two are often lumped together and referred to as IOM.
Monitoring requires sufficient expertise by the team and good communication with the surgeon. Details depend upon the particular techniques, especially on whether the aim is testing or monitoring.
Testing to identify a particular neurological structure usually requires personal involvement of a clinical neurophysiologist. This is the case for ECoG, or for localization of language or motor cortex. These require professional judgment about what tissue to resect, a level of skill greater than that of a technologist alone. The clinical neurophysiologist is in the room to make decisions and discuss interpretations and recommendations personally with the surgeon. This is referred to as personal supervision.
After training, the fellow should understand
- basic sciences of anatomy, physiology, and pharmacology of pathways monitored in surgery
- various neurophysiological techniques used in surgical monitoring and testing
- how to use IOM to predict and prevent adverse neurologic outcomes
- how to use IOM to locate and identify neurologic structures during surgery
- effects of anesthesia, systemic disorders, and co- morbidities
- how to train and supervise technologists to conduct monitoring
- normal variations and criteria for abnormality and alarms
The fellow should have extensive clinical experience in use of the common IOM techniques including
- EEG monitoring
- ECoG
- somatosensory EP spinal cord monitoring
- somatosensory EP motor cortex localization
- motor EP spinal cord monitoring
- brainstem auditory EP monitoring
- EMG cranial nerve monitoring
- EMG monitoring of trunk and limbs
- pedicle screw stimulation
- NCV peripheral testing
- deep brain stimulator placement
- monitoring from remote sites
The fellow should have extensive clinical experience with common clinical uses of IOM including
- cerebral aneurysm clipping
- cerebral tumor resection
- epilepsy surgery
- deep brain stimulator implantation
- cranial base and posterior fossa tumor resection
- scoliosis correction
- cervical myelopathy decompression
- peripheral nerve repair or decompression
- CEA
- aortic and cardiac surgery
Overall, the field of intraoperative neurophysiology has developed from disparate difficult techniques in its earlier days three decades ago. Now, it has become a discipline with a large number of techniques, applied to a variety of surgical and other procedures. The monitoring teams encompass many types of specialists contributing their own expertise. The goals remain the same as before: to enhance patient care, avoid neurological deficits, allow for more complete procedures, allow procedures even on some high-risk patients, and provide feedback to the surgeon about actions that could injure the nervous system.
Burke, D, Hicks, R, Stephen, J, Woodforth, I and Craw- ford, M (1992) Assessment of corticospinal and somato- sensory conduction simultaneously during scoliosis surgery. Electroencephalogr. Clin. Neurophysiol., 85: 388–396.
Engler, GL, Speilholz, NI, Bernhard, WN, Danziger, F, Merkin, H and Wolf, T (1978) Somatosensory evoked potentials during Harrington instrumentation for scolio- sis. J. Bone Joint Surg. (Am), 60: 528–532.
Grundy, BL (1982) Monitoring of sensory evoked poten- tials during neurosurgical operations: methods and applications. Neurosurgery, 11: 556–575.
Hicks, RG, Burke, DJ and Stephen, JP (1991) Monitoring spi- nal cord function during scoliosis surgery with Cotrel- Dubousset instrumentation. Med. J. Aust., 154: 82–86.
Imai, T (1976) Human electrospinogram evoked by direct stimulation on the spinal cord through epidural space. J. Japanese Ortho. Assoc., 50: 1037–1056.
Jasper, HH (1949) Electrocorticograms in man. Electroen- cephalogr. Clin. Neurophysiol., Suppl. 2: 16–29.
Jones, SJ, Edgar, MA and Ransford, AO (1982) Sensory nerve conduction in the human spinal cord: epi- dural recordings made during scoliosis surgery. J. Neurol. Neurosurg. Psychiatr., 45: 446–451.
Marshall, C and Walker, AE (1949) Electrocorticography. Bull. Johns Hopkins Hosp., 85: 344–359.
Mller, AR (1988) Evoked Potentials in Intraoperative Moni- toring, Williams & Wilkins, Baltimore, 224 pp.
Mller, MB and Mller, AR (1985) Loss of auditory func- tion in microvascular decompression for hemifacial spasm. Results in 143 consecutive cases. J. Neurosurg., 63: 17–20.
Nash, CL, Jr. and Brodkey, JS (1977) Clinical Application of Spinal Cord Monitoring for Operative Treatment of Spinal Disease. Case Western Reserve University, Cleveland, 140 pp.
Nash, CL, Jr., Schatzinger, L and Lorig, R (1974) Intraoper- tive monitoring of spinal cord function during scoliosis spine surgery. J. Bone Joint Surg. (Am), 56: 1765.
Nash, CL, Jr., Lorig, RA, Schatzinger, LA and Brown, RH (1977) Spinal cord monitoring during operative treatment of the spine. Clin. Orthop., 126: 100–105.
Nuwer, MR (Ed.) (1986) Evoked Potential Monitoring in the Operating Room. Raven Press, New York, 246 pp.
Nuwer, MR and Dawson, EC (1984) Intraoperative evoked potential monitoring of the spinal cord: enhanced stabil- ity of cortical recordings. Electroencephalogr. Clin. Neurophysiol., 59: 318–327.
Nuwer, JM and Nuwer, MR (1997) Neurophysiologic sur- gical monitoring staffing patterns in the USA. Electro- encephalogr. Clin. Neurophysiol., 103: 616–620.
Penfield, W and Boldrey, E (1937) Somatic motor and sen- sory representation in the cerebral cortex of man as stud- ied by electrical stimulation. Brain, 37: 389–443.
Sharbrough, FW, Messick, JM, Jr. and Sundt, TM, Jr. (1973) Correlation of continuous electroencephalo- grams with cerebral blood flow measurements during carotid endarterectomy. Stroke, 4: 674–683.
Shimoji, K, Higashi, H and Kano, T (1971) Epidural recording of spinal electrogram in man. Electroence- phalogr. Clin. Neurophysiol., 30: 236–239.
Speilholz, NI, Benjamin, MV, Engler, GL and Ransohoff, J (1979) Somatosensory evoked potentials during decom- pression and stabilization of the spine: methods and findings. Spine, 4: 500–505.
Sundt, TM, Jr., Sharbrough, FW, Anderson, RE and Michen- felder, JD (1974) Cerebral blood flow measurements and electroencephalograms during carotid endarterectomy. J. Neurosurg., 41: 310–320.
Tamaki, T, Yamashita, T, Kobayashi, H and Hirayama, H (1972) Spinal cord monitoring. J. Jpn. Electroencepha- logr. Electromyogr., 1: 196.
Tamaki, T, Tsuji, H, Inoue, S and Kobayashi, H (1981) The prevention of iatrogenic spinal cord injury utiliz- ing the evoked spinal cord potential. Int. Orthop., 4: 313–317.
Thompson, JE (1968) Surgery for Cerebrovascular Insuf- ficiency (Stroke) with Special Emphasis on Carotid Endarterectomy. Charles C Thomas, Springfield, IL, 96 pp.
Wylie, EJ and Ehrenfeld, WK (1970) Extracranial OcclusiveCerebrovascular Disease: Diagnosis and Management. W.B. Saunders Company, Philadelphia, 231 p.
Engler, GL, Speilholz, NI, Bernhard, WN, Danziger, F, Merkin, H and Wolf, T (1978) Somatosensory evoked potentials during Harrington instrumentation for scolio- sis. J. Bone Joint Surg. (Am), 60: 528–532.
Grundy, BL (1982) Monitoring of sensory evoked poten- tials during neurosurgical operations: methods and applications. Neurosurgery, 11: 556–575.
Hicks, RG, Burke, DJ and Stephen, JP (1991) Monitoring spi- nal cord function during scoliosis surgery with Cotrel- Dubousset instrumentation. Med. J. Aust., 154: 82–86.
Imai, T (1976) Human electrospinogram evoked by direct stimulation on the spinal cord through epidural space. J. Japanese Ortho. Assoc., 50: 1037–1056.
Jasper, HH (1949) Electrocorticograms in man. Electroen- cephalogr. Clin. Neurophysiol., Suppl. 2: 16–29.
Jones, SJ, Edgar, MA and Ransford, AO (1982) Sensory nerve conduction in the human spinal cord: epi- dural recordings made during scoliosis surgery. J. Neurol. Neurosurg. Psychiatr., 45: 446–451.
Marshall, C and Walker, AE (1949) Electrocorticography. Bull. Johns Hopkins Hosp., 85: 344–359.
Mller, AR (1988) Evoked Potentials in Intraoperative Moni- toring, Williams & Wilkins, Baltimore, 224 pp.
Mller, MB and Mller, AR (1985) Loss of auditory func- tion in microvascular decompression for hemifacial spasm. Results in 143 consecutive cases. J. Neurosurg., 63: 17–20.
Nash, CL, Jr. and Brodkey, JS (1977) Clinical Application of Spinal Cord Monitoring for Operative Treatment of Spinal Disease. Case Western Reserve University, Cleveland, 140 pp.
Nash, CL, Jr., Schatzinger, L and Lorig, R (1974) Intraoper- tive monitoring of spinal cord function during scoliosis spine surgery. J. Bone Joint Surg. (Am), 56: 1765.
Nash, CL, Jr., Lorig, RA, Schatzinger, LA and Brown, RH (1977) Spinal cord monitoring during operative treatment of the spine. Clin. Orthop., 126: 100–105.
Nuwer, MR (Ed.) (1986) Evoked Potential Monitoring in the Operating Room. Raven Press, New York, 246 pp.
Nuwer, MR and Dawson, EC (1984) Intraoperative evoked potential monitoring of the spinal cord: enhanced stabil- ity of cortical recordings. Electroencephalogr. Clin. Neurophysiol., 59: 318–327.
Nuwer, JM and Nuwer, MR (1997) Neurophysiologic sur- gical monitoring staffing patterns in the USA. Electro- encephalogr. Clin. Neurophysiol., 103: 616–620.
Penfield, W and Boldrey, E (1937) Somatic motor and sen- sory representation in the cerebral cortex of man as stud- ied by electrical stimulation. Brain, 37: 389–443.
Sharbrough, FW, Messick, JM, Jr. and Sundt, TM, Jr. (1973) Correlation of continuous electroencephalo- grams with cerebral blood flow measurements during carotid endarterectomy. Stroke, 4: 674–683.
Shimoji, K, Higashi, H and Kano, T (1971) Epidural recording of spinal electrogram in man. Electroence- phalogr. Clin. Neurophysiol., 30: 236–239.
Speilholz, NI, Benjamin, MV, Engler, GL and Ransohoff, J (1979) Somatosensory evoked potentials during decom- pression and stabilization of the spine: methods and findings. Spine, 4: 500–505.
Sundt, TM, Jr., Sharbrough, FW, Anderson, RE and Michen- felder, JD (1974) Cerebral blood flow measurements and electroencephalograms during carotid endarterectomy. J. Neurosurg., 41: 310–320.
Tamaki, T, Yamashita, T, Kobayashi, H and Hirayama, H (1972) Spinal cord monitoring. J. Jpn. Electroencepha- logr. Electromyogr., 1: 196.
Tamaki, T, Tsuji, H, Inoue, S and Kobayashi, H (1981) The prevention of iatrogenic spinal cord injury utiliz- ing the evoked spinal cord potential. Int. Orthop., 4: 313–317.
Thompson, JE (1968) Surgery for Cerebrovascular Insuf- ficiency (Stroke) with Special Emphasis on Carotid Endarterectomy. Charles C Thomas, Springfield, IL, 96 pp.
Wylie, EJ and Ehrenfeld, WK (1970) Extracranial OcclusiveCerebrovascular Disease: Diagnosis and Management. W.B. Saunders Company, Philadelphia, 231 p.
AM
Intro
IOM associated with reducing risk of postoperative neurological deficits in operations where the nervous system is at risk of being permanently injured. Maintain the integrity of the brain and spinal cord through electrophysiologic techniques.
surgical manipulations such as stretching, compressing, or heating from electrocoagulation are insults that can injure neural tissue, as can ischemia caused by impairment of blood supply resulting from surgical manipulations or intentional clamping of arteries, that could also result in permanent injury to neural structures causing a risk of noticeable postoperative neural deficits. function decreases for the time of the insult and at the other end the nervous tissue is permanently damaged and normal function never recovers thus causing permanent postoperative deficits. there is total or partial recovery, up to a certain degree of injury there can be a total recovery but thereafter the neural function might be affected for some time after a more severe injury the recovery of normal function not only takes a longer time but the final recovery would only be partial with the degree of recovery depending on the nature degree and duration of the insult. injuries acquired during operations that result in a permanent neurological deficit will most likely reduce the quality of life for the patient for many years to come and maybe for a lifetime. therefor it is important that the person responsible for interpreting the results of monitoring is aware that the neurophysiologist has a great degree of responsibility together with the surgeon and the anesthesiologist in reducing the risk of injury to the patient during the operation
general principle is to apply a stimulus and then record the electrical response from specific neural structures along the neural pathway that are at risk of being injured. this is done by recording near-field evoked potentials by placing a recording electrode on a specific neural structure that becomes exposed during the operation or as more commonly done by recording the far-field evoked potentials from electrodes placed on the surface of the scalp
Sensory Systems: widely practiced since the 80's, sensory evoked potentials for diagnostic purposes; monitored by applying an appropriate stimulus and recording the response from the ascending neural pathway usually by placing recording electrodes on the surface of the scalp to pick up far-field potentials from nerve tracts and nuclei in the brain. In the operating room, it is only changes in the recorded potentials that occur during the operation that are of interest. The results obtained in the operating room must be interpreted instantly, which places demands on the personnel who are responsible for intraoperative neurophysiological monitoring that differ from those working in the clinical laboratory.
The use of evoked potentials in intraoperative neurophysiological monitoring for the purpose of reducing the risk of postoperative permanent sensory deficits is based on the following: 1. electrical potentials can be recorded in response to a stimulus 2. these potentials change in a noticeable way as a result of surgically induced changes in function 3. proper surgical intervention, such as reversal of the manipulation that caused the change, will reduce the risk that the observed change in function develops into a permanent neurological deficit or at least will reduce the degree of the postoperative deficits
monitoring of spinal motor systems also makes use of recordings directly from the descending motor pathways of the spinal cord. spinal motor systems are often monitored by recording EMG potentials from specific muscles in response to electrical or magnetic stimulation of the motor cortex.
Motor Systems - gained wide use in the 90s after obstacles in activating descending spinal motor pathways were resolved
Peripheral Nerves
Recording of muscle activity that is elicited by mechanical stimulation of a motor nerve or by injury to a motor nerve are important parts of many forms of monitoring of the motor system. such muscle activity is monitored by continuous recording EMG potentials (free-running EMG) when such activity is made audible, it can provide important feedback to the surgeon and the surgeon can then modify their operative technique accordingly; electromyographical potentials from muscles that are innervated by specific motor nerves
Interpretation of neuroelectric potentials
the success of intraoperative neurophysiological monitoring depends greatly on the correct interpretation of the recorded neuroelectrical potentials; usefulness depends on the person that is watching the display, makes the interpretation of what information should be given to the surgeon. imperative for success in iom monitoring that the person who is responsible for the monitoring be well trained. it is also important that they are familiar with the different steps of the operation and well informed in advance about the patient who is to be monitored
important that information about changes in potentials be presented in a way that contributes specific interpreted detail that the surgeon will find useful and actionable. surgeons are not neurophysiologists and the knowledge of neurophysiology varies among surgeons; must present their skilled interpretation of the recorded potentials. keep in mind that the surgeon may not understand and therefore appreciate certain data because they might not understand. monitoring is of no value if the surgeon does not take action accordingly. if the surgeon doesn't understand, there is little chance that he/she will take appropriate action
correct and prompt interpretation of changes is essential for monitoring to be useful; ABR, SSEP, etc are often complex and consist of a series of peaks and troughs that represent the electrical activity that is generated by successively activated nerve tracts and nuclei of the ascending neural pathways of the sensory system. exact interpretation of the changes in such potentials that could occur as a result of various kinds of surgical insult therefore require thorough knowledge of the anatomy and physiology of the systems that are monitored and of how the recorded potentials are generated.
different ways to reduce the time necessary to obtain an interpretable recording will be discussed and the specific techniques that are suitable for intraoperative neurophysiological monitoring of the auditory, somatosensory, and visual systems.
The design of the monitoring system and the way the recorded potentials are processed are important factors in facilitating proper interpretation of the recorded neuroelectric potentials as is the way the recorded potentials are displayed. the proper choice of stimulus parameters and the selection of the location along the nervous pathways where the responses are recorded also facilitate prompt interpretation of recorded neuroelectrical potentials.
When recording EMG potentials, it is often advantageous to make the recorded response audible, so that the neurophysiologist responsible for the monitoring and the surgeon can hear the response and make his/her own interpretation.
STILL, THE POSSIBILITIES TO PRESENT THE RECORDED POTENTIALS DIRECTLY TO THE SURGEON ARE CURRENTLY FEW AND IT IS QUESTIONABLE WHETHER IT WOULD BE ADVANTAGEOUS. Few surgeons are physiologists and most surgeons want results of monitoring to be presented n an interpreted form rather than raw data.
the importance of being able to detect a change in function as soon as possible cannot be emphasized enough. prompt interpretation of changes in recorded potentials makes it possible for the surgeon to accurately identify the step in the operation that caused the change which is a prerequisite for proper and prompt surgical intervention and thus the ability to reduce the risk of postoperative neurological deficits.
correct identification of the step in an operation that entails a risk of complications might make it possible to modify the way such an operation is carried out in the future and thereby makes it possible to reduce the risk of complications in subsequent operations. in this way, intraoperative neurophysiological monitoring can contribute to the development of safer operating methods by making it possible to identify which steps in an operation might cause neurological deficits and it thereby naturally also plays an important role in teaching surgical residents and fellows.
When to inform the surgeon
should the information that is gained be used only as a warning that implies that if no intervention is made there is a likelihood that the patient will get a permanent postoperative neurological deficit or should all information about changes in function be conveyed to the surgeon
the degree and the nature of the change and the length of time that the adverse effect has lasted are all factors that are likely to affect the outcome and the effect of these factors on the risk of postoperative neurological deficits are largely unknown. Individual variation in susceptibility to surgical insults to the nervous system and many other factors affect the risk of neurological deficits mostly unknown ways and degrees.
The surgeon can use such information in the planning and the decision of how to proceed with the operation and intraoperative neurophysiological monitoring can thereby effectively decrease the risk of neurological deficits. it is beneficial to the surgeon to be informed whenever his or her actions have resulted in a noticeable change in the recorded neuroelectrical potentials. in that way, iom provides information rather than warnings. changes in the recorded potentials that are larger than the normal variations of the potentials in question should be reported to the surgeon if there is reasonable certainty that these changes are related to surgical manipulations.
if the surgeon is made aware of any change in the potentials it can help them to carry out the operation in an optimal way with as little risk of adverse affect on neural function as possible. information can alter his or her course of action in a wide range of time. it is likely that the surgeon would be able to reverse the effect by a slight change in the surgical approach.
It would be difficult for the surgeon to determine which step in the surgical procedure caused the adverse effect if information about a change in the recorded potentials is withheld until the change in the recorded electrical potentials has increased greatly
the more knowledge that is gathered about the effect of mechanical manipulation on the nerves, the more it seems apparent that even slight changes in measures of electrical activity might be signs of permanent injury. Still, relatively little is known about the degree to which a nerve can be stretched, heated or deprived of oxygen before a permanent injury results but there is no doubt that different nerves respond in different ways to injury because of mechanical manipulations, heat, or lack of oxygen
present information about changes in recorded electrical potentials as soon as they reach a level where they are detectable, has an educational benefit that tells the surgeon precisely which steps in an operation might result in neurological deficit.
important that it be made clear to the surgeon that information represents guidance details. the surgeon should be informed of the possibility of a surgically induced injury even in cases in which the change could be caused by equipment or electrode malfunction. thus only after assuming that the problem is biological in nature can equipment failure be considered as a possible cause.
False Alarms
false positive and false negative responses; false positive response meant that the surgeon was alerted of a situation that would not have led to any noticeable risk of deficit if no action had been taken
false positive - test showed the presence of a disease when no disease was present
false-negative test that the test failed to show that a certain individual in fact had disease present
purpose of IOM is to show changes in function that indicate a risk of causing neurological deficits. no serious consequence to this
the occurrence of a false negative means that a serious risk has occurred without being noticed, indicate a failure in reaching the goal of intraoperative neurophysiological monitoring and it might have serious consequences.
purpose of IOM is not to identify an individual with a neurological deficit but to identify signs that have a certain risk of leading to such deficits if no action is taken
Non surgical causes of changes in recorded potentials
technical problems do occur, remain vigilant and hyper aware of the connections and distribution of of your cables and connections to equipment; loss of contact of electrodes, first step is an electrode impedance check.
technical stuff is usually different from surgical information, can usually can be quickly troubleshooted and maintain consistent real-time monitoring with goal of zero minutes downtime for critical period of surgery; 60 hz noise in one channel, loss of one channel, loss of one pod, loss of all pods, method of coding to ensure pods are in the correct order; put knots in electrodes, run them along the b for protection and better signaling, have junction area at the table with a loop of loose tension extra slack for stretching, run cables away from equipment and to a path that you can quickly access them in case it's necessary for a move in operating position (anterior to posterior/posterior to anterior). Highly improbably that a biological change and a technical change would manifest the same. better to alert the surgeon if you have a technical problem, a false alarm is better than a negative outcome. pays to know your system and have confidence in your setup. time is precious, could increase the change in function of a neural structure if not vigilant, especially during critical periods. Have a methodology or a system to your monitoring,
electrodes: red/red/white, orange/orange/white, yellow/yellow/white, green/green/white, blue/blue/white, 4 pairs to stimulate, L&R UL/MN, L&R PTIB for SSEP, Cz-Fpz-C3-C4 SSEP recording electrodes, C1-C2 mep electrodes, Mastoid GND
keep equipment failures to a minimum, have a system in the operating room and successfully navigate around the table and operating room, know your environment.
pre examine patient and as pre and post operative so changes can be quantified, testing and examination of the patient before the surgery, get a quality assessment of their neurological deficits and history of surgery and history of pain; chronic, trauma, mva, degeneration,
"reducing the risk of any measurable or noticeable deficit as much as possible must be the goal of intraoperative neurophysiological monitoring" Moller
abnormal muscle response in patients undergoing microvascular decompression operations to relieve hemi-facial spasm. abnormal muscle response to stimulation amplitude decreases when the facial nerve is adequately decompressed *posterior to the facial nerve, is possible to identify the blood vessel or blood vessels that caused the symptoms of HFS as well as to ensure that the facial nerve has been adequately decompressed.
Electrophysiological guidance for placement of lesions in the basal ganglia and the thalamus for the treatment of movement disorders and pain is absolutely essential for the success of such treatment. deep brain stimulation is coming.
the future is bright, need for people with surgical neurophysiology will increase especially when new methods to aid surgeons and acceptance is gained.
Working In The Operating Room
interfere minimally with other activities in the operating room. operating room has a lot of dynamic activity , careful planning is necessary to ensure that iom doesn't interfere with other forms of monitoring and the use of life support equipment.
Intro
IOM associated with reducing risk of postoperative neurological deficits in operations where the nervous system is at risk of being permanently injured. Maintain the integrity of the brain and spinal cord through electrophysiologic techniques.
surgical manipulations such as stretching, compressing, or heating from electrocoagulation are insults that can injure neural tissue, as can ischemia caused by impairment of blood supply resulting from surgical manipulations or intentional clamping of arteries, that could also result in permanent injury to neural structures causing a risk of noticeable postoperative neural deficits. function decreases for the time of the insult and at the other end the nervous tissue is permanently damaged and normal function never recovers thus causing permanent postoperative deficits. there is total or partial recovery, up to a certain degree of injury there can be a total recovery but thereafter the neural function might be affected for some time after a more severe injury the recovery of normal function not only takes a longer time but the final recovery would only be partial with the degree of recovery depending on the nature degree and duration of the insult. injuries acquired during operations that result in a permanent neurological deficit will most likely reduce the quality of life for the patient for many years to come and maybe for a lifetime. therefor it is important that the person responsible for interpreting the results of monitoring is aware that the neurophysiologist has a great degree of responsibility together with the surgeon and the anesthesiologist in reducing the risk of injury to the patient during the operation
general principle is to apply a stimulus and then record the electrical response from specific neural structures along the neural pathway that are at risk of being injured. this is done by recording near-field evoked potentials by placing a recording electrode on a specific neural structure that becomes exposed during the operation or as more commonly done by recording the far-field evoked potentials from electrodes placed on the surface of the scalp
Sensory Systems: widely practiced since the 80's, sensory evoked potentials for diagnostic purposes; monitored by applying an appropriate stimulus and recording the response from the ascending neural pathway usually by placing recording electrodes on the surface of the scalp to pick up far-field potentials from nerve tracts and nuclei in the brain. In the operating room, it is only changes in the recorded potentials that occur during the operation that are of interest. The results obtained in the operating room must be interpreted instantly, which places demands on the personnel who are responsible for intraoperative neurophysiological monitoring that differ from those working in the clinical laboratory.
The use of evoked potentials in intraoperative neurophysiological monitoring for the purpose of reducing the risk of postoperative permanent sensory deficits is based on the following: 1. electrical potentials can be recorded in response to a stimulus 2. these potentials change in a noticeable way as a result of surgically induced changes in function 3. proper surgical intervention, such as reversal of the manipulation that caused the change, will reduce the risk that the observed change in function develops into a permanent neurological deficit or at least will reduce the degree of the postoperative deficits
monitoring of spinal motor systems also makes use of recordings directly from the descending motor pathways of the spinal cord. spinal motor systems are often monitored by recording EMG potentials from specific muscles in response to electrical or magnetic stimulation of the motor cortex.
Motor Systems - gained wide use in the 90s after obstacles in activating descending spinal motor pathways were resolved
Peripheral Nerves
Recording of muscle activity that is elicited by mechanical stimulation of a motor nerve or by injury to a motor nerve are important parts of many forms of monitoring of the motor system. such muscle activity is monitored by continuous recording EMG potentials (free-running EMG) when such activity is made audible, it can provide important feedback to the surgeon and the surgeon can then modify their operative technique accordingly; electromyographical potentials from muscles that are innervated by specific motor nerves
Interpretation of neuroelectric potentials
the success of intraoperative neurophysiological monitoring depends greatly on the correct interpretation of the recorded neuroelectrical potentials; usefulness depends on the person that is watching the display, makes the interpretation of what information should be given to the surgeon. imperative for success in iom monitoring that the person who is responsible for the monitoring be well trained. it is also important that they are familiar with the different steps of the operation and well informed in advance about the patient who is to be monitored
important that information about changes in potentials be presented in a way that contributes specific interpreted detail that the surgeon will find useful and actionable. surgeons are not neurophysiologists and the knowledge of neurophysiology varies among surgeons; must present their skilled interpretation of the recorded potentials. keep in mind that the surgeon may not understand and therefore appreciate certain data because they might not understand. monitoring is of no value if the surgeon does not take action accordingly. if the surgeon doesn't understand, there is little chance that he/she will take appropriate action
correct and prompt interpretation of changes is essential for monitoring to be useful; ABR, SSEP, etc are often complex and consist of a series of peaks and troughs that represent the electrical activity that is generated by successively activated nerve tracts and nuclei of the ascending neural pathways of the sensory system. exact interpretation of the changes in such potentials that could occur as a result of various kinds of surgical insult therefore require thorough knowledge of the anatomy and physiology of the systems that are monitored and of how the recorded potentials are generated.
different ways to reduce the time necessary to obtain an interpretable recording will be discussed and the specific techniques that are suitable for intraoperative neurophysiological monitoring of the auditory, somatosensory, and visual systems.
The design of the monitoring system and the way the recorded potentials are processed are important factors in facilitating proper interpretation of the recorded neuroelectric potentials as is the way the recorded potentials are displayed. the proper choice of stimulus parameters and the selection of the location along the nervous pathways where the responses are recorded also facilitate prompt interpretation of recorded neuroelectrical potentials.
When recording EMG potentials, it is often advantageous to make the recorded response audible, so that the neurophysiologist responsible for the monitoring and the surgeon can hear the response and make his/her own interpretation.
STILL, THE POSSIBILITIES TO PRESENT THE RECORDED POTENTIALS DIRECTLY TO THE SURGEON ARE CURRENTLY FEW AND IT IS QUESTIONABLE WHETHER IT WOULD BE ADVANTAGEOUS. Few surgeons are physiologists and most surgeons want results of monitoring to be presented n an interpreted form rather than raw data.
the importance of being able to detect a change in function as soon as possible cannot be emphasized enough. prompt interpretation of changes in recorded potentials makes it possible for the surgeon to accurately identify the step in the operation that caused the change which is a prerequisite for proper and prompt surgical intervention and thus the ability to reduce the risk of postoperative neurological deficits.
correct identification of the step in an operation that entails a risk of complications might make it possible to modify the way such an operation is carried out in the future and thereby makes it possible to reduce the risk of complications in subsequent operations. in this way, intraoperative neurophysiological monitoring can contribute to the development of safer operating methods by making it possible to identify which steps in an operation might cause neurological deficits and it thereby naturally also plays an important role in teaching surgical residents and fellows.
When to inform the surgeon
should the information that is gained be used only as a warning that implies that if no intervention is made there is a likelihood that the patient will get a permanent postoperative neurological deficit or should all information about changes in function be conveyed to the surgeon
the degree and the nature of the change and the length of time that the adverse effect has lasted are all factors that are likely to affect the outcome and the effect of these factors on the risk of postoperative neurological deficits are largely unknown. Individual variation in susceptibility to surgical insults to the nervous system and many other factors affect the risk of neurological deficits mostly unknown ways and degrees.
The surgeon can use such information in the planning and the decision of how to proceed with the operation and intraoperative neurophysiological monitoring can thereby effectively decrease the risk of neurological deficits. it is beneficial to the surgeon to be informed whenever his or her actions have resulted in a noticeable change in the recorded neuroelectrical potentials. in that way, iom provides information rather than warnings. changes in the recorded potentials that are larger than the normal variations of the potentials in question should be reported to the surgeon if there is reasonable certainty that these changes are related to surgical manipulations.
if the surgeon is made aware of any change in the potentials it can help them to carry out the operation in an optimal way with as little risk of adverse affect on neural function as possible. information can alter his or her course of action in a wide range of time. it is likely that the surgeon would be able to reverse the effect by a slight change in the surgical approach.
It would be difficult for the surgeon to determine which step in the surgical procedure caused the adverse effect if information about a change in the recorded potentials is withheld until the change in the recorded electrical potentials has increased greatly
the more knowledge that is gathered about the effect of mechanical manipulation on the nerves, the more it seems apparent that even slight changes in measures of electrical activity might be signs of permanent injury. Still, relatively little is known about the degree to which a nerve can be stretched, heated or deprived of oxygen before a permanent injury results but there is no doubt that different nerves respond in different ways to injury because of mechanical manipulations, heat, or lack of oxygen
present information about changes in recorded electrical potentials as soon as they reach a level where they are detectable, has an educational benefit that tells the surgeon precisely which steps in an operation might result in neurological deficit.
important that it be made clear to the surgeon that information represents guidance details. the surgeon should be informed of the possibility of a surgically induced injury even in cases in which the change could be caused by equipment or electrode malfunction. thus only after assuming that the problem is biological in nature can equipment failure be considered as a possible cause.
False Alarms
false positive and false negative responses; false positive response meant that the surgeon was alerted of a situation that would not have led to any noticeable risk of deficit if no action had been taken
false positive - test showed the presence of a disease when no disease was present
false-negative test that the test failed to show that a certain individual in fact had disease present
purpose of IOM is to show changes in function that indicate a risk of causing neurological deficits. no serious consequence to this
the occurrence of a false negative means that a serious risk has occurred without being noticed, indicate a failure in reaching the goal of intraoperative neurophysiological monitoring and it might have serious consequences.
purpose of IOM is not to identify an individual with a neurological deficit but to identify signs that have a certain risk of leading to such deficits if no action is taken
Non surgical causes of changes in recorded potentials
technical problems do occur, remain vigilant and hyper aware of the connections and distribution of of your cables and connections to equipment; loss of contact of electrodes, first step is an electrode impedance check.
technical stuff is usually different from surgical information, can usually can be quickly troubleshooted and maintain consistent real-time monitoring with goal of zero minutes downtime for critical period of surgery; 60 hz noise in one channel, loss of one channel, loss of one pod, loss of all pods, method of coding to ensure pods are in the correct order; put knots in electrodes, run them along the b for protection and better signaling, have junction area at the table with a loop of loose tension extra slack for stretching, run cables away from equipment and to a path that you can quickly access them in case it's necessary for a move in operating position (anterior to posterior/posterior to anterior). Highly improbably that a biological change and a technical change would manifest the same. better to alert the surgeon if you have a technical problem, a false alarm is better than a negative outcome. pays to know your system and have confidence in your setup. time is precious, could increase the change in function of a neural structure if not vigilant, especially during critical periods. Have a methodology or a system to your monitoring,
electrodes: red/red/white, orange/orange/white, yellow/yellow/white, green/green/white, blue/blue/white, 4 pairs to stimulate, L&R UL/MN, L&R PTIB for SSEP, Cz-Fpz-C3-C4 SSEP recording electrodes, C1-C2 mep electrodes, Mastoid GND
keep equipment failures to a minimum, have a system in the operating room and successfully navigate around the table and operating room, know your environment.
pre examine patient and as pre and post operative so changes can be quantified, testing and examination of the patient before the surgery, get a quality assessment of their neurological deficits and history of surgery and history of pain; chronic, trauma, mva, degeneration,
"reducing the risk of any measurable or noticeable deficit as much as possible must be the goal of intraoperative neurophysiological monitoring" Moller
abnormal muscle response in patients undergoing microvascular decompression operations to relieve hemi-facial spasm. abnormal muscle response to stimulation amplitude decreases when the facial nerve is adequately decompressed *posterior to the facial nerve, is possible to identify the blood vessel or blood vessels that caused the symptoms of HFS as well as to ensure that the facial nerve has been adequately decompressed.
Electrophysiological guidance for placement of lesions in the basal ganglia and the thalamus for the treatment of movement disorders and pain is absolutely essential for the success of such treatment. deep brain stimulation is coming.
the future is bright, need for people with surgical neurophysiology will increase especially when new methods to aid surgeons and acceptance is gained.
Working In The Operating Room
interfere minimally with other activities in the operating room. operating room has a lot of dynamic activity , careful planning is necessary to ensure that iom doesn't interfere with other forms of monitoring and the use of life support equipment.
How to Reduce the Risk of Mistakes in Intraoperative Monitoring
Monitor correctly, appropriate dermatomal levels for somatosensory are understood; Median Nerve and Ulnar Nerve enter the spinal cord above the thoracic level. know your left from your right, i remember hearing a surgeon repeatedly saying the direction and type of surgery over and over. take steps to isolate what side you are working on; needles correctly placed, screen correctly set up to identify different directions, motor evoked potential on one side at a time, look for emg activity on all muscle channels, do the opposite side. use emg activity to guide you, upper left ssep should stimulate on the right hand, upper right ssep should stimulate on the left hand, lower left should stimulate on the right foot, lower right ssep should stimulate from the left foot. motor evoked potential should resonate on all muscles on left and right. Monitoring the wrong ABR could have catastrophic consequences of a nerve after failing to detect any change. mistakes can only be avoided if it is physically possible to make the mistake. example, if need to test one ear, test one, not two. if need two ears, clearly mark the R and L and ensure proper connection and computer setup. monitoring the wrong side of the spinal cord could cause serious neurological deficit. test connections after have them set up, have a methodology, use red and black for tcmep stimulation because that is how they are connected at the tcmep motor box. Red for Motor, Blue for Sensory; Blue/Blue/White, Green/Green/White;
Keep it Simple!
Don't add risk, electrical safety/wet, be knowledgeable about your equipment and how it functions, be vigilant about your placement of stimulating and recording electrodes and especially those electrodes that are to be placed intracranially for recording or stimulation.
the patient gains the most out of monitoring, many severe postoperative neurological deficits that were common before iom are now rare. better surgical technique and various technological advancements have provided progress. much less brutal than 25 years ago, Wilder Penfield would be proud!
surgeons who have experienced the advantages of intraoperative neurophysiological monitoring are reluctant to deprive their patients of the benefits provided by an aid in the operation that they believe can improve the outcome. increased feeling of security
Surgeons at all levels of experience can benefit from intraoperative monitoring but the degree of benefit depends on the experience of the surgeon in the particular kind of operation being performed. many experienced surgeons are not willing to operate without the use of monitoring. reduces risk of pos operative neurological deficits as well as by its ability to provide the surgeon with a feeling of security from knowing that he/she will know when neural tissue is being adversely manipulated.
Monitor correctly, appropriate dermatomal levels for somatosensory are understood; Median Nerve and Ulnar Nerve enter the spinal cord above the thoracic level. know your left from your right, i remember hearing a surgeon repeatedly saying the direction and type of surgery over and over. take steps to isolate what side you are working on; needles correctly placed, screen correctly set up to identify different directions, motor evoked potential on one side at a time, look for emg activity on all muscle channels, do the opposite side. use emg activity to guide you, upper left ssep should stimulate on the right hand, upper right ssep should stimulate on the left hand, lower left should stimulate on the right foot, lower right ssep should stimulate from the left foot. motor evoked potential should resonate on all muscles on left and right. Monitoring the wrong ABR could have catastrophic consequences of a nerve after failing to detect any change. mistakes can only be avoided if it is physically possible to make the mistake. example, if need to test one ear, test one, not two. if need two ears, clearly mark the R and L and ensure proper connection and computer setup. monitoring the wrong side of the spinal cord could cause serious neurological deficit. test connections after have them set up, have a methodology, use red and black for tcmep stimulation because that is how they are connected at the tcmep motor box. Red for Motor, Blue for Sensory; Blue/Blue/White, Green/Green/White;
Keep it Simple!
Don't add risk, electrical safety/wet, be knowledgeable about your equipment and how it functions, be vigilant about your placement of stimulating and recording electrodes and especially those electrodes that are to be placed intracranially for recording or stimulation.
the patient gains the most out of monitoring, many severe postoperative neurological deficits that were common before iom are now rare. better surgical technique and various technological advancements have provided progress. much less brutal than 25 years ago, Wilder Penfield would be proud!
surgeons who have experienced the advantages of intraoperative neurophysiological monitoring are reluctant to deprive their patients of the benefits provided by an aid in the operation that they believe can improve the outcome. increased feeling of security
Surgeons at all levels of experience can benefit from intraoperative monitoring but the degree of benefit depends on the experience of the surgeon in the particular kind of operation being performed. many experienced surgeons are not willing to operate without the use of monitoring. reduces risk of pos operative neurological deficits as well as by its ability to provide the surgeon with a feeling of security from knowing that he/she will know when neural tissue is being adversely manipulated.