Department of Neurology
Monitoring of spinal cord function during spinal surgery was initiated at the Adelaide Children's Hospital (now known as the Women's and Children's Hospital) in the early nineteen eighties. This coincided with the phasing out of the old Harrington Rod spinal instrumentation system and the introduction of new more forceful systems for treatment of scoliosis, the first of which was the Cotrel-Debousset instrumentation. The head of Orthopaedic Surgery, Sir Denis Paterson was keen to implement some form of monitoring to guard against potential neurological sequelae resulting from the large distractional forces which the new systems produced on the spine. He was aware of movements in the United States to introduce spinal cord monitoring for medical and medico-legal protection (The International Spinal Monitoring Society started in the US in 1979 for clinicians interested in monitoring and protecting the spinal cord, meeting biannually). Dr James Manson as head of Neurology, agreed for me as the Medical Scientist in Neurology to assist in providing a monitoring service.
Sensory Evoked Potentials
Our initial attempts at monitoring involved stimulation of the nerves in the lower limbs and recording of cortical somatosensory evoked potentials from surface EEG electrodes on the head. These attempts were marred by a naive lack of knowledge of the optimal conditions for monitoring and a lack of suitable equipment. With the kind generosity of the Morialta Trust of SA , in 1985 we were able to purchase a Medelec Monitor 91. This simple single channel unit remained our primary means of monitoring until 1997. Apart from some initial difficulties with amplifier damage from diathermy Radio-Frequency Interference (RFI) which was overcome with an pre-amplifier upgrade, the Monitor 91 proved robust and reliable. We added some switches to allow quick changes of stimulation for right and left legs and to allow switching between various inputs.
However, even with our new machine we had difficulty in recording cortical somatosensory evoked potentials (SEPs) under anaesthetic. A significant problem appeared to be the anaesthetic conditions. Literature of the time suggested that halothane or enflurane levels should be less than 0.5-0.4% for successful recording of cortical SEPs while our anaesthetist liked to run closer to 1.0%. The surgeons still remember the day I suggested we needed lower levels of volatile agents for successful cortical SEPs. The anaesthetist (who has long since retired) gruffly told me that he was there to do his job and I should get along and do mine. Clearly with this level of cooperation, an alternative approach to monitoring was needed.
Direct recording of sensory evoked potentials from the spinal cord, thus circumventing most affects of anaesthetic agents, appeared to offer a viable alternative. Medelec supplied bone screw and hook electrodes for direct spinal recordings. We found the signals from these to be too small, unstable and subject to interference. After a few trials we settled on the bipolar cardiac pacing electrode recommended by the spinal unit at Royal National Orthopaedic Hospital at Stanmore in the UK where Steve Jones had developed the technique. Not only did we not have to worry about the activities of an unsympathetic anaesthetist but we could stimulate considerably faster allowing much more rapid feedback on spinal cord function. In addition, the multi-component epidural response allowed visualization of separate nerve-pathway function. The minority of patients who were having anterior procedures still required cortical recordings and these were generally regarded as problematic (Disclaimer: many monitoring people, particularly in the US use only cortical SEPs or sometimes sub-cortical SEPs, whatever the instrumentation. While this may be feasible with suitable anaesthesia, you will understand from this article that I use these techniques only when an epidural lead is not available or not working and motor evoked potentials are not possible.).
While epidural recordings largely removed the anaesthetic variable, there was none the less a teething period before successful recordings could be obtained routinely. Training the surgeons to respect the integrity of the recording lead took time. We also encountered patients with previously undetected neurological disorders in whom monitoring was unsuccessful or less than optimal. Consequentially, pre-operative clinical examinations were introduced together with pre-operative somatosensory evoked potentials and later MRI of the spine and pre-operative motor evoked potentials (MEPs).
The pre-operative neurophysiology done the day before, or on the morning of the surgery allowed us to fix electrodes to the scalp and to mark stimulation and recording points on the legs. This made for efficiencies in the OR and the pre-operative assessments made for improved confidence in obtaining reliable monitoring.
Motor Evoked Potentials
From 1987, motor evoked potential monitoring was added to our protocol in an effort to screen for possible motor deficits which could be missed by simple sensory monitoring.
Although sensory and motor pathways in the spinal cord are physically in close proximity to each other and a major spinal cord insult would probably affect both systems, there is the theoretical and indeed the reported possibility of post-operative paraplegia with normal sensory monitoring. This admittedly, has been only in a small number of cases and generally when recording cortical sensory rather than epidural sensory potentials (Ginsburg et al. 1985, Zornow et al. 1990, Lesser et al. 1986, Ben-David et al. 1987). Not only are the former susceptible to anaesthetic conditions but animals studies have shown that at least three quadrants of the spinal cord must be damaged before there is a consistent loss of cortical SEPs (Gains et al. 1984). By contrast, the multi-component nature of the latter (epidural SEPs), should make it possible to observe changes in function of smaller groups of spinal neurons which would normally be masked in cortical SEPs due to the mass effect of the spinal input and by cortical amplification. Whether this explains the predominance of these reports in the cortical SEP literature over the epidural SEP literature or the greater use of cortical SEPs internationally, I cannot say. None the less the main concern of spinal cord monitoring in scoliosis is to avoid paraplegia and motor monitoring would appear to provide the ultimate reassurance in this respect.
The non electrophysiolgical approach to this problem had been the Stagnara Wake-Up Test (a somewhat problematic test of the patient’s ability to move their legs while partially anaesthetised which was continued at the WCH until the mid-90's). This could only be reasonably done once during the surgery, usually after the instrumentation was complete. A less disruptive technique which could be performed repeatedly throughout the operation was needed. Motor evoked potentials in response to transcranial electrical stimulation as reported by Boyd et al (1986) appeared to offer a method to motor function continuously.
A Digitimer D180A single pulse transcranial electrical stimulator was purchased for the purpose. I spent six months of 1987 in London working with Dr Stewart Boyd at the Hospital for Sick Children, Great Ormond Street and at the Royal National Orthopaedic Hospital, Stanmore trying to record motor potentials from the epidural space and EMG responses from the lower limb musculature using the D180 as a stimulator. MEPs were found to be useful but were, like the cortical SEPs, frequently confounded by technical factors. Epidural recordings were not reliable below the T12 spinal vertebra and did not differentiate between right and left legs. Peripheral EMG was very susceptible to the effects of the volatile anaesthetic agents including nitrous oxide.
Despite these limitations we continued to employ MEPs as part of our monitoring protocol and MEP recordings from the legs provided a useful adjunct to the wake-up test, often demonstrating motor conduction before voluntary movement was elicited. The replacement of the D180 with the D185 multipulse stimulator in 2000 has subsequently much improved the reliability of the recording peripheral EMG responses to transcranial cortical stimulation under anaesthetic and the wake-up test has been discontinued as a routine procedure.
Current Monitoring Practice
Monitoring of spinal cord function during surgery for treatment of scoliosis or other spinal pathology is routinely provided by the Department of Neurology. Monitoring involves recording of Sensory and Motor Evoked Potentials using surface electrodes on the head, legs and with epidural electrodes situated above and below the surgical site. The philosophy of our department is to provide a comprehensive monitoring service which is flexible in its approach and readily adaptable to the needs of the individual patient and particular surgical conditions.
Pre-operatively, patients routinely receive a work-up which includes 1) a clinical neurological assessment, sometimes supplemented with an MRI study of the spine. 2) cerebral SEPs and optionally, 3) trans-cranial electrical MEPs. Patients with a history of epilepsy, head injury or intra-cranial surgery are not accepted for MEP monitoring and are monitored by SEPs only. Pre-operative evoked potential tests are performed the day prior or on the morning of surgery. At this time scalp electrodes are fastened with collodion glue and remain in position until surgery is completed. We use custom made 2cm diameter silver cupped discs with studs suitable for standard ECG type lead attachment.These electrodes avoid tangling wires and allow self-conscious patients to wear a cap to conceal them. The scalp electrodes are placed at the vertex and 6cms lateral / 2cms forward of a line joining the vertex and respective external auditory meatii. The position of nerve stimulation electrodes on the legs is also marked at this time.
Our current monitoring unit as of 2010, is a Medelec Synergy ten channel averaging system although most of our earlier studies were done with a Medelec Premier Plus, Medelec Monitor 91 and occasionally, a Cadwell Spectrum 32. In the past, various switch boxes have been added to improve the flexibility and user friendliness of the systems however the main addition for our current set-up is a switch to allow us to use the same head electrodes for stimulating and for recording (see methodology) purposes. Motor stimuli are generated by a Digitimer D185 multipulse transcranial electrical stimulator which is triggered from the Synergy.
Our configuration for SEP monitoring during spinal instrumentation for scoliosis is summarized in the above figure. Stimulation is given unilaterally, to the peroneal (aka lateral popliteal) nerve in the popliteal fossa or the posterior tibial nerve at the ankle. In recent times, we have become aware that excessive stimulation of the peroneal nerve may possibly potentiate a tibialis anterior compartment syndrome in certain susceptible individuals. This is unproven, but to reduce the likelihood of this eventuality, we have decided to reduce the rate of stimulation of the peroneal nerve and preferentially stimulate the posterior tibial nerve at the ankle where that gives a satisfactory response. Cortical SEPs are recorded from the scalp electrodes, either C4’-C3’ or Cz’-Fpz or whichever gives the best response. Subcortical SEPs may be recorded with a scalp to mastoid montage. Epidural recordings are made from a bipolar cardiac pacing electrode (size 3 or 4F) inserted by the surgeon above and where possible below the surgical site. The isolated ground is a diathermy-type ground electrode placed on the thigh. These leads may be used for both epidural SEP and MEP recordings. The MEP configuration is also summarized above. Transcranial electrical stimulation is given via the same electrodes as used in recording cerebral SEPs (either C4’-C3’ or Cz’ to C4’/C3’ or whichever gives clear responses for low stimulus intensities) while recordings are from epidural leads and tibialis anterior referred to the anterior foot. We use the 2cm diameter stimulating electrodes to reduce current density in accordance with the safety guidelines of Agnew and McCreery (1987).
Intra-operative monitoring begins as soon as the patient is on the operating table and electrodes are connected. Baseline cortical SEPs and peripheral MEPs are recorded. There is a period when monitoring is not possible with this equipment while diathermy is in use. Following exposure of the spine the surgeon inserts the epidural lead(s) and recording of epidural SEPs and MEPs continues until closure when recording of cerebral SEPs resumes. Peripheral MEPs are recorded pre- and post-instrumentation, the latter sometimes (very infrequently now) in conjunction with the wake-up test.
Responses are visually monitored for stability by the operator. Monitoring is performed continuously with periodic storage of sample traces. Any reduction in amplitude of 40-50% is investigated for possible technical causes.
In the event of an altered signal... "Don't Panic!"
There are numerous equipment related problems which can interfere with monitoring. These can result in a range of signal changes from mild to fluctuating to complete loss of response. A reduction in latency of upper epidural SEPs for example would suggest that the lead has been partially pulled out (ie toward the stimulator electrodes). The operator also needs to ensure that stimulating electrode contact is maintained. Other failures of monitoring can be caused by interference from other equipment in or even near the O.R., broken wires and amplifier malfunction. Worn or dirty switches can cause intermittent loss of signal.
Depending on the monitoring technique, there are several patient related factors which can influence the ability to monitor effectively. Pre-existing neurological impairment may result in very poor signals and may prevent any useful monitoring at all. During the operation, signals may change as a result of alterations in anaesthetic agent regimes, blood pressure, body temperature and muscle relaxation. There is also a certain amount of idiopathic normal variation even with epidural sensory recordings (acc. Loughnan et al, 1989. 10-30%).
Successful peripheral MEP monitoring requires minimal and preferably no muscle relaxant. If there is doubt that an EMG is present then recording tibialis anterior responses to peroneal stimulation can confirm that the peripheral neuromuscular system and recording apparatus is functioning.
When there is observed an increase in the latency of the epidural SEPs with a decrease in amplitude of all or part of the responses the possibility of compromise of neurological function is raised. The operator must quickly and efficiently investigate possible technical causes. Discussion with the anaesthetist may provide invaluable information about changes in the status of the patient which could be influencing the responses. If epidural SEPs fall to less than 50% with no apparent technical explanation the surgeons are alerted and possible causes and responses are discussed. It should be emphasized that this is a team effort requiring open communication between monitoring, anaesthetic and surgical parties.
The 50% amplitude level has long been suggested as a significant value for indicating potential neurological compromise for epidural SEP recordings (Jones, 1994; Kurthen & Schramm, 1994). It should be emphasized that this is most meaningful if one has established stable baseline over an extended period of monitoring. Other techniques such as cortical SEPs and peripheral MEPs are more subject to variation and lower amplitude minimums are probably acceptable (Neuloh & Schramm 2002). Certainly, anaesthetic factors are much more significant in these forms of monitoring and there is a strong argument for using TIVA when using these techniques.
Surgical options include, no action, wait and see, changing the instrumentation, removing the instrumentation while the anaesthetist may consider actions to improve spinal cord perfusion. For more detailed discussion of spinal monitoring techniques I have included some published titles below. If you are actively involved in intra-operative monitoring I also recommend joining the Neuro-Monitoring Discussion List run by Jerry Larson. He has some recommendations on anaesthetic regimes at his site. You may also like to refer to the American Society of Neurophysiological Monitoring web page for useful links and reference information.
For those of us who were unable to attend the IXth International Spinal Monitoring Symposium held in Rome, May 3-6 2004, Jerry Larson has kindly allowed me to pass on his impressions of the meeting which he originally posted on the Neuromonitoring Discussion List. See Jerry's report.
Neurophysiology in Neurosurgery, Eds. Deletis and Shils, Publ. Academic Press (2002), ISBN 0-12-209036-5
Intraoperative Spinal Cord Monitoring with Myogenic Transcranial Motor Evoked Potentials, Ubags LH (1998) Thesis. Available from Digitimer Ltd.
Motor evoked potential monitoring during spinal surgery: responses of distal limb muscles to transcranial cortical stimulation with pulse trains. Jones SJ et al (1996), EEG and Clin Neurophysiol 100, 375-383
Handbook of Spinal Cord Monitoring, Proc 5th Int Symp on Spinal Cord Monit. London 1992, Eds Jones et al. Publ. Kluwer Acad Pub (1994), ISBN-0-7923-8833-X
Spinal Cord Monitoring 1989. Loughnan BA and Hall GM. Br J Anaesth 1989; 63, 587-594 Evoked Potential Monitoring In the Operating Room, Nuwer M, Publ. Raven Press (1986), ISBN 0-88167-230-0
Spinal Cord Monitoring, Eds Schramm J & Jones SJ, Publ. Sringer-Verlag (1985), ISBN 0-387-15774-3 (US)
There are of course many more learned and important texts on the subject of spinal cord monitoring many of which are referred to in the pages of the above and may be accessed via Pub Med or similar.
Agnew WF, McCreery DB. Considerations for safety in the use of extracranial stimulation for motor evoked potentials Neurosurgery, 1987; 20(1): 143-147
Boyd SG, Rothwell JC, Cowan JMA, Webb PJ, Morley T, Asselman P, Marsden CD. A method for monitoring function in corticospinal pathways during scoliosis surgery with a note on motor conduction velocities. J Neurol. Neurosurg & Psychiat, (1986); 49: 251-257
Ben-David B, Haller G, Taylor P. Anterior spinal fusion complicated by paraplegia. A case report of a false-negative somatosensory-evoked potential. Spine. 1987 Jul-Aug;12(6):536-9.
Gaines R, York DH, Watts C. Identification of spinal cord pathways responsible for the peroneal-evoked response in the dog. Spine. 1984 Nov-Dec;9(8):810-4.
Ginsburg HH, Shetter AG, Raudzens PA. Postoperative paraplegia with preserved intraoperative somatosensory evoked potentials. Case report. J Neurosurg. 1985 Aug;63(2):296-300.
Halonen JP, Jones SJ, Edgar MA, Ransford AO. Conduction properties of epidurally recorded spinal cord potentials following lower limb stimulation in man. Electroencephalogr Clin Neurophysiol. 1989 May-Jun;74(3):161-74.
Jones SJ, Edgar MA, Ransford AO and Thomas NP. A system for electrophysiological monitoring of the spinal cord during operations for scoliosis. J Bone Joint Surg., 1983; 65B, 134-9
Jones SJ. Somatosensory evoked potentials and their use for spinal cord monitoring, Handbook of spinal cord monitoring, Proceedings of the 5th international symposium on spinal cord monitoring, London, June 1992. Eds Jones, Boyd, Hetreed, Smith. Kluwer Academic Publ. 1994.
Kurthen M & Schramm J. Application of intra-operative spinal cord monitoring to neurosurgery Handbook of spinal cord monitoring , Proceedings of the 5th international symposium on spinal cord monitoring, London, June 1992. Eds Jones, Boyd, Hetreed, Smith. Kluwer Academic Publ. 1994.
Lesser RP, Raudzens P, Luders H, Nuwer MR, Goldie WD, Morris HH 3rd, Dinner DS, Klem G, Hahn JF, Shetter AG, et al. Postoperative neurological deficits may occur despite unchanged intraoperative somatosensory evoked potentials. Ann Neurol. 1986 Jan;19(1):22-5.
Loughnan BA, King MJ, Grundy EM, Young DL and Hall GM. Effects of halothane on somatosensory evoked potentials recorded in the extradural space. Br J Anaesth. 1989;62, 297-300.
Neuloh G, Schramm J. Intraoperative neurophysiological mapping and monitoring for supratentorial procedures. From Neurophysiology in Neurosurgery, Eds. Deletis and Shils, Publ. Academic Press, 2002.
Zornow MH, Grafe MR, Tybor C, Swenson MR. Preservation of evoked potentials in a case of anterior spinal artery syndrome. Electroencephalogr Clin Neurophysiol. 1990 Mar-Apr;77(2):137-9.
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last modified: 08 Apr 2016