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Intraoperative monitoring of corticospinal function
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Electroencephalography and clinical Neurophysiology 94 (1995) 89-90
Q
Letters to the Editor Intraoperative monitoring of corticospinal function David Burke a,b,*, Richard Hicks b a Prince of Wales Medical Research Institute, High Stree~ Randwick, NSW 2031, Australia Department of Clinical Neurophysiology, The Prince Henry and Prince of Wales Hospitals, Sydney 2031, Australia Accepted for publication: 2 November 1994
We have read with considerable interest the letter by Dr. Kofler and colleagues, and the reply by Dr. Nuwer in the May issue of this journal (Kofler et al., 1994; Nuwer, 1994). Dr. Nuwer's response raises a number of issues that should be addressed, lest it discourage readers from using one of the simplest, safest and most reproducible means of monitoring spinal cord function. In defending the value of somatosensory evoked potentials during scoliosis surgery, Dr. Nuwer implies that the risks of weakness or paraplegia are relatively minor: " T h e reported cases are generally mild in degree, outside of the monitored pathways and have other extenuating circumstances." We are aware of 5 reports covering 10 patients who developed weakness; or paraplegia not detected by intraoperative monitoring using sornatosensory evoked potentials (Ginsburg et al., 1985; Lesser et al., 1986; Molaie, 1986; Ben-David et al., 1987; Manninen et al., 1988). Dr. Nuwer was a co-author on the paper by Lesser et al. (1986). Admittedly, neurological deficits after scoliosis surgery are rare, but they can be devastating, particularly when they occur in a patient who was neurologically normal pre-operatively. We acknowledge that the deficit occurred in some of these patients (3 in the report by Lesser et al., 1986) after ~Lheoperation and the monitoring had been concluded. However, this is not the case with all patients. The common feature in these 5 reports was the reliance on scalp-recorded somatosensory potentials, sometimes supplemented by ,;urface recordings of peripheral volleys. In none of the patients were epidural recordings made, as has been advocated by the British school (for example, Jones et al., 1982, 1983). When epidural record-
* Corresponding author. Tel.: + 61.2.3992671; Fax: + 61.2.3991119.
ings can be made, we believe that they provide a better monitoring service than potentials recorded from the skin or scalp. Dr. Nuwer implies that transcranial stimuli of 1500 V would be used in routine monitoring. Such stimuli usually create so much stimulus artefact that it is impossible to record corticospinal volleys using epidural electrodes. Stimuli of 300-400 V are usually quite adequate to produce large well-formed and clearly defined corticospinal volleys in epidural recordings. In a recent, as yet unpublished, study of the trial-to-trial reproducibility of different components of the evoked corticospinal volley, the co-efficient of variation of the amplitude of simple D waves as measured for 100 consecutive responses was found to be between 3% and 8%. Averaging reduced the co-efficient further, and we believe that, given stable anaesthetic and operative conditions, we can detect changes in the amplitude of the D wave of 10-20% quite reliably in averages of < 10 responses. Paradoxically, the components of the corticospinal volley have greater variability if stronger stimuli are used. This is because, when the strength of the cortical stimulus is increased, some corticospinal axons are excited at subcortical sites, and the D wave becomes bifid or trifid. The co-efficients of variation for the amplitudes of the different components of such a complex D wave volley are high, up to 51%. To use stimuli of 1500 V is therefore not "best practice," at least when monitoring the response from the spinal cord using epidural electrodes. This is not to say that we have not used such intense stimuli. In a recent study, we managed to record epidural volleys in response to graded stimuli up to 1500 V in 12 neurologically normal subjects undergoing surgery for scoliosis (Rothwell et al., 1994). However, these stimulus intensities were used to help define the different sites at
0013-4694/95/$09.50 © 1995 Elsevier Science Ireland Ltd. All rights reserved
SSDI 0 0 1 3 - 4 6 9 4 ( 9 4 ) 0 0 3 0 2 - 5
EEG 94136
90
D. Burke, R. Hicks/Electroencephalography and clinical Neurophysiology 94 (1995) 89-90
which electrical stimulation can activate corticospinal axons. The routine monitoring in these patients was undertaken, using less intense stimuli. Dr. Nuwer suggests that intraoperative monitoring of corticospinal volleys requires that one " a v o i d neuromuscular junction blocking medication." This is not necessary when the corticospinal volleys are recorded from the spinal cord using epidural electrodes, as is, we believe, the preferred technique. Under these circumstances, muscle relaxation is a distinct advantage, not something to be avoided. Even if one is recording the response as a compound muscle action potential, it is n o t necessary to avoid neuromuscular junction blocking medications completely, but clearly the subjects cannot be totally paralysed. However, there is considerable variability of the compound muscle action potential evoked by transcranial stimulation, even when anaesthetic and muscle relaxation are kept constant, and we cannot recommend this method o f intraoperative monitoring of corticospinal function if epidural recording can be undertaken. Finally, we would like to point to a further advantage of epidural recording: namely, the ability to monitor ascending somatosensory volleys and descending motor cortex volleys simultaneously using the same recording electrodes. In the technique described recently in this journal (Burke et al., 1992), we demonstrated that simultaneous stimulation of peripheral nerve and motor cortex sets up an ascending volley and a descending volley, both of which can be recorded in the same sweep. The ability to identify an artefactual change in either the ascending sensory or the descending corticospinal volley is enhanced by recording at two levels from the spinal cord.
References Ben-David, B., Hailer, G. and Taylor, P. Anterior spinal fusion complicated by paraplegia. A case report of a false-negative somatosensoryevoked potential. Spine, 1987, 12: 536-539. Burke, D., Hicks, R., Stephen, J., Woodforth, I. and Crawford, M. Assessment of corticospinal and somatosensory conduction simultaneously during scoliosis surgery. Electroenceph. clin. Neurophysiol., 1992, 85: 388-396. Ginsburg, H.H., Shetter, A.G. and Randzens, P.A. Postoperative paraplegia with preserved intraoperative somatosensory evoked potentials. J. Neurosurg., 1985, 63: 296-300. Jones, S.J., Edgar, M.A. and Ransford, A.O. Sensory nerve conduction in the human spinal cord: epidural recordings made during scoliosis surgery. J. Neurol. Neurosurg. Psychiat., 1982, 45: 446-451. Jones, S.J., Edgar, M.A., Ransford, A.O. and Thomas, N.P. A system for the electrophysiological monitoring of the spinal cord during operations for scoliosis. J. Bone Jt. Surg., 1983, 65B: 134-139. Kofler, M., Kr'2an, M., Zgur, T., Vodu~ek, D.B. and Deletis, V. Neuromonitoring during surgery - a comment. Electroenceph. clin. Neurophysiol., 1994, 90: 388-389. Lesser, R.P., Raudzens, P., Liiders, H., Nuwer, M.R., Goldie, W.D., Morris, H.H., Dinner, D.S., Klem, G., Hahn, J.F., Shetter, A.G., Ginsburg, H.H. and Gurd, A.R. Postoperative neurological deficits may occur despite unchanged intraoperative somatosensory evoked potentials. Ann. Neurol., 1986, 19: 22-25. Manninen, P.H., Lam, A.M. and Nantan, W.E. Monitoring of somatosensory evoked potentials during temporary arterial occlusion in cerebral aneurysm surgery. Anesth. Analg., 1988, 67: S139. Molaie, M. False negative intraoperative somatosensory evoked potentials with simultaneous bilateral stimulation. Clin. Electroenceph., 1986, 17: 6-9. Nuwer, M.R. Response to Kofler eta|. on neuromonitoring during surgery. Electroenceph. clin. Neurophysiol., 1994, 90: 390. Rothwell, J.C., Burke, D., Hicks, R., Stephen, J., Woodforth, I. and Crawford, M. Transcraniai electrical stimulation of the motor cortex in man: further evidence for the site of activation. J. Physiol. (Lond.), 1994, 481: 243-250.
Electroencephalography and clinical Neurophysiology 94 (1995) 89-90
Q
Letters to the Editor Intraoperative monitoring of corticospinal function David Burke a,b,*, Richard Hicks b a Prince of Wales Medical Research Institute, High Stree~ Randwick, NSW 2031, Australia Department of Clinical Neurophysiology, The Prince Henry and Prince of Wales Hospitals, Sydney 2031, Australia Accepted for publication: 2 November 1994
We have read with considerable interest the letter by Dr. Kofler and colleagues, and the reply by Dr. Nuwer in the May issue of this journal (Kofler et al., 1994; Nuwer, 1994). Dr. Nuwer's response raises a number of issues that should be addressed, lest it discourage readers from using one of the simplest, safest and most reproducible means of monitoring spinal cord function. In defending the value of somatosensory evoked potentials during scoliosis surgery, Dr. Nuwer implies that the risks of weakness or paraplegia are relatively minor: " T h e reported cases are generally mild in degree, outside of the monitored pathways and have other extenuating circumstances." We are aware of 5 reports covering 10 patients who developed weakness; or paraplegia not detected by intraoperative monitoring using sornatosensory evoked potentials (Ginsburg et al., 1985; Lesser et al., 1986; Molaie, 1986; Ben-David et al., 1987; Manninen et al., 1988). Dr. Nuwer was a co-author on the paper by Lesser et al. (1986). Admittedly, neurological deficits after scoliosis surgery are rare, but they can be devastating, particularly when they occur in a patient who was neurologically normal pre-operatively. We acknowledge that the deficit occurred in some of these patients (3 in the report by Lesser et al., 1986) after ~Lheoperation and the monitoring had been concluded. However, this is not the case with all patients. The common feature in these 5 reports was the reliance on scalp-recorded somatosensory potentials, sometimes supplemented by ,;urface recordings of peripheral volleys. In none of the patients were epidural recordings made, as has been advocated by the British school (for example, Jones et al., 1982, 1983). When epidural record-
* Corresponding author. Tel.: + 61.2.3992671; Fax: + 61.2.3991119.
ings can be made, we believe that they provide a better monitoring service than potentials recorded from the skin or scalp. Dr. Nuwer implies that transcranial stimuli of 1500 V would be used in routine monitoring. Such stimuli usually create so much stimulus artefact that it is impossible to record corticospinal volleys using epidural electrodes. Stimuli of 300-400 V are usually quite adequate to produce large well-formed and clearly defined corticospinal volleys in epidural recordings. In a recent, as yet unpublished, study of the trial-to-trial reproducibility of different components of the evoked corticospinal volley, the co-efficient of variation of the amplitude of simple D waves as measured for 100 consecutive responses was found to be between 3% and 8%. Averaging reduced the co-efficient further, and we believe that, given stable anaesthetic and operative conditions, we can detect changes in the amplitude of the D wave of 10-20% quite reliably in averages of < 10 responses. Paradoxically, the components of the corticospinal volley have greater variability if stronger stimuli are used. This is because, when the strength of the cortical stimulus is increased, some corticospinal axons are excited at subcortical sites, and the D wave becomes bifid or trifid. The co-efficients of variation for the amplitudes of the different components of such a complex D wave volley are high, up to 51%. To use stimuli of 1500 V is therefore not "best practice," at least when monitoring the response from the spinal cord using epidural electrodes. This is not to say that we have not used such intense stimuli. In a recent study, we managed to record epidural volleys in response to graded stimuli up to 1500 V in 12 neurologically normal subjects undergoing surgery for scoliosis (Rothwell et al., 1994). However, these stimulus intensities were used to help define the different sites at
0013-4694/95/$09.50 © 1995 Elsevier Science Ireland Ltd. All rights reserved
SSDI 0 0 1 3 - 4 6 9 4 ( 9 4 ) 0 0 3 0 2 - 5
EEG 94136
90
D. Burke, R. Hicks/Electroencephalography and clinical Neurophysiology 94 (1995) 89-90
which electrical stimulation can activate corticospinal axons. The routine monitoring in these patients was undertaken, using less intense stimuli. Dr. Nuwer suggests that intraoperative monitoring of corticospinal volleys requires that one " a v o i d neuromuscular junction blocking medication." This is not necessary when the corticospinal volleys are recorded from the spinal cord using epidural electrodes, as is, we believe, the preferred technique. Under these circumstances, muscle relaxation is a distinct advantage, not something to be avoided. Even if one is recording the response as a compound muscle action potential, it is n o t necessary to avoid neuromuscular junction blocking medications completely, but clearly the subjects cannot be totally paralysed. However, there is considerable variability of the compound muscle action potential evoked by transcranial stimulation, even when anaesthetic and muscle relaxation are kept constant, and we cannot recommend this method o f intraoperative monitoring of corticospinal function if epidural recording can be undertaken. Finally, we would like to point to a further advantage of epidural recording: namely, the ability to monitor ascending somatosensory volleys and descending motor cortex volleys simultaneously using the same recording electrodes. In the technique described recently in this journal (Burke et al., 1992), we demonstrated that simultaneous stimulation of peripheral nerve and motor cortex sets up an ascending volley and a descending volley, both of which can be recorded in the same sweep. The ability to identify an artefactual change in either the ascending sensory or the descending corticospinal volley is enhanced by recording at two levels from the spinal cord.
References Ben-David, B., Hailer, G. and Taylor, P. Anterior spinal fusion complicated by paraplegia. A case report of a false-negative somatosensoryevoked potential. Spine, 1987, 12: 536-539. Burke, D., Hicks, R., Stephen, J., Woodforth, I. and Crawford, M. Assessment of corticospinal and somatosensory conduction simultaneously during scoliosis surgery. Electroenceph. clin. Neurophysiol., 1992, 85: 388-396. Ginsburg, H.H., Shetter, A.G. and Randzens, P.A. Postoperative paraplegia with preserved intraoperative somatosensory evoked potentials. J. Neurosurg., 1985, 63: 296-300. Jones, S.J., Edgar, M.A. and Ransford, A.O. Sensory nerve conduction in the human spinal cord: epidural recordings made during scoliosis surgery. J. Neurol. Neurosurg. Psychiat., 1982, 45: 446-451. Jones, S.J., Edgar, M.A., Ransford, A.O. and Thomas, N.P. A system for the electrophysiological monitoring of the spinal cord during operations for scoliosis. J. Bone Jt. Surg., 1983, 65B: 134-139. Kofler, M., Kr'2an, M., Zgur, T., Vodu~ek, D.B. and Deletis, V. Neuromonitoring during surgery - a comment. Electroenceph. clin. Neurophysiol., 1994, 90: 388-389. Lesser, R.P., Raudzens, P., Liiders, H., Nuwer, M.R., Goldie, W.D., Morris, H.H., Dinner, D.S., Klem, G., Hahn, J.F., Shetter, A.G., Ginsburg, H.H. and Gurd, A.R. Postoperative neurological deficits may occur despite unchanged intraoperative somatosensory evoked potentials. Ann. Neurol., 1986, 19: 22-25. Manninen, P.H., Lam, A.M. and Nantan, W.E. Monitoring of somatosensory evoked potentials during temporary arterial occlusion in cerebral aneurysm surgery. Anesth. Analg., 1988, 67: S139. Molaie, M. False negative intraoperative somatosensory evoked potentials with simultaneous bilateral stimulation. Clin. Electroenceph., 1986, 17: 6-9. Nuwer, M.R. Response to Kofler eta|. on neuromonitoring during surgery. Electroenceph. clin. Neurophysiol., 1994, 90: 390. Rothwell, J.C., Burke, D., Hicks, R., Stephen, J., Woodforth, I. and Crawford, M. Transcraniai electrical stimulation of the motor cortex in man: further evidence for the site of activation. J. Physiol. (Lond.), 1994, 481: 243-250.