Expansion thoracoplasty is a new procedure for correction of congenital scoliosis aiming at inducing lung growth on the concave-sided hemithorax (7). Although experience with this technique is limited and the incidence of spinal cord or brachial plexus damage is unknown at present, isolated cases have been reported (R. M. Campbell, personal communication), making electrophysiological monitoring (including MEP testing) desirable.
The present case is unique because the patient served as his own control during two consecutive procedures using two different anesthetic techniques. Our standard anesthetic technique for electrophysiological testing during spine surgery consists of small-dose propofol and 50% nitrous oxide. With this technique, reliable MEP signals can usually be obtained in children older than 8 years. In younger children, however, we found it increasingly difficult to obtain MEPs. This observation is in accordance with a reported inverse relationship between the height range 70–180 cm and threshold stimulus intensity required for evoking muscle responses in normal subjects, although electromagnetic stimulation had been used in this study (8).
Although we could not record signals during the first operation when using propofol-based anesthesia (Fig 1), MEP monitoring was successful throughout the second operation when a ketamine-based anesthesia was used (Figs. 2 and 3). Ketamine was reported to improve the intraoperative monitoring of SSEPs in a neurologically impaired pediatric patient (9). In a primate model, using transcranial magnetic stimulation, epidurally recorded descending neural motor volleys were well maintained during ketamine infusions (10,11). Favorable effects on MEPs with either unchanged or increased amplitudes have been documented with ketamine in volunteers with transcranial magnetic stimulation (12) and in patients undergoing spine surgery with transcranial electrical stimulation (13–16). Ketamine increases intracranial pressure, salivary and tracheal secretions, skeletal muscle tone, and sympathoadrenal activity. In addition, unpleasant dreaming and hallucinations during emergence from anesthesia are well known potential drawbacks when using ketamine. We did not observe any of these effects during the second anesthesia; however, further systematic studies are needed to determine the usefulness and side effects of ketamine in pediatric patients when MEP monitoring is indicated.
It is notable that reliable evoked potentials could be recorded only when the cortical stimulus was preceded by a train of stimuli to the plantar arch within the receptive field of the withdrawal reflex of the tibialis anterior muscle of the same side (Fig 2). This technique has been shown to result in reliable MEPs after single TES in most adult patients who underwent spinal surgery whereas, without the use of a facilitatory stimulus, recording was successful in only half of these patients (5). In a recent report, facilitation was achieved by using a preconditioning train of transcranial electrical pulses 10–35 milliseconds before the second train of pulses was applied and resulted in magnified MEPs in most patients (17). It seems that various different facilitatory techniques may improve MEP monitoring.
We conclude that, in young children, sophisticated electrophysiological techniques combined with a ketamine-based anesthesia may be necessary to achieve reliable MEPs. We speculate that this approach might also be useful to improve the signal quality of MEPs in patients with preexisting neurological or muscular disease.
The authors would like to thank G. Andersson, MD, Department of Clinical Neurophysiology, Lund University Hospital, Lund, Sweden for his support in electrophysiologic monitoring and Joan Etlinger, Department of Anesthesiology, University Hospitals of Basel, Basel, for her editorial advice.
1. Ginsburg HH, Shetter AG, Raudzens PA. Postoperative paraplegia with preserved intraoperative somatosensory evoked potentials. J Neurosurg 1985;63:296–300.
2. Lesser RP, Raudzens P, Luders H, et al. Postoperative neurological deficits may occur despite unchanged intraoperative somatosensory evoked potentials. Ann Neurol 1986;19:22–5.
3. Kalkman CJ, Ubags LH. Motor evoked potential monitoring. Curr Opin Anesthesiol 1997;10:327–32.
4. De Haan P, Kalkman CJ, de Mol BA, et al. Efficacy of transcranial motor-evoked myogenic potentials to detect spinal cord ischemia during operations for thoracoabdominal aneurysms. J Thorac Cardiovasc Surg 1997;113:87–100.
5. Andersson G, Ohlin A. Spatial facilitation of motor evoked responses in monitoring during spinal surgery. Clin Neurophysiol 1999;110:720–4.
6. Nezu A, Kimura S, Uehara S, et al. Magnetic stimulation of motor cortex in children: Maturity of corticospinal pathway and problem of clinical application. Brain Dev 1997;19:176–80.
7. Campbell RM Jr., Smith MD, Hell-Vocke AK. Expansion thoracoplasty: The surgical technique of opening-wedge thoracostomy. J Bone Joint Surg Am 2004;86:51–64.
8. Eyre JA, Miller S, Ramesh V. Constancy of central conduction delays during development in man: Investigation of motor and somatosensory pathways. J Physiol (Lond) 1991;434:441–52.
9. Agarwal R, Roitman KJ, Stokes M. Improvement of intraoperative somatosensory potentials by ketamine. Paediatr Anaesth 1998;8:263–6.
10. Ghaly RF, Stone JL, Aldrete JA, Levy WJ. Effects of incremental ketamine hydrochloride doses on motor evoked potentials following transcranial magnetic stimulation: A primate study. J Neurosurg Anesthesiol 1990;2:79–85.
11. Ghaly RF, Ham JH, Lee JJ. High-dose ketamine hydrochloride maintains somatosensory and magnetic motor evoked potentials in primates. Neurol Res 2001;23:881–6.
12. Kalkman CJ, Drummond JC, Patel PM, et al. Effects of droperidol, pentobarbital, and ketamine on myogenic transcranial magnetic motor-evoked responses in humans. Neurosurgery 1994;35:1066–71.
13. Yang LH, Lin SM, Lee WY, Liu CC. Intraoperative transcranial electrical motor evoked potential monitoring during spinal surgery under intravenous ketamine or etomidate anaesthesia. Acta Neurochir (Wien) 1994;127:191–8.
14. Ubags LH, Kalkman CJ, Been HD, et al. The use of ketamine or etomidate to supplement sufentanil/N2O anesthesia does not disrupt monitoring of myogenic transcranial motor evoked responses. J Neurosurg Anesthesiol 1997;9:228–33.
15. Kawaguchi M, Sakamoto T, Inoue S, et al. Low dose propofol as a supplement to ketamine-based anesthesia during intraoperative monitoring of motor-evoked potentials. Spine 2000;25:974–9.
16. Inoue S, Kawaguchi M, Kakimoto M, et al. Amplitudes and intrapatient variability of myogenic motor evoked potentials to transcranial electrical stimulation during ketamine/N2O- and propofol/N2O-based anesthesia. J Neurosurg Anesthesiol 2002;14:213–7.
17. Journee HL, Polak HE, de Kleuver M, et al. Improved neuromonitoring during spinal surgery using double-train transcranial electrical stimulation. Med Biol Engl Comput 2004;42:110–3.