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Improvement of Motor-Evoked Potentials by Ketamine and Spatial Facilitation During Spinal Surgery in a Young Child

Erb, Thomas O. MD, MHS*; Ryhult, Sven E. CRNA*; Duitmann, Ewald CRNA*; Hasler, Carol MD*; Luetschg, Juerg MD; Frei, Franz J. MD*

doi: 10.1213/01.ANE.0000149896.52608.08
Pediatric Anesthesia: Case Report

Monitoring motor evoked potentials is desirable during spine surgery but may be difficult to obtain in small children. In addition, the recording of reliable signals is often hampered by the presence of various anesthetics. We report the case of a young child whose motor evoked potentials were successfully monitored using a ketamine-based anesthesia and a newly introduced stimulation technique consisting of combined spatial and temporal facilitation.

IMPLICATIONS: Continuous monitoring of corticospinal motor pathways is essential during spine surgery, but respective signals are difficult to obtain in small children. We present a 3-yr and 8-mo old child, in which motor evoked potentials could successfully be monitored with specific anesthetic and electrophysiologic techniques.

*Department of Pediatric Orthopedic Surgery, †Department of Pediatric Neurology and Neurophysiology. University Children’s Hospital Beider Basel, Basel, Switzerland

Accepted for publication October 22, 2004.

Address correspondence and reprint requests to Franz J. Frei, MD, Associate Professor, Department of Pediatric Anesthesiology, University Children’s Hospital Beider Basel, UKBB, Roemergasse 8, CH-4058 Basel, Switzerland. Address e-mail to Franz-J.Frei@unibas.ch.

Spinal surgery bears the risk of spinal cord injury with neurological sequelae. Impending damage that is limited to the corticospinal motor pathways or the anterior horn may remain undetected by somatosensory-evoked potentials (SSEP) (1,2). Motor pathway integrity can be monitored by recording motor-evoked potentials (MEPs) from transcranial electrical stimulation (TES) of the motor cortex (3). However, this technique is not reliable in all cases, as the responses may be easily suppressed to varying degrees by different anesthetics. Although modifications of the stimulation modalities, such as temporal and spatial facilitation, have improved recording of reliable MEPs, signal suppression by anesthetics may still prevent reliable intraoperative monitoring (4,5). The immature corticospinal motor pathway in small children is an additional factor decreasing the likelihood of successfully recording MEP responses (6). We report the case of a young child whose MEPs were successfully monitored when a ketamine-based anesthesia was used although an attempt 4 months earlier was unsuccessful when a small-dose propofol-based anesthesia using the same stimulation modalities was used.

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Case Report

A 13-kg, 3-yr and 8-mo old male, initially presenting with a congenital thoracic scoliosis of 95° Cobb angle, was scheduled to undergo a left-sided expansion thoracoplasty (7). His medical history was otherwise unremarkable; in particular, a normal neurological status was recorded. Preanesthetic electrophysiological testing was not done. The patient was premedicated with 4 mg midazolam rectally. Anesthesia was induced with sevoflurane and 50% nitrous oxide. After fentanyl 50 μg was administered, tracheal intubation was performed without muscle relaxants. Sevoflurane was stopped and propofol (50–100 μg · kg−1 · min−1) and remifentanil (2 μg · kg−1 · min−1) were started. For MEP monitoring, TESs (consisting of a train-of-five pulses, 500–700V; time constant, 100 μs; interstimulus interval, 2 ms) were applied with a Digitimer D185 stimulator (Digitimer Ltd, Hertfordshire, UK) triggered by a Viking IV D apparatus (Nicolet Biomedical, Madison, WI). The anode was placed at the Cz position and a ring of 4 cathodes approximately 6 cm apart. A facilitatory stimulus (train-of-10 pulses at 500 Hz; pulse duration 0.5 ms; strength 20–60 mA) to the medial border of the plantar arch was delivered 60 ms before TES was started (5). No MEPs could be recorded in the tibialis anterior muscles (Fig. 1). Postoperative motor function was normal.

Figure 1

Figure 1

Four months later, the patient had to undergo a planned lengthening procedure of the titanium rib implant during which monitoring of MEPs was applied as described above. After premedication with 4 mg midazolam rectally, anesthetic induction and tracheal intubation were performed with sevoflurane and fentanyl 50 μg. Subsequently, ketamine 20 mg as a bolus followed by an infusion of 4 mg · kg−1 · h−1 and remifentanil, 2 μg · kg−1 · min−1 was administered. Nitrous oxide 50% was used during the entire procedure. MEP responses with amplitudes of at least 50 μV could be obtained in the lower extremities throughout the procedure, provided that spatial facilitation was applied (Fig. 2). Reliable MEP monitoring was possible in the upper extremities without spatial facilitation (Fig. 3).

Figure 2

Figure 2

Figure 3

Figure 3

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Discussion

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.

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