Improvement of Motor-Evoked Potentials by Ketamine and
Spatial Facilitation During Spinal Surgery in a Young Child
Thomas O. Erb, MD, MHS*, Sven E. Ryhult, CRNA*, Ewald Duitmann, CRNA*,
Carol Hasler, MD*, Juerg Luetschg, MD†, and Franz J. Frei, MD*
*Department of Pediatric Orthopedic Surgery, †Department of Pediatric Neurology and Neurophysiology. University
Children’s Hospital Beider Basel, Basel, Switzerland
Monitoring motor evoked potentials is desirable dur-
ing spine surgery butmay 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.
(Anesth Analg 2005;100:1634–6)
S
pinal surgery bears the risk of spinal cord
injury with neurological sequelae. Impending
damage that is limited to the corticospinal mo-
tor pathways or the anterior horn may remain un-
detected by somatosensory-evoked potentials
(SSEP) (1,2). Motor pathway integrity can be moni-
tored 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 anes-
thetics. 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 cor-
ticospinal motor pathway in small children is an
additional factor decreasing the likelihood of suc-
cessfully recording MEP responses (6). We report
the case of a young child whose MEPs were success-
fully monitored when a ketamine-based anesthesia
was used although an attempt 4 months earlier was
unsuccessful when a small-dose propofol-based an-
esthesia using the same stimulation modalities was
used.
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 med-
ical history was otherwise unremarkable; in particular, a nor-
mal neurological status was recorded. Preanesthetic electro-
physiological testing was not done. The patient was
premedicated with 4 mg midazolam rectally. Anesthesia was
induced with sevoflurane and 50% nitrous oxide. After fenta-
nyl 50
g was administered, tracheal intubation was per-
formed without muscle relaxants. Sevoflurane was stopped
and propofol (50–100
g·kg

1
· min

1
) and remifentanil (2

g·kg

1
· min

1
) were started. For MEP monitoring, TESs
(consisting of a train-of-five pulses, 500–700V; time constant,
100
s; interstimulus interval, 2 ms) were applied with a Digi-
timer D185 stimulator (Digitimer Ltd, Hertfordshire, UK) trig-
gered by a Viking IV D apparatus (Nicolet Biomedical, Madi-
son, 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 de-
livered 60 ms before TES was started (5). No MEPs could be
recorded in the tibialis anterior muscles (Fig. 1). Postoperative
motor function was normal.
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, anes-
thetic induction and tracheal intubation were performed
with sevoflurane and fentanyl 50
g. Subsequently, ket-
amine 20 mg as a bolus followed by an infusion of
4mg·kg

1
·h

1
and remifentanil, 2
g·kg

1
· min

1
was
administered. Nitrous oxide 50% was used during the entire
procedure. MEP responses with amplitudes of at least 50
V
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).
Accepted for publication October 22, 2004.
Address correspondence and reprint requests to Franz J. Frei, MD,
Associate Professor, Department of Pediatric Anesthesiology, Univer-
sity Children’s Hospital Beider Basel, UKBB, Roemergasse 8, CH-4058
Basel, Switzerland. Address e-mail to Franz-J.Frei@unibas.ch.
DOI: 10.1213/01.ANE.0000149896.52608.08
2005 by the International Anesthesia Research Society
1634 Anesth Analg 2005;100:1634–6 0003-2999/05

Discussion
Expansion thoracoplasty is a new procedure for cor-
rection of congenital scoliosis aiming at inducing lung
growth on the concave-sided hemithorax (7). Al-
though experience with this technique is limited and
the incidence of spinal cord or brachial plexus damage
is unknown at present, isolated cases have been re-
ported (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 tech-
niques. Our standard anesthetic technique for elec-
trophysiological testing during spine surgery con-
sists of small-dose propofol and 50% nitrous oxide.
With this technique, reliable MEP signals can usu-
ally be obtained in children older than 8 years. In
younger children, however, we found it increas-
ingly difficult to obtain MEPs. This observation is in
accordance with a reported inverse relationship be-
tween the height range 70–180 cm and threshold
stimulus intensity required for evoking muscle re-
sponses in normal subjects, although electromag-
netic stimulation had been used in this study (8).
Although we could not record signals during the
first operation when using propofol-based anesthe-
sia (Fig 1), MEP monitoring was successful through-
out 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 pa-
tient (9). In a primate model, using transcranial
magnetic stimulation, epidurally recorded descend-
ing neural motor volleys were well maintained dur-
ing ketamine infusions (10,11). Favorable effects on
Figure 1. Motor evoked potentials of the lower extremities during
the first operation. A facilitation stimulus to the left (upper record-
ing) and right (lower recording) plantar arch was administered
followed 60 ms later by transcranial electrical stimulation. No sig-
nals could be recorded in the tibialis anterior muscles.
Figure 2. Motor evoked potentials of the lower extremities during
the second operation. A spatial facilitation stimulus to the left
plantar arch followed 60 ms later by transcranial electrical stimula-
tion evoked a motor potential of approximately 200 mV on the left
side after 24 ms, whereas the potential on the right side was only
about 40 mV (upper recording). The spatial facilitation stimulus to
the right plantar arch evoked a potential in the right tibialis anterior
muscle of approximately 350 mV, whereas it was not present in the
left tibialis anterior muscle (lower recording).
ANESTH ANALG CASE REPORTS 1635
2005;100:1634–6