Original Article
Somatosensory- and motor-evoked potential monitoring during spine
and spinal cord surgery
P Costa*,1, A Bruno2, M Bonzanino1, F Massaro3, L Caruso2, I Vincenzo4, P Ciaramitaro1 and E Montalenti5
1Section of Clinical Neurophysiology, CTO Hospital, Torino, Italy; 2Section of Spine Surgery, CTO Hospital, Torino,
Italy; 3Section of Neurosurgery, CTO Hospital, Torino, Italy; 4Section of Anesthesiology, CTO Hospital, Torino, Italy;
5Department of Neurosciences, University of Torino, Torino, Italy
Study design: Prospective, observational study.
Setting: Regional Trauma Center, Torino, Italy.
Objectives: Complex spinal surgery carries a significant risk of neurological damage. The aim
of this study is to determine the reliability and applicability of multimodality motor-evoked
potentials (MEPs) and somatosensory-evoked potentials (SEPs) monitoring during spine and
spinal cord surgery in our institute.
Methods: Recordings of MEPs to multipulse transcranial electrical stimulation (TES) and
cortical SEPs were made on 52 patients during spine and spinal cord surgery under propofol/
fentanyl anaesthesia, without neuromuscular blockade.
Results: Combined MEPs and SEPs monitoring was successful in 38/52 patients (73.1%),
whereas only MEPs from at least one of the target muscles were obtained in 12 patients (23.1%);
both MEPs and SEPs were absent in two (3.8%). Significant intraoperative-evoked potential
changes occurred in one or both modalities in five (10%) patients. Transitory changes were
noted in two patients, whereas three had persistent changes, associated with new deficits or
a worsening of the pre-existing neurological disabilities. When no postoperative changes in
MEP or MEP/SEP modalities occurred, it was predictive of the absence of new motor deficits
in all cases.
Conclusion: Intraoperative combined SEP and MEP monitoring is a safe, reliable and sensitive
method to detect and reduce intraoperative injury to the spinal cord. Therefore, the authors
suggest that a combination of SEP/MEP techniques could be used routinely during complex
spine and/or spinal cord surgery.
Spinal Cord (2007) 45, 86–91. doi:10.1038/sj.sc.3101934; published online 2 May 2006
Keywords: spine surgery; spinal cord surgery; intraoperative neurophysiologic monitoring;
transcranial, electrically elicited motor-evoked potentials; somatosensory-evoked
potentials
Introduction
Spine and spinal cord surgery carries a significant risk of
neurological impairment.1 The incidence of severe post-
operative neurologic sequelae has been reported to be
0.46% for anterior cervical discectomy,2 0.25–3.2% for
scoliosis surgery3,4 and 23.8–65.4% for intramedullary
spinal cord tumour surgery.5,6
Over the last decade, intraoperative monitoring with
somatosensory-evoked potentials (SEPs) has proven
to be a reliable tool in the assessment of the spinal
cord function during complex surgery. Moreover, it is
also possible to identify any evolving iatrogenic spinal
cord injury, thus reducing the risk of postoperative
deficits.7 However, as SEPs are mediated primarily by
the dorsal sensory spinal cord tracts, they cannot assess
the spinal motor pathways, which may be independently
damaged.8 Consequently, the use of transcranial,
electrically-elicited, motor evoked potentials (MEPs)
has been introduced so as to assess the integrity of the
motor pathways during such procedures as the removal
of spinal cord tumours, correction of scoliosis and
cervical spine surgery.8–15
This study sought to determine the reliability and
applicability of multimodality MEP and SEP moni-
toring during spine and spinal cord surgeries in our
institution.
*Correspondence: P Costa, Section of Clinical Neurophysiology, CTO
Hospital, Via Zuretti 29, Torino 10126, Italy
Spinal Cord (2007) 45, 86–91
& 2007 International Spinal Cord Society All rights reserved 1362-4393/07 $30.00
www.nature.com/sc

Materials and methods
MEP and SEP monitoring was attempted on a total of
52 patients (30 men, 22 women, average age 50.9719.6
year, range 16–81 years).
The surgery was carried out for trauma in 10 cases,
tumour resection in 20, spondylosis in 14, scoliosis in
five, correction of vascular abnormality of the spinal
cord in two cases and multiple dorsal echinococcus
cysts in one. Table 1 reports the procedures used, 25
cervical (48.08%), 20 (38.46%) thoracic and seven
(13.46%) lumbosacral.
Preoperative mild to severe neurological disability
was present in 32 (61.5%) of the 52 patients, whereas
20 (38.5%) had a normal preoperative neurological
examination.
All patients gave their informed consent after being
informed that potential risks included seizures, skin
burns from stimulating electrodes, tongue bites, inad-
vertent injury caused by transcranial electrical stimula-
tion (TES)-induced patient movement.
Continuous spinal cord monitoring was performed,
as from the induction of anaesthesia until the end of
surgical manoeuvres.
The anaesthetic protocol used during surgery included
a combination of the two drugs, remifentanil and
propofol, with total intravenous anaesthesia (TIVA).
Induction was obtained with a continuous infusion of
remifentanil at 0.15–0.25 mg/kg/min and maintained
with 0.25–0.40 mg/kg/min. Target-controlled infusion
was used for propofol with a plasma concentration for
induction of 3–4 mg/ml and maintenance with 3–4.5 mg/
ml. No muscle relaxants were used after induction and
intubation.
Cortical SEPs were elicited by a 100 or 200 ms square-
wave electrical pulse presented sequentially to the poste-
rior tibial and/or median nerves at a rate of 7.1/s.
Stimulus intensity was adjusted individually and ranged
from 14 to 40mA. Cortical potentials were recorded
from monopolar needle electrodes placed at Cz0 for
posterior tibial nerve stimulation, C30 or C40 for median
nerve stimulation and referenced to Fpz (international
10–20 EEG system). Commercially available neuro-
physiology instrumentation (Nicolet Endeavor; Nicolet
Biomedical, Madison, WI, USA) was used for SEPs
stimulation and recording. Filtering was typically 30–
1000Hz, with a 50 or 100ms analysis time; averaging
was stopped manually at such times as potentials were
clearly reproducible and the responses repeatedly com-
pared to that of the baseline (after induction and
positioning).
MEPs were elicited with a brief duration of transcra-
nially applied electrical pulses (pulse width¼ 50 ms),
high-voltage (200–700V) anodal electrical stimulus train
(N¼ 3–5, interstimulus interval 4ms), delivered with
two corkscrew-type electrodes inserted over motor
cortex regions at C3 and C4 (international 10–20 EEG
system). Stimuli were delivered through a commercially
available IOM electrical stimulator (D185; Digitimer,
Welwyn Garden City, UK) with responses recorded on
the same system used for monitoring SEPs. In order to
avoid bite or tongue bites, a bite block consisting of
rolled gauze were used.
Right extremity MEPs were monitored after left-
cranium anodal stimulation and vice versa. MEPs were
recorded with a needle electrode placed in the muscle
with a belly-tendon montage. Although the choice of
muscles used differed according to the pathology, those
most commonly chosen were responses from the
abductor pollicis brevis or the first dorsal interosseus
muscle in the upper extremities and both tibialis anterior
and abductor hallucis muscles in the lower extremities.
The time base was 100–200ms and the filter bandpass
30–3000Hz, occasionally making use of a restricted
bandpass so as to reduce artefacts. Cortical SEP ampli-
tude change was defined as an amplitude alteration
occurring abruptly or as a trend clearly exceeding trial-
to-trial variability, excluding technical problems that is,
a persistent unilateral or bilateral amplitude loss of
at least 50% was used as a warning criteria.
MEPs were interpreted in a similar manner, but as
there was a large trial-to-trial variability of the normal
background, persistent amplitude decrements of more
than 60% of baseline values were considered indicative
of significant change.
The surgical team was immediately informed of any
significant EP change.
Results
Successful combined MEP and SEP monitoring was
obtained in 38 (73.1%); only MEPs from at least one of
the target muscles were obtained in 12 patients (23.1%).
Both MEPs and SEPs were absent in two patients
(3.8%), who presented marked preoperative lower-limb
weakness and cannot walk without assistance.
It was possible to record both SEPs and MEPs in the
20 patients who had a normal preoperative neurological
examination; whereas SEPs were unsuitable for intra-
operative monitoring in 12 (37.5%) and MEPs in two
(17.8%), in the neurologically compromised group of
patients, no patient with absent MEP had preserved
SEP; therefore, the data herein reported refer to the 50
patients in whom it was possible to carry out some form
of monitoring.
Table 1 Diagnostic categories of the 52 patients
Pathology
Total Level
No. % Cervical Thoracic Lumbo-sacral
Trauma 10 19.2 8 2
Tumour 20 28.5 5 10 5
Spondylosis 14 26.9 12 2
Scoliosis 5 9.6 4 1
AVM 2 3.9 1 1
Echinococcus 1 1.9 1
Total 52
SEP and MEP monitoring during spine and spinal cord surgery
P Costa et al
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Spinal Cord