The Effect of Age on Motor Evoked Potentials
in Children Under Propofol/Isoflurane Anesthesia
Jeremy A, Russ Lyon, John Feiner, et al.
• MEPs may identify and help prevent injury to
• MEPs can be detected intraoperatively in
neurologically intact children , but these signals
may be missed in 32%–61% of children with
preexisting neurologic deficits.
• The depression of MEPs caused by inhaled
anesthetics or by the sedative-hypnotic propofol.
• Age may also influence our ability to elicit MEPs.
• We retrospectively analyzed data obtained from
pediatric patients (2–18 yr of age) who underwent
spine surgery while using transcranial electric
stimulation MEPs monitoring.
• We specifically evaluated how age affected
stimulation variables necessary to elicit MEPs to
during general anesthesia.
• 56 neurologically intact subjects who underwent
correction for idiopathic scoliosis and who were
monitored using transcranial myogenic MEPs.
• excluded patients who had myelopathic,
neuromuscular, and congenital spine pathologies.
levels as needed for
Isoflurane (0.75%~1.0%) clinical reasons.
Propofol(50~75µg/kg·min) and Fentanyl No BIS.
no Muscle relaxant
Stimuli locus:C3,C4 started at 150 V and
Record locus: thenar-hypothenar muscles was increased by 25 V
tibialis anterior increments up to
extensor hallucis longus, amaximum of 400 V
abductor hallucis muscles
Train stim: 5 pulses
Duration: 0.5ms after increments of
Recording and filtering parameters were typically 50–75 V, an additional
30–1000 Hz, with a time base of 100 ms. pulse was added
reproducible response waveforms with amplitudes
of at least 50 V in all muscle groups.
If MEP responses were unobtainable after all technical adjustments
had been exhausted, the anesthesiologist will reduce the dose of the
volatile anesthetic drug.
All recordings were done before surgical incision .
• Data analysis:
– Statistical analysis was performed using JMP 4.0.
– We performed multiple linear regression to develop a
model of the change in threshold voltage and to investigate
the contribution of multiple factors, including age, height,weight,
gender, BMI, BSA, and anesthetic dose.
– χ2analyses were used to compare categorical variables
• MEP stimulation threshold voltage was higher
in younger patients and decreased with
increasing age (P ＜ 0.0001).
• There was a similar, but weaker relationship
between threshold voltage and increasing BSA
([R2=0.42 versus 0.53 for age], P ＜0.0001),
weight ( [R2=0.36], P＜0.0001) and height
• BMI was weakly associated with threshold voltage
(data not shown; [R2=0.12], P＜0.05). Gender was
not a significant factor.
• Younger patients also required more stimulating
pulses, as compared with older patients, to elicit
reproducible MEP responses (R2=0.53, P＜0.0001).
• Younger patients received smaller isoflurane doses,
measured as both absolute end-tidal isoflurane
• MEP stimulation threshold was higher in subjects
receiving smaller isoflurane doses (R2=0.24, P＜
Result (Multivariate analysis)
• Age was the dominant predictor of threshold
voltage (P＜ 0.0001).
• Isoflurane dose, Weight, height, BMI, and BSA
were not statistically significant, after accounting
• The age-related difference in stimulating voltage
cannot be attributed to larger concentrations of
suppressive anesthetics administered to younger
– Significantly less isoflurane was administered to the
– Range of propofol dosage has not been shown to
substantially depress MEP responses.
• Total anesthetic depth is not known. No BIS.
• Arterial blood pressure was not consistently
• Temperature, another physiologic factor that
affects MEP responses , did not differ by age at
the time MEP measurements were made.
• Perhaps the key limitation of this study is that it
was not designed to test maturational effects of
• Our analysis focused on defining the minimum
parameters needed to generate interpretable
responses rather than on examining the
characteristics of the responses.
Discussion (other supports)
• Higher thresholds are needed when using
transcranial magnetic MEPs in awake children
• Parano et al. observed diminished evoked
response amplitudes in infancy and childhood
compared to adults; this difference was more
pronounced in the first 2 years of life.
• Reliable MEP responses may be unobtainable in
children younger than 6 years, even when they
• Our study results can best be explained by
immaturity of the central nervous system.
– Cortical changes with aging may affect MEP
– Hagelthorn et al. observed decreased evoked
potential inter-hemispheric transmission time
with increasing age (7–17 years), suggesting
increased corpus callosal myelination and
integration during childhood.
• Spinal cord motor pathways also undergo a
prolonged period of maturation.
– Nezu et al. estimated that electrophysiologic maturity of
the corticospinal tracts (CST) innervating the hand
muscles was complete by 13 years of age.
– At birth, the conduction velocity of central motor fibers of
the spinal cord are approximately 10 m/s, whereas adult
values are in the range of 50–70 m/s.
• We cannot conclude that these observations
are attributable solely to maturation effects.
– Younger patients may have enhanced sensitivity to
suppression by volatile anesthetics or propofol.
– Technical challenges.
• younger children are at greater risk for diminished
or lost MEP responses during surgery when using
this anesthetic regimen.
– 90% of our patients 2 to 10 years old----above 300V
– 30% of children older than 10 years required 300V
• Threshold voltage requirements may further
increase during the course of surgery (20,37),
especially with changes in physiologic variables,
such as arterial blood pressure, temperature, and
• The selection of the anesthetic regimen for
any child must consider all of the desired
anesthetic and surgical goals, including the
effects on MEPs.
– Combined propofol and fentanyl anesthesia has been
used successfully for obtaining both myogenic and
epidurally recorded MEPs in a pediatric population as
young as 8 to 12 months.
– Improved MEP stimulation techniques may be
especially useful for obtaining MEP responses in very
young subjects .
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