The Effect of Age on Motor Evoked Potentials in Children Under by dffhrtcv3


									   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
  motor pathways.

• 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.

                               Muscle relaxant
                                                                         Adjusted anesthetic
                                                                         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
              Stimulation intensity:150~400V
              Interstimulus Interval:2ms
              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
    across groups.
• 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
  ([R2=0.27], P<0.0001).

• 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
  concentration (P<0.0001).

• 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
  for age.

• 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
    younger subjects.

  – Range of propofol dosage has not been shown to
    substantially depress MEP responses.
Discussion         (limitation)

• 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.
Discussion         (limitation)

• Perhaps the key limitation of this study is that it
  was not designed to test maturational effects of
  MEP responses.

• 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
  versus adults.

• 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
  are awake.
Discussion          (explain)

• 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.
 Discussion            (explain)

• 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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