Transcranial Motor Evoked Potential Monitoring for Pediatric Spine Surgery

1
Transcranial Motor Evoked Potential Monitoring
for Pediatric Spine Surgery

• Children Hospital and Regional Medical Center of
  Seattle
• K. Song, MD D. Emerson, MD M. Balvin, MS N J.
  Chen, MD A. Bergeson, BA N. Jiminez, MD J.
  Slimp, MD

2
Introduction

• There is a recognized risk of neurologic injury
  with spine surgery in children
• True incidence unknown
• Range 0.2-5
• Gold standard to assess motor function has been,
  wake-up test
• Direct testing of motor function
• Skilled team, cooperative patient
• Single point in time
• Late 1970s, early 1980s, continuous monitoring
  of brain/spinal activity developed with the goal
  being to provide for early detection of
  neurologic change during surgical manipulation
  and to allow for countermeasures to change the
  outcome
• Various types of monitoring, SSEP, EMG, H-reflex

3
Neural Monitoring

• Monitoring options have been
• SSEP - somatosensory evoked potentials
• False negative rate 0.13
• False positive rate 1.5
• Motor monitoring
• Late 1980s
• NMEP - neurogenic motor evoked potentials
• Antidromic signal via sensory pathways
• False negative reports
• Transcranial Motor Evoked Potentials
• Developed in late 1980s, early 1990s.
  Initially intra-cranial procedures
• Allows true monitoring of cortico-spinal pathways
• Magnetic or electrical stimulation
• Upper extremities as controls
• All or none response
• Intersynaptic transmission means need to use
  total intravenous anesthesia (TIVA)

4
Purpose

• Review early experience and learning curve using
  TcMEP
• Identify factors related to positive changes
• Identify reversal strategies for positive changes
• Determine sensitivity compared to SSEP if ture
  positive changes

5
Methods

• 8/03 - 4/05 - 139 spinal deformity/tumor cases
• 84 attempted MEP/SSEP (78 spine deformity 6
  tumor)
• Did not attempt to perform monitoring for
• Known seizure disorder
• Nonamb., incontinent spastic quadriparesis
• Paraparetic myelodysplasia
• Spondylolisthesis/spondylolysis
• Idiopathic scoliosis 35
• Congenital scolisis 4
• Neuromuscular scoliosis 29
• Acquired kyphosis 5
• Congenital kyphosis 5
• Intra canal tumor/syrinx 6
• Technique
• CV2 stimulator (Caldwell laboratories) Separate
  consent - FDA approved 2/05
• Stimulation sites Left/Right cortex C3 and C4
  sites
• Recording sites
• Thenar - wrist, Tibialis anterior - ankle, Toe
  flexors - heel

6
Anesthesia

• This requires total intravenous anesthesia
• Propofol most commonly used
• Titratable
• Short acting
• Propofol infusion syndrome
• Opiates as adjunct
• Fentanyl/Remifentanyl
• Inhalational agents - interfere with monitoring.
  Need minimal dose and only at initation of case
  or will have problems
• Benzodiazepines
• Controlled hypotension more difficult
• Propofol Infusion Syndrome
• Is a fatal complication of high dose Propofol.
  Causes
• Metabolic acidosis
• Lipemic serum (common)
• Irreversible bradycardia - asystole
• Associated with rate of infusion gt 4.5 mg/kg/hr
• 200?g/kg/min - 50kg female 24mg/kg/hr
• Associated with infusions gt 24 hours
• Generally seen in ICU settings

7
Results

• Significant SSEP change definition
• 50 ? amplitude
• 10 ? latency
• Significant MEP change definition
• Complete loss, intact uppers
• Degradation gt 75 with lack of response by
  voltage increase of 100 volts and adjustment of
  anesthesia
• Neuro Status
• 49 - Preop Normal ? Postop Normal
• 32 - Neuro abnormal preop ? No change postop
• 3 - Neurologically worse postop Intrapinal
  tumor, congenital kyphosis

• MEP loss 100 predictive deficit
• 1 MEP absent stable SSEP
• 5 SSEP absent stable MEPs 2 SSEP absent, MEP
  absent-both with neuro deficit

8
Results

• 17 pts. (20) with variable/loss MEP - no deficit
• A/P fusion, Length of surg., MAP (plt0.08)
• 2/17 had abnormal SSEP
• Successful strategies to recover TcMEP
• Increase number of trains of stimulus
• Increase voltage of stimulus
• Raise MAP to gt 50
• Decrease Propofol infusion rate to lt
  200?g/kg/min.
• Release correction

Loss No Recovery
Loss Recovery
LIGATION SEGMENTAL ARTERY With release
MAP 51 MAP 60 Propofol 200?150
LOSS AFTER LAMINECTOMY
9
MEP Learning curve versus use of inhalational
agentsAs we used less inhalational agents,
positive MEP cases decreased relative to total
number of cases.
Pos MEP
cases
Inhalational
TIVA
10
The impact of Inhalational Anesthetics
Voltage required to generate MEP greater with
higher number of trains If inhalational agents
used.
For a given age with stable BP and uncomplicated
case Younger children require higher number of
trains and more variable voltage to generate MEP
stimulation
11
Summary

• TcMEP is a useful, predictable, safe technique
  for motor monitoring
• There is a steep learning curve
• You need good anesthesia/monitoring with
  communication between them
• There can be a high rate of positives which may
  or may not be false, but are associated with
• Low MAP
• High propofol flow
• Use of inhalational anesthesia
• Age of patient, younger gt older for variability
• High sensitivity, ? High specificity
• Propofol infusion syndrome is a risk, but
  incidence is unknown. Alternative agents may
  include agents such as Etomidate?