Transcranial Motor Evoked Potential Monitoring for Pediatric Spine

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					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

Introduction
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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 1970’s, early 1980’s, 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

Neural Monitoring
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Monitoring options have been SSEP - somatosensory evoked potentials » False negative rate 0.13% » False positive rate 1.5% Motor monitoring » Late 1980’s » NMEP - neurogenic motor evoked potentials – Antidromic signal via sensory pathways
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False negative reports

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Transcranial Motor Evoked Potentials
Developed in late 1980’s, early 1990’s. 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)

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Purpose
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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

Methods
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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
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

Technique
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Anesthesia
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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 Is a fatal complication of high dose Propofol. Causes: » Metabolic acidosis » Lipemic serum (common) » Irreversible bradycardia - asystole Associated with rate of infusion > 4.5 mg/kg/hr » 200g/kg/min - 50kg female = 24mg/kg/hr Associated with infusions > 24 hours Generally seen in ICU settings Case reports exist for short cases 3 hours

Propofol Infusion Syndrome
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Results
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Significant SSEP change definition » 50%  amplitude » 10%  latency Significant MEP change definition » Complete loss, intact uppers » Degradation > 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 Absent at Variable start of case Baseline SSEP 7(8.%)* 5 Stable No ∆ 68 Lost Recovery 3 Lost no Recovery 1

MEP
• • •

3(4%)^

10

61

7

3

MEP loss 100% predictive deficit ^1 MEP absent stable SSEP *5 SSEP absent stable MEPs; 2 SSEP absent, MEP absent-both with neuro deficit

Results
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17 pts. (20%) with variable/loss MEP - no deficit » A/P fusion, Length of surg., MAP (p<0.08) 2/17 had abnormal SSEP Successful strategies to recover TcMEP » Increase number of trains of stimulus » Increase voltage of stimulus » Raise MAP to > 50 » Decrease Propofol infusion rate to < 200g/kg/min. » Release correction

Loss Recovery

Loss No Recovery

LIGATION SEGMENTAL ARTERY With release

MAP 51 MAP 60 Propofol 200150

LOSS AFTER LAMINECTOMY

MEP Learning curve versus use of inhalational agents: As we used less inhalational agents, % positive MEP cases decreased relative to total number of cases.
MEP Positive - No Deficit
40 35 30 25 20 15 10 5 0 8 /03 - 1 2/0 3 1 /04 - 5 /0 4 6 /04 - 1 0/0 4 1 1 /0 4 - 3/0 5

Series 2 Series 1

Pos MEP # cases

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%
gSe 07 p0 O 7 ct N 07 ov D 07 ec -0 Ja 7 n0 Fe 8 bM 08 ar -0 Ap 8 rM 08 ay -0 Ju 8 n0 Ju 8 lAu 08 gSe 08 p0 O 8 ct N 08 ov D 08 ec -0 8

Inhalational

TIVA

Au

The impact of Inhalational Anesthetics
M EP STIM ULATION: TIV A AGES 2-21
10 8

ISO/SEVO-N2O,PROPOFOL: ALL AGES
10 8

TRAIN

TRAIN

6 4 2 0 0 100 200 300 400 500 600 700 VOLTAGE Series1

6 4 2 0 0 100 200 300 400 500 600 700 800 900 VOLTAGE Series1

Voltage required to generate MEP greater with higher number of trains If inhalational agents used.
M EP STIM ULATION: TIV A: AGES 9-21
10 8
10 8

MEP STIMULATION: TIVA AGES 2-7

TRAIN

TRAIN

6 4 2 0 0 100 200 300 400 500 600 700 VOLTAGE Series1

6 4 2 0 0 100 200 300 VOLTAGE Series1 400 500 600

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

Summary
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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 > 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?


				
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