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Neuromonitoring in the Operating Room Dr. Gary Simon Introduction Define neuromonitoring Risk of CNS damage Spinal cord anatomy Mechanisms of injury Introduction Stagnara and Clonus testing Somatosensory evoked potentials Motor evoked potentials Electromyography

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neuromonitoring in the operating room

Neuromonitoring in the Operating Room

Dr. Gary Simon

LHSC Anesthesiology

introduction
Introduction
  • Define neuromonitoring
  • Risk of CNS damage
  • Spinal cord anatomy
  • Mechanisms of injury

LHSC Anesthesiology

introduction3
Introduction
  • Stagnara and Clonus testing
  • Somatosensory evoked potentials
  • Motor evoked potentials
  • Electromyography

LHSC Anesthesiology

define neuromonitoring
Define neuromonitoring
  • Ensure that functional integrity is maintained
  • Identify anatomical structures
  • Detect injury patterns

LHSC Anesthesiology

risk of cns damage
Risk of CNS damage
  • Elective AAA (0.16%-0.25%)
  • Scoliosis instrumentation (0.4%-1.6%)
  • Coarctation of the aorta (0.4%-1.5%)
  • Thoraco-abdominal aneurysm (16%-25%)
    • Risk factors: dissection, hypotension, long period of aortic crossclamping, increased ICP, sacrifice of critical intercostal or lumbar arteries, and extent of aortic disease

LHSC Anesthesiology

high risk scoliosis procedures
High risk scoliosis procedures
  • Combined anterior and posterior repair
  • Hyperkyphosis
  • Significant rigid curves
  • Neuromuscular vs idiopathic

LHSC Anesthesiology

spinal cord injury
Spinalcord injury
  • Ischemia, disruption, compression, concussion, or distraction.
  • Type, intensity, location and duration of injury determines the extent of damage.
  • Experienced neuromonitoring teams can decrease deficits by 50% for scoliosis surgery (ie 0.46% vs 1.04 %)

LHSC Anesthesiology

spinal cord and brain anatomy
Spinal cord and brain anatomy
  • Gray and white matter
  • Motor and sensory
  • Anterior and posterior spinal arteries
  • Low thoracic/lumbar blood supply
  • Anterior horn cell

LHSC Anesthesiology

brain sensory motor areas
Brain sensory & motor areas

LHSC Anesthesiology

brain sensory motor areas10
Brain sensory & motor areas

LHSC Anesthesiology

brain anatomy
Brain anatomy

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spine anatomy
Spine Anatomy

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slide13

Spine Anatomy

LHSC Anesthesiology

sensory and motor pathways to and from the brain
Sensory and motor pathways to and from the brain

Three sensory modalities:

  • Discriminative touch
    • Touch, pressure and vibration
  • Pain and temperature
    • Free nerve endings are responsive to nerve damage, prostaglandins, histamine and substance P
  • Proprioception

LHSC Anesthesiology

discriminative touch
Discriminative touch
  • Enter the cord and ascend on the ipsilateral side
  • Synapse in the medulla and cross to the contralateral side
  • Secondary afferents project to the ventroposterior lateral nucleus where they synapse and ascend to the cortex

LHSC Anesthesiology

slide16

Pain and temperature

  • Afferents enter the dorsal horn and synapse within one or two segments
  • Cross to the opposite side and ascend in the spinothalamic tract
  • Synapse again in the ventro- posterior lateral nucleus and project to the cortex

LHSC Anesthesiology

slide17

Motor pathways

  • Large pyramidal neurons send axons into the brain that finally end up in the medulla
  • They form pyramids which cross at the caudal part of the medulla
  • 15-20% of fibres do not cross descend
  • in the ant. corticospinal tract
  • Majority of fibres decussate and
  • descend in the lat corticospinal tract
  • Fibres enter the grey matter at the
  • appropriate level and synapse
  • Alpha motor neuron sends axon to
  • muscle

LHSC Anesthesiology

spinal cord vascular supply
Spinal cord vascular supply
  • Arteries of the brain and spinal branches of the subclavian, aorta and iliac arteries
  • Anterior spinal artery supplies the ventral 2/3
  • Posterior spinal arteries supplies the dorsal sensory part of the spine
  • No communicating branches between the anterior and posterior spinal arteries

LHSC Anesthesiology

spinal cord vascular supply19
Spinal cord vascular supply

LHSC Anesthesiology

spinal cord vascular supply20
Spinal cord vascular supply

LHSC Anesthesiology

anterior horn cell
Anterior horn cell is the most sensitive structure in the cord to ischemia

Axons (white matter) are relatively resistant to ischemia

Anterior horn cell

LHSC Anesthesiology

scoliosis etiology
Scoliosis etiology
  • Idiopathic
  • Neuromuscular (flaccidity, spasticity, dyskinesis)
    • Neuropathic
      • Upper motor neuron
        • C.P., spinocerebellar deg., springomyelia, tumour, trauma
      • Lower motor neuron
        • Polio, trauma, spinal muscle atrophy, dysautonomia
    • Myopathic
      • Arthrogryphosis, muscle and myotonic dystrophy, cong. hypotonia, fibre type disproportion

LHSC Anesthesiology

scoliosis etiology23
Scoliosis etiology
  • Congenital
    • 20% have congenital GU malformations;10-15% have congenital heart disease; high association with spinal dysraphism
  • Syndromes
    • Neurofibromatosis
    • Marfan syndrome
  • Compensatory
    • Leg-length discrepancy

LHSC Anesthesiology

mechanisms of injury
Mechanisms of injury
  • Ischemia, disruption, compression, concussion or distraction
  • Distraction or compression affects region
  • Extent of damage determined by-

type, intensity, location and duration of insult

  • Recovery depends on interval of time and reversal of precipitating events.

LHSC Anesthesiology

stagnara wakeup test
Stagnara wakeup test
  • Stagnara wakeup – Clin Orthop 1973
  • Gold standard
  • Difficulties
    • Unsuitable patients
    • Not able to provide ongoing monitoring
    • Limited repetition capabilities
    • Air embolism
    • Extubation
    • Recall

LHSC Anesthesiology

clonus test
Clonus test
  • Motor response to stretch reflex
  • Clonus to rapid dorsiflexion of ankle
    • Normal CNS prevents clonus by sending inhibitory potentials to that reflex area in the spinal cord
    • Recovering from GA depresses central inhibition and allows clonus
  • Problems – timing is everything!

LHSC Anesthesiology

clonus test27
Clonus test
  • The ankle clonus test is not a clinically useful measure of spinal cord integrity in children. Ewen A - Can J Anaesth - 01-MAY-2005; 52(5): 524-9
  • Conclusion: We conclude that the specificity of the ankle clonus test is too low to be clinically useful as a measure of spinal cord integrity in children, both when isoflurane and sevoflurane are used as the primary anesthetic agent.

LHSC Anesthesiology

somatosensory evoked potentials
SomatoSensory Evoked Potentials
  • History
  • Time locked neural patterns vs EEG
  • Comparison of electrical potentials:
    • EKG: 1000µV
    • EEG: 10-100µV
    • SSEP: 0.1-10 µV

LHSC Anesthesiology

homunculus
Homunculus

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international 10 20 markings
International 10-20 markings

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international 10 20 markings31
International 10-20 markings

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somatosensory evoked potentials32
Somatosensory evoked potentials
  • Repetition and averaging
  • Stimulating and recording electrodes
  • Latency and amplitude

LHSC Anesthesiology

somatosensory evoked potentials33
SomatoSensory Evoked Potentials
  • Approximately 100 – 500 stimulus repetitions.
  • Nerve conduction velocity is fairly constant
  • Evoked potentials can be monitored from the periphery to the surface of brain.

LHSC Anesthesiology

somatosensory evoked potentials34
Somatosensory evoked potentials
  • Negative predictive value – 99.9%
  • Positive predictive value – 42%
  • Factors that can alter traces:
    • Technical
    • Physiologic: temperature, hemoglobin, BP
    • Anesthetic effect on traces:

Pentothal, propofol, narcotics and benzodiazepines

LHSC Anesthesiology

somatosensory evoked potentials35

c) Isoflurane

Isoflurane

Somatosensory evoked potentials
  • Factors that can alter traces:
    • Anesthetic: inhalational agents and N2O
    • Ketamine and etomidate (familalmyoclonic epilepsy)

LHSC Anesthesiology

somatosensory evoked potentials39
Somatosensory evoked potentials
  • Warning – latency increase by 10%
    • Amplitude decrease by 50%
  • Interventions
    • Check equipment, physiology, anesthesia
    • Increase blood pressure, reverse manipulation
  • Limitations
    • Afferent impulses are conducted nonsynaptically. Ten plus minutes needed for SSEP’s to deteriorate.
    • Posterior columns only are monitored

LHSC Anesthesiology

motor evoked potentials
Motor evoked potentials
  • Electrical or magnetic energy
  • Stimulating sites:
    • Transcranial electrical stimulation (TCES)
    • Neurogenic “motor” evoke potentials
      • Cervical or thoracic spinal cord
    • Nerve root or fibre
      • Free running or triggered EMG

LHSC Anesthesiology

motor evoked potentials tces
Motor evoked potentials-TCES
  • Exclusively motor potential
  • Paralysis
  • Specific risk to motor cord
    • Intramedullary tumour
    • Vascular abnormalities
    • Thoraco-abdominal aneurysm
    • Certain spinal corrections

LHSC Anesthesiology

motor evoked potentials tces42
Motor evoked potentials-TCES
  • Stimulation over pre-central motor strip
  • 3-5 short duration but high voltage pulses
    • Facilitation of cortical neurons
    • Spatial and temporal summation
    • Amplification of myogenic response
    • Not ECT – seizure should not occur

LHSC Anesthesiology

motor evoked potentials tces43
Motor evoked potentials-TCES
  • Stimulus through the skull to motor cortex.
  • Response can be monitored in epidural space or at the level of muscle.

LHSC Anesthesiology

motor evoked potentials tces44
Motor evoked potentials-TCES
  • Recording sites-
    • Epidural: resistant to anesthetic, more consistent
    • Muscle: anterior horn cell, lateralizing information
  • Anesthesia agents
    • Inhalational agents, N2O
    • TIVA drugs
    • Muscle Relaxants

LHSC Anesthesiology

motor evoked potentials tces45
Motor evoked potentials-TCES
  • Warning signs – present/absent
  • Risks include:
    • Seizures, skin burns, electrode site infections, patient movement, tongue lacerations and jaw fracture
  • Contraindications:
    • Epilepsy, convexity skull defects, increased ICP, significant cardiac disease, intracranial electrodes, vascular clips, shunts, pacemakers or other biomedical devices

LHSC Anesthesiology

motor evoked potentials tces49
Motor evoked potentials-TCES

Charles Dong: Ann Thoracic Surgery 74:S1873-6,2002. Intraoperative Spinal Cord Monitoring During Descending Thoracic and Thoracoabdominal Aneurysm Surgery.

  • 16 of 56 showed MEP evidence of spinal cord ischemia while only four of them had delayed associated SSEP changes.
  • 13 had reversal of MEP changes with (a) implantation of more segmental arteries (b) increasing blood flow (c) increase BP or (d) DHCA. None were paraplegic post-op.
  • 3 had no recovery of MEP traces and all awoke paraplegic (SSEP’s relatively preserved)

LHSC Anesthesiology

motor evoked potentials emg
Motor evoked potentials- EMG
  • Monitor individual nerve roots
  • Electrodes are placed in muscle at risk
    • Quadriceps femoris: L2-L4
    • Anterior tibialis: L4-L5
    • Biceps femoris: L5-S1
    • Gastrocnemius: S1-S2
  • Patients are not (or partially) paralysed

LHSC Anesthesiology

motor evoked potentials emg51
Motor evoked potentials- EMG
  • Free running EMG is based on irritated or injured nerves having neural discharges
    • Burst activity suggesting transient irritation
    • Sustained or train activity suggesting more significant injury

LHSC Anesthesiology

motor evoked potentials emg52
Motor evoked potentials- EMG
  • Triggered EMG can determine proximity to a nerve root
  • Electrical is applied to a pedicle screw or probe. Intact pedicle wall provides a barrier to transmission.
  • EMG activity at less than 4 mA suggests direct contact with the nerve root.

LHSC Anesthesiology

neuromonitoring in the operating room53
Neuromonitoring in the Operating Room
  • Define neuromonitoring
  • Risk of CNS damage
  • Spinal cord anatomy
  • Mechanisms of injury
  • Stagnara and Clonus testing
  • Somatosensory evoked potentials
  • Motor evoked potentials

LHSC Anesthesiology