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British Medical Books Health Issues Computerized Tomography

Spinal Cord Monitoring

Risk Factors

  • that make neurologic injury more likely
    1. surgical factors
      --sublaminar wiring, spinal distraction, spinal osteotomy, skeletal traction
    2. pathophysiologic factors
      --severe and rigid curves, congenital scoliosis, kyphosis, postradiation curves, pre-existing neurologic
      deficits, neurofibromatosis
    3. anesthetic factors
      -- hemodilution and induced hypotension
      - no decrease in spinal cord flow with nitroprusside-induced hypotension

Mechanisms of spinal cord damage

  1. compession to blunt trauma
  2. stretching or disruption of neural elements
  3. ischemia
  • believed that the latter is the most common explanation
    • Dolan and colleagues --> experimental spinal distraction caused cord ischemia, which was already severe at the point that the SEP became significantly delayed and attenuated
    • Further distraction abolished the SEP as well as cord blood flow at the distraction site
  • early detection and reversal of ischemia ---> crucial in diminishing or preventing neurologic injury

Spinal cord monitoring

  • divided into two categories - clinical and electrophysiologic

Clinical testing

  • physical examination
    • the most important form of monitoring pre- and postoperatively
  • the wake-up test
  • the ankle clonus test

Electrophysiologic testing

  • measures the nervous system response to and ability to convey stimuli (typically electrical)
  • described in terms of the stimulus site
    • somatosensory evoked potentials (SEP)
    • spinal evoked potentials (SpEP)
    • motor evoked potentials (MEP)
    • cerebellar evoked potentials (CEP)

The Wake-up Test ( Stagnara )

  • lightening the anesthetic state to the point that the patient can respond to a command
    • asked to squeeze her hand --> demonstrating that she responding --> then to move her feet and toes
    • if unable to move feet --> distraction reduced and the test repeated until a safe level of distraction demonstrated
    • during scoliosis surgery stretch of neural & neurovascular structures leads to ischaemia
  • gold standard of intraoperative spinal cord monitoring for scoliosis surgery
    • simple to perform & requires no additional equipment or personnel --> inexpensive and widely applicable
    • most importantly --> reliable test of motor function
  • potential hazards
    • accidental extubation, dislodgment of orthopedic instrumentation, self injury, broncho-spasm, recall of intraoperative events and psychologic trauma, air embolism, and cardiac ischemia
      • use midazolam (Dormicum) as premed --> amnaesia
      • contraindicated in IHD
    • false negative tests
      • rare
        • isolated posterior cord injury
        • isolated nerve root injury
    • not applicable in young children, deaf, retarded

Somatosensory Evoked Potentials ( SEP )

  • electrophysiologic response of the nervous system to sensory stimulation
    • from electrical stimulation at a peripheral nerve the signal travels via peripheral nerve through plexuses to nerve root to ipsilateral dorsal column and also spinothalamic and spinocerebellar pathways
    • the signal crosses over at the level of the brain stem and progresses rostrally to thalamus and on to thalamocortical projections on
      primary sensory cortex
    • stimulation --> median or post.tibial nerve
    • recording --> needle electrode in interspinous ligament, epior subdurally
      • "significant change from baseline"