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Spinal cord monitoring during spinal surgery (Motor evoked potentials…
Spinal cord monitoring during spinal surgery
Purpose - attempt to detect neurological injury and prevent devastating, irreversible damage (injuries from instrumentation, distraction injury and reduced perfusion)
Somatosensory evoked potentials
Dorsal columns
Stimulate peripheral nerve
Posterior tibial nerve (electrodes in popliteal, c-spine and scalp - proximal site ensures recording system functioning)
Median nerve of arm (electrodes over brachial plexus in supraclavicular fossa, rostral C-Spine, scalp)
Direct response within epidural or scalp electrodes - level of surgery determines the choice of stimulation and recording sites
Spinal pathways must span site of surgery
Evoked potentials averaged ~5mins to eliminate background noise, then displayed as voltage against time
Nerve injury may be indicated by decreased amplitude or increased latency (<50% amplitude, >10% increase in latency)
Motor evoked potentials
Anteriolateral columns (anterior spinal artery)
Rely on corticospinal tract integrity
Number of techniques
Transcranial electrical stimulation (most common) - corkscrew electrode in scalp overlying motor areas, multiple pulse stimulation
Transcranial magnetic impulses
Direct stimulation of rostral spinal cord
Signal is detected with epidural electrodes or as compound muscle action potentials
Recordings from subcutaneous or intramuscular needles in multiple muscles in arms and legs
May have simultaneous recording from 8 sites bilaterally
More consistent responses from more distal muscles (tibialis anterior, abductor hallucis longus, intrinsic hand muscles
Stimulation will produce mvmt
Warn surgeon when stimulus train is going to be delivered (avoid mvmt during critical part of surgery)
Activates masseter muscles (risks tongue laceration, tooth fracture - use bite block)
Can also correlate activity with nerve root discharge
Contraindications - epilepsy, cortical lesions, skull defects, raised ICP, surgically implanted cranial devices, PPM, implantable pump)
Influence of anaesthetic
Volatiles depress MEPs (and to lesser extent SSEPs)
Propofol and N20 depress SSEPs and MEPs
Opioids have little effect
Decreased BP and temp also depress signals
NMB agents reduce background noise in SSEPs, but profound block prevents compound muscle action potentials
Baseline recordings made after induction of anaesthesia (in conjunction with neuro tech)
Must have electrical protection
Wake up test
If change in recorded evoked potential occurs and injury is suspected, consider performing 'wake up' test
Snapshot of spinal cord motor function
Surgery halted, volatile switched off, emergence allowed
Patient asked to move feet
Once this occurs, recommence anaesthesia
If paraplegia, all implants removed, hypotension and anaemia corrected, high dose of methylpred commenced