During posterior spinal instrumentation for adolescent idiopathic scoliosis, multichannel neuromonitoring is in use. Midway through rod placement, bilateral lower limb motor evoked potentials disappear completely, while somatosensory evoked potentials recorded from the posterior tibial nerves remain unchanged. What does this pattern indicate?
- A An artefact, because motor and sensory pathways are always affected together
- B Isolated compromise of the corticospinal (motor) tracts, typically supplied by the anterior spinal artery, with dorsal column function still intact ✓
- C Isolated compression of the dorsal columns with motor pathways spared
- D Peripheral nerve injury from malpositioned stimulating electrodes
Correct answer: B. Isolated compromise of the corticospinal (motor) tracts, typically supplied by the anterior spinal artery, with dorsal column function still intact
Explanation
MEPs elicited by transcranial electrical stimulation travel down the corticospinal tract in the anterior cord, supplied mainly by the anterior spinal artery. SSEPs test the dorsal column medial lemniscus pathway in the posterior cord. Because these territories differ, isolated MEP loss signals anterior cord or motor tract compromise and demands immediate surgical response, even though SSEPs remain normal. MEPs are generally more sensitive than SSEPs for anterior cord ischaemia during scoliosis correction.
Reference: Miller's Anesthesia, 9th ed.
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Written and medically reviewed by the StethoPrep medical team.