Spinal Cord Monitoring (Intraoperative Neuromonitoring: SSEP, MEP, EMG, Wake-up Test)

The purpose of IONM is a real-time warning of an evolving neurological injury — from distraction/correction, direct trauma, implant malposition or ischaemia — so it can be reversed before it becomes permanent. Different modalities watch different tracts, and the spinal-cord cross-section is the key: the posterior (dorsal) columns carry sensation (monitored by SSEP), while the corticospinal tracts carry motor output (monitored by MEP). Because these lie in different parts of the cord with different blood supplies, no single modality is sufficient.

• A single anterior spinal artery supplies the anterior two-thirds of the cord — the corticospinal (motor) tracts, the spinothalamic (pain/temperature) tracts and the anterior horns.
• Paired (two) posterior spinal arteries supply the posterior third — the dorsal columns (proprioception/vibration).
• The dominant thoracolumbar feeder is the artery of Adamkiewicz (arteria radicularis magna), usually arising on the left between about T9 and T12 and supplying the lumbar enlargement; the mid-thoracic cord (~T4–T9) is a poorly-perfused watershed.
• Why it matters for IONM: an anterior-cord ischaemic insult (correction/distraction stretching the feeders, or anterior-spinal-artery compromise) knocks out MOTOR (MEP) while sparing the dorsal columns (SSEP stays normal) — the exact "normal SSEP, lost MEP" pattern MEP was added to catch — and it is why raising the MAP to perfuse the anterior cord is the first response.
• Anterior cord syndrome: bilateral loss of motor and pain/temperature below the level with preserved proprioception/vibration/light touch — the worst-prognosis incomplete cord syndrome.

Transcranial MEP is powerful but not free of hazards — examinable safety points the topic should state:

• Relative contraindications: epilepsy/seizure disorder, cortical lesions or skull defects near the stimulating electrodes, implanted intracranial electrodes/vascular clips/shunts, a cardiac pacemaker or implantable defibrillator, and cochlear/other electronic implants.
• Complications: the commonest is a tongue or lip laceration (and dental/jaw injury) from the masseter/temporalis contraction the stimulus evokes — so a soft bite block is mandatory; the limb twitch can interrupt surgery at the critical moment; and rarely seizures, cardiac arrhythmias or scalp burns at the electrode sites.
• Limitations: MEP is unreliable in young children (the corticospinal tract is not fully myelinated in the first few years) and a baseline is hard to obtain in a pre-existing motor deficit / established myelopathy — so confirm a reliable baseline at the start before trusting later changes.

For MEP (and EMG) to work, the anaesthetic must preserve neuromuscular transmission and cortical excitability: use total intravenous anaesthesia (TIVA), avoid or minimise volatile (inhalational) agents (which suppress MEP), and avoid long-acting muscle relaxants once motor monitoring is running. Maintain stable physiology — adequate mean arterial pressure, normothermia and haemoglobin — because hypotension, hypothermia and anaemia themselves reduce signals and threaten cord perfusion.