Motor Evoked Potentials (MEPs): Orthopaedics & Neurology

What Are MEPs?

MEPs in Orthopaedic Surgery

Role in Intraoperative Neurophysiological Monitoring (IONM)

According to Bailey & Love's Short Practice of Surgery (28th Ed., p. 541): During surgery for scoliosis, continuous spinal cord monitoring uses SSEPs, MEPs, and free-run/stimulated EMG activity to minimise the risk of neurological damage. The risk of neurological injury is 0.4% (1 in 250).

Surgeries Requiring MEP Monitoring

MEP vs SSEP: Complementary Roles

Key point: SSEPs alone miss anterior cord syndrome. MEPs are essential to detect motor pathway injury independently of SSEP changes.

Warning Criteria (Intraoperative Alerts)

• Amplitude decrease ≥50% from baseline — the most widely used criterion
• Complete loss of MEP — most serious; requires immediate response
• Increased stimulation threshold (voltage needed to elicit response rises significantly)

Response to MEP Alerts

1. Inform surgeon immediately
2. Check anaesthetic depth (volatile agents suppress MEPs significantly)
3. Raise mean arterial pressure (MAP) to ≥80 mmHg
4. Reverse any recent instrumentation or distraction
5. If MEPs do not recover → consider wake-up test (Stagnara test)

Anaesthetic Considerations

• Volatile agents (sevoflurane, isoflurane) — markedly suppress MEPs; avoid or use <0.5 MAC
• Propofol + remifentanil TIVA — preferred regimen for MEP monitoring
• Neuromuscular blockers — abolish muscle MEPs; avoid or use only partial block
• Nitrous oxide — additive suppression; generally avoided

MEPs in Neurology

Diagnostic Applications

Key Measurements

MEPs in Specific Neurological Conditions

1. Multiple Sclerosis (MS)

• Most established neurological use
• Prolonged CMCT reflects demyelination of corticospinal tracts
• MEPs can be abnormal when clinical exam and MRI are equivocal
• Used to objectively quantify disability and monitor progression

2. Amyotrophic Lateral Sclerosis (ALS)

• Upper motor neuron (UMN) signs can be subtle clinically
• Prolonged CMCT or absent MEP supports UMN involvement
• Helps distinguish ALS from pure lower motor neuron (LMN) syndromes
• Part of updated Awaji criteria evidence base

3. Stroke / Cerebrovascular Disease

• MEP presence or absence after stroke is a strong predictor of motor recovery
• Absent MEP in acute stroke → poor hand/arm recovery prognosis
• Serial MEPs track cortical reorganisation during rehabilitation
• TMS mapping identifies eloquent cortex before neurosurgery

4. Cervical Myelopathy

• CMCT prolongation occurs before severe clinical symptoms
• Helps differentiate cervical myelopathy from peripheral neuropathy
• Supports surgical decision-making in borderline cases

5. Parkinson's Disease

• Short cortical silent period; reduced MEP inhibition
• Motor cortex hyperexcitability reflected in lowered threshold
• Used in research; less routine diagnostically

6. Hereditary Spastic Paraplegia (HSP)

• Markedly prolonged CMCT or absent lower limb MEPs
• Useful in mild/subclinical cases

7. Friedreich's Ataxia

• MEPs often absent in lower limbs
• Helps differentiate from other ataxias

Therapeutic TMS (rTMS)

• Depression: FDA-approved; 10 Hz rTMS over left dorsolateral prefrontal cortex at 120% motor threshold (Bailey & Love; rTMS guideline data, p. 14)
• Stroke rehabilitation: Low-frequency rTMS to contralesional hemisphere suppresses interhemispheric inhibition, facilitating ipsilesional motor recovery
• Chronic pain / neuropathic pain: Motor cortex stimulation protocols
• OCD: FDA-approved rTMS protocol

MEP Recording Technique

Stimulation (Intraoperative TES)

• Corkscrew scalp electrodes at C3/C4 or Cz positions
• Short train stimuli (5–7 pulses, interstimulus interval 2–4 ms)
• Avoids single-pulse cardiac/movement artefacts

Recording

• Needle electrodes in target muscles bilaterally
• Upper limb: abductor pollicis brevis (APB), abductor digiti minimi (ADM)
• Lower limb: tibialis anterior, abductor hallucis
• Recorded as compound muscle action potential (CMAP)

Summary: MEPs at a Glance

Motor Evoked Potentials (MEPs) in Orthopaedic Surgery

Why MEPs Are Used

According to Bailey & Love's Short Practice of Surgery (28th Ed., p. 541): Continuous spinal cord monitoring using SSEPs, MEPs, and free-run/stimulated EMG is standard practice during corrective spinal surgery. The risk of neurological injury is 0.4% (1 in 250) — a risk MEP monitoring aims to prevent or detect early.

Anatomical Basis

• Mechanical compression by instrumentation
• Anterior spinal artery ischaemia
• Distraction forces during deformity correction

Surgeries Where MEP Monitoring Is Used

MEP Technique in the Operating Theatre

Stimulation

• Transcranial electrical stimulation (TES) via corkscrew scalp electrodes
• Electrode positions: C1/C2 or C3/C4 (international 10-20 system) — contralateral placement targets opposite limbs
• Short train stimuli: 4–7 pulses, interstimulus interval 2–4 ms, high voltage (up to 200–400 V)
• Short trains allow the stimulus to summate and traverse injured/partially blocked cord segments

Recording

• Surface or intramuscular needle electrodes in target muscles
• Recorded as compound muscle action potential (CMAP)

MEP vs SSEP: Critical Comparison

Alert Criteria (Warning Signs)

The 50% amplitude drop rule is the universally accepted alert criterion. Complete loss mandates immediate surgical action.

Response Protocol to MEP Alerts

1. Notify the surgeon immediately — pause instrumentation
2. Check anaesthetic depth — volatile agents, bolus propofol, or recent neuromuscular blockade can cause false positives
3. Optimise haemodynamics — raise MAP to ≥80 mmHg (or >10 mmHg above baseline) to improve cord perfusion
4. Correct mechanical cause — remove distraction, back off implant, decompress if applicable
5. Irrigate wound with warm saline (cord temperature matters)
6. Reassess MEPs after each corrective step
7. If MEPs do not recover → perform wake-up test (Stagnara test) to clinically confirm motor function

Anaesthetic Management for MEP Monitoring

The Wake-Up Test (Stagnara Test)

• Anaesthesia is lightened until the patient can follow commands
• Patient asked to move hands, then feet
• Positive response (movement preserved) = reassuring
• Limitations: patient cooperation required, one-time snapshot, awareness risk, cannot be repeated frequently

Specific Orthopaedic Contexts

Scoliosis Surgery

• Highest-risk spinal operation for neurological injury
• Distraction forces across multiple levels can compromise anterior cord perfusion
• MEPs + SSEPs + EMG used together
• Alert during rod rotation or final compression/distraction manoeuvres

Cervical Spine Surgery

• Risk of cord injury from retraction, instability, or direct compression
• Upper limb MEPs (APB, deltoid) are particularly important
• Myelopathic patients may have reduced baseline MEP amplitudes — establishing a good baseline before incision is critical

Pedicle Screw Placement

• Stimulated EMG (not MEP per se) is used to detect medial wall breach
• A screw stimulation threshold <6–8 mA suggests medial breach into the canal
• Free-run EMG detects nerve root irritation in real time

Thoracic Surgery / Aortic Cross-Clamping

• Anterior spinal artery watershed zone (T4–T9) is most vulnerable
• MEP loss during aortic cross-clamping → reposition clamp, insert intercostal reimplantation, raise MAP

Limitations of MEP Monitoring

Summary