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Evoked Potentials — SSEP, MEP, BAEP

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Monitoring II · 9 min read

When the surgeon is operating near the spinal cord or brainstem, evoked potentials test the pathway in real time. The anesthetic constrains the signal.

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Distinguish SSEP, MEP, and BAEP modalities.
Choose anesthetic technique that preserves the signal.
Respond to signal loss intraop.
Recall the surgical indications.

SSEP — somatosensory

KeyStimulate peripheral nerve (posterior tibial, median), record from spinal cord + cortex.

Tests dorsal column-medial lemniscus pathway.

Used in scoliosis, aortic surgery (Adamkiewicz region), aneurysm clipping.

Tolerates volatile up to 0.5-1 MAC; opioids minimal effect.

Sensitivity for cord injury 92%; specificity 99%.

Alarm criterion: amplitude drop >50% or latency increase >10%.

Spinal cord blood supply and ischemia syndromes: 1 anterior spinal artery (motor, sensory pain/temp), 2 posterior spinal arteries (dorsal columns); artery of Adamkiewicz T9-T12; anterior spinal artery syndrome from cross-clamp or hypotension.

MEP — motor evoked

KeyStimulate motor cortex transcranially or via epidural electrode, record from peripheral muscle.

Tests corticospinal tract.

Highly anesthetic-sensitive — even 0.5 MAC volatile abolishes useful signal.

TIVA (propofol + remifentanil + low-dose ketamine) is standard.

NMB precluded during MEP monitoring (need motor signal).

Alarm criterion: amplitude drop >75% or new asymmetry.

Critical for spine surgery + brain tumor resection near motor cortex.

BAEP — brainstem auditory

KeyClick stimulus to ear, record wave I-V from scalp electrodes.

Tests cranial nerve VIII + brainstem auditory pathway.

Most anesthetic-resistant of the evoked potentials — tolerates 1 MAC volatile.

Used in posterior fossa surgery (acoustic neuroma, brainstem tumor, microvascular decompression).

Wave V latency + amplitude tracked.

Loss of wave V = retrocochlear pathology — alert surgeon, look for retraction or vascular compromise.

Volatile and N₂O effects

KeySensitivity gradient (most to least): MEP > VEP > SSEP > BAEP.

Volatile MAC limit for monitoring: MEPs 0 MAC (TIVA only), SSEPs 0.5-1 MAC, BAEPs 1+ MAC.

N₂O suppresses similarly.

Opioids minimal effect — high-dose remifentanil compatible with all monitoring.

Ketamine actually enhances SSEPs/MEPs slightly.

Dexmedetomidine well-tolerated.

NMB OK during SSEP, banned during MEP.

TIVA preferred for MEPs

KeyStandard regimen: propofol 75-150 mcg/kg/min + remifentanil 0.1-0.3 mcg/kg/min + low-dose ketamine 0.15 mg/kg bolus + 0.15 mg/kg/hr infusion.

Avoid NMB after intubation dose wears off.

Maintain BIS 40-60 if monitored.

Hemodynamic management: maintain MAP within 20% baseline; phenylephrine first-line for MAP support without affecting signals.

Vasoactive drugs minimal effect.

Signal loss algorithm

KeySudden amplitude drop or latency increase — pause + investigate.

Sequential intervention

1) notify surgeon — may need to release distraction or hardware

2) raise MAP (vasopressor + volume)

3) maintain Hgb >9-10 (transfuse if needed)

4) normalize PaCO₂

5) verify position no compression

6) confirm anesthetic not over-deep (check BIS, light volatile if any)

Recovery within 15-20 min reassures; persistent loss prompts wake-up test (Stagnara) or case termination.

Visual evoked potentials (VEP) + EMG monitoring

KeyVEP: light flash to retina, record from occipital cortex — used in pituitary surgery (transsphenoidal) to monitor optic nerve.

Highly anesthetic-sensitive (more than SSEP, less than MEP) — TIVA preferred.

EMG (electromyography continuous): used in posterior fossa cranial nerve surgery (acoustic neuroma, microvascular decompression for trigeminal neuralgia, parotid surgery to identify facial nerve) — alerts surgeon to nerve proximity by spontaneous activity in monitored muscle.

NMB CONTRAINDICATED for EMG monitoring (need motor unit signal).

Train monitor neurophysiologist on what surgical phases to expect activity.

False alarms + interpretation

KeyTEMPERATURE drops cause amplitude drop + latency increase — confounds true ischemia signals.

PHARMACOLOGIC changes: bolus of induction agent or volatile dial-up causes acute drop; communicate with team about anesthetic adjustments BEFORE making them so neurophys monitor can interpret.

POSITIONAL changes during surgery cause transient signal changes.

PROBE DISLODGEMENT or cable disconnect mimics signal loss — verify hardware.

The neurophysiology team interprets in real-time; trust them but verify with the algorithm above before declaring true cord/nerve injury.

⚠ Common pitfalls

Volatile >0.5 MAC during MEP monitoring — signal suppressed.
Standard NMB during MEP — paralyzes the response; reduce or pause NMB.
Ignoring temperature + BP changes — signal also varies with physiology.
Reading 'signal loss' as artifact when it's surgical injury — work it up before assuming.

💎 Clinical pearls

TIVA (propofol + remi) preserves both SSEP and MEP — the safest combo.
MEP signal loss: raise MAP first, check temperature + Hb, then surgical team alerts.
BAEP unaffected by volatile + NMB — useful in cerebellar/posterior fossa surgery.
SSEP usable under sub-MAC volatile; MEP needs TIVA almost universally.

Recap

TIVA (propofol + remi) preserves both SSEP and MEP — the safest combo.
MEP signal loss: raise MAP first, check temperature + Hb, then surgical team alerts.
BAEP unaffected by volatile + NMB — useful in cerebellar/posterior fossa surgery.
SSEP usable under sub-MAC volatile; MEP needs TIVA almost universally.

Mark each section done to complete the module.