Neuromonitoring: SSEPs, MEPs and anesthetic choice
Stem
A 15-year-old girl, 52 kg, is scheduled for posterior spinal fusion from T2 to L3 for a 68-degree adolescent idiopathic scoliosis. She is otherwise healthy. The surgeon requests somatosensory and transcranial motor evoked potential monitoring throughout, and expects four to five hours of surgery with moderate blood loss.
Question
How will you design your anesthetic so that the monitoring works?
Ideal answer
HeadlineA stable, low-interference total intravenous anesthetic, because MEPs are the most anesthetic-sensitive signal; a change must mean the cord, not my drugs.1
- Agree baselines and communication with the neurophysiologist before induction, so changes reach the surgeon instantly.
- Propofol, fentanyl, one intubating dose of rocuronium, never redosed, because blockade must be gone before baseline MEPs; if needed, at least two of four twitches.2
- Propofol plus remifentanil 0.1–0.5 mcg/kg/min, little effect on either signal; no nitrous oxide, any added volatile below 0.5 MAC,2 infusions not boluses, constant through correction.
- Soft bite blocks between molars after positioning, because transcranial stimulation clenches the jaw onto tongue and teeth.2
- MAP around 80, temperature within 2 °C, hemoglobin near baseline, ventilation normal, because ischemia, cooling and anemia depress signals.2
Watch for: every drug, pressure or blood-loss change, told to the neurophysiologist.
Breaking down the answer
- Know the tracts — SSEPs travel the dorsal columns and MEPs the corticospinal tract; MEP changes often precede SSEP changes, buying time to act.1
- Volatile versus TIVA — halogenated agents and nitrous oxide raise latency and cut amplitude, worst for MEPs; opioids, ketamine and etomidate have little MEP effect.1 A full MAC of sevoflurane with nitrous leaves no usable MEPs.
- Relaxants block MEPs, not SSEPs — they abolish the muscle response MEPs record, so relaxants are avoided after induction when MEPs are monitored.1,3
- Physiology masquerades as injury — hypotension, hypothermia, anemia, a pH below 7.2 or a PaCO2 below 20 change signals globally.2 Changes if: some teams allow a lower MAP during exposure and raise it before correction (see scoliosis-surgery), so I agree the target.
- Global versus focal — all four limbs points to drugs or physiology; one limb or below the surgical level points to the surgery or position.2
- Dexmedetomidine with care — it depresses MEPs at higher doses, so low-dose adjunct only.1
Follow-up question
During rod derotation the neurophysiologist reports that both lower-extremity MEPs have fallen by more than 80%, with upper-extremity MEPs and all SSEPs unchanged. Nothing in your anesthetic has changed. What do you do?
Follow-up answer
HeadlineA real alarm: over 80% amplitude loss meets MEP criteria, legs only is focal, the cord is at risk.2 Surgeon stops and releases correction.
In parallel I raise the mean pressure above her baseline with phenylephrine or norepinephrine, increase inspired oxygen, check a gas and hemoglobin and transfuse if anemic, and confirm temperature and that no bolus was given.2 The neurophysiologist checks the leads. If signals do not recover, I lighten the anesthetic for a wake-up test asking her to move her feet, and the team discusses removing the hardware. I document the times.
Pitfalls that cost points
- Bolusing propofol during correction and creating a false alarm.
- Calling a signal loss a technical glitch before raising pressure.
Rapid-fire
Q: What are the SSEP alarm criteria? A: Amplitude down more than 50% or latency up more than 10%.2
Q: Which agents increase SSEP amplitude? A: Ketamine and etomidate.1
Q: Which modality is least affected by anesthetics? A: Brainstem auditory evoked potentials.1
Q: What triggered-EMG threshold suggests a breached pedicle screw? A: A response below 10 mA.2
Q: How much volatile can SSEP-only monitoring tolerate? A: Up to about 1 MAC, with sevoflurane or desflurane preferred over isoflurane.2
Numbers to know
| Item | Value |
|---|---|
| SSEP alarm | Amplitude ↓ >50% or latency ↑ >10%2 |
| MEP alarm | Amplitude ↓ >80% or loss2 |
| Volatile with MEPs | <0.5 MAC2 |
| Remifentanil | 0.1–0.5 mcg/kg/min2 |
| MAP in spine surgery | Often ~80 mmHg2 |
| Temperature | Within 2–2.5 °C of baseline2 |
Links
- Scoliosis surgery, Postoperative visual loss and prone positioning, Volatile agents and nitrous oxide: when and when not, Neuromuscular blockade, monitoring and reversal, Subarachnoid hemorrhage and aneurysm clipping
- OpenAnesthesia: evoked potential alarm criteria and anesthetic effects
Sources
- 1.Evoked potentials: modalities and anesthetic effects · OpenAnesthesia (IARS) · openanesthesia.org
- 2.Evoked potentials: alarm criteria, differential diagnosis and recommendations · OpenAnesthesia (IARS) · openanesthesia.org
- 3.Anesthesia for the surgical treatment of cerebral aneurysms · Colombian Journal of Anesthesiology (Lecours, Gelb), 2015 · elsevier.esCited for avoiding relaxants after induction when MEPs are monitored.
Facts on this page were checked against these sources on Oct 15, 2026. Guidelines change; confirm doses against current institutional protocols.