Intraoperative Neuromonitoring

What is intraoperative neuromonitoring?

It is a system of continuous electrophysiological assessment of spinal cord and nerve function throughout surgery, while the patient is under anesthesia.

Using specialized electrodes placed on the limbs and scalp, we stimulate or record neural activity and visualize signal stability in real time. Significant changes in amplitude or latency may indicate altered spinal cord perfusion, mechanical stress, positioning issues, or surgical manipulation— prompting immediate corrective action.

In which brain and spine procedures is it most commonly used?

Not all surgeries require neuromonitoring, but it is invaluable in complex, high-risk operations.

Common indications include:

• Scoliosis and kyphosis correction (especially in pediatric and adolescent patients).
• Extensive decompressions and multilevel stabilizations with significant blood loss.
• Combined anterior–posterior approaches or staged reconstructions.
• Craniotomies for brain and skull base tumors.
• Intramedullary and paraspinal tumors (e.g., glioma, ependymoma).
• Congenital brain and spine disorders (e.g., posterior fossa decompression for Chiari malformation, detethering for lipomyelomeningocele).
• Revision spine surgery with scarring, deformity, or severe stenosis.
• Minimally invasive lateral approaches near neural plexuses (e.g., transpsoas approaches in the lumbar spine).

Which signal modalities are used (SSEP, MEP, EMG)?

A multimodal approach is used to assess sensory pathways, motor pathways, and individual nerves.

• SSEP (somatosensory evoked potentials) – assess dorsal column pathways (proprioception, vibration, touch). Peripheral nerves (e.g., median, tibial) are stimulated, and signal amplitude and conduction time are measured. A >50% amplitude decrease or >10% latency increase from baseline is considered concerning.
• MEP (motor evoked potentials) – evaluate corticospinal tract integrity. The motor cortex is stimulated via scalp electrodes, and muscle responses in the upper and lower limbs are recorded. A 70–80% amplitude reduction or complete loss without recovery is associated with a risk of paresis or paralysis.
• EMG (electromyography) – records muscle activity.
  • Spontaneous EMG: high-frequency discharges warn of mechanical nerve root irritation.
  • Triggered EMG (tEMG): stimulation of pedicle screws, instruments, or dilators to detect nerve proximity.

How do anesthesia and surgical factors affect the signals?

Anesthetic management is critical—certain agents can suppress signals without true neurologic injury.

Volatile anesthetics and high doses of propofol markedly suppress MEPs. Therefore, TIVA (total intravenous anesthesia) with propofol ± ketamine and minimal or no inhalational agents is commonly used in complex spine surgery with neuromonitoring.

Other factors affecting signal reliability include:

• Hypotension or low mean arterial pressure (MAP).
• Hypothermia, electrolyte imbalance, anemia.
• Neuromuscular blockers (especially for MEPs).
• Electrical interference from operating room equipment.

How are changes in SSEPs interpreted?

SSEPs primarily reflect dorsal column function and spinal cord perfusion.

Continuous monitoring focuses on amplitude and latency. A greater than 50% amplitude drop or latency increase >10% from baseline triggers immediate evaluation using a structured alert protocol.

Brief, isolated changes may relate to anesthesia or noise; persistent or progressive alerts require prompt intervention.

How are changes in MEPs interpreted?

MEPs reflect corticospinal tract integrity—sudden loss is a critical warning sign.

A 70–80% reduction or complete loss of MEPs, especially if asymmetric, constitutes an urgent alert. Recovery after corrective measures suggests low risk of permanent deficit; lack of recovery increases the likelihood of transient or permanent weakness.

What is EMG and its role in instrumentation and surgical approaches?

EMG helps detect irritation or injury of individual nerve roots and peripheral nerves.

In posterior spinal fixation, triggered EMG assesses pedicle screw placement:

• < 2.8 mA → confirmed cortical breach.
• < 4 mA → high suspicion of breach.
• 4–8 mA → possible breach; requires imaging correlation and judgment.
• > 8 mA → screw likely safely contained.

In lateral transpsoas approaches, directional tEMG maps the lumbar plexus within the psoas muscle. Thresholds < 5 mA indicate direct nerve contact; ≥ 11 mA suggest a safer corridor.

What do “safety thresholds” mean for pedicle screws and lateral approaches?

Numerical thresholds are guidelines—not absolute rules—supporting intraoperative decision-making.

Surgeons integrate EMG data with imaging (fluoroscopy, O-arm, navigation) and anatomical expertise. Acceptable thresholds may differ depending on patient anatomy and clinical context.

What does the team do when MEP/SSEP changes occur?

At Neuroknife, a structured MEP/SSEP alert checklist ensures a rapid, coordinated response.

Neurosurgeon

• Immediately pauses surgical manipulation.
• Assesses for mechanical compression, overcorrection, or hardware issues.
• Performs additional decompression or reverses the last step if needed.

Neurophysiologist

• Confirms signal validity and electrode integrity.
• Analyzes symmetric vs asymmetric changes.
• Continuously updates the team on signal trends.

Anesthesiologist

• Checks neuromuscular blockade and anesthetic depth.
• Optimizes MAP, oxygenation, temperature, and hemoglobin.
• Adjusts anesthetic agents to facilitate signal recovery.

What are the practical benefits and limitations?

Neuromonitoring reduces—but does not eliminate—risk and requires expert interpretation.

Benefits include:

• Early detection of spinal cord ischemia or compression.
• Opportunity for immediate corrective action.
• Safer, more effective deformity correction.
• Improved accuracy of instrumentation placement.

Limitations:

• No absolute class-I evidence in all scenarios.
• False positives and negatives may occur.
• Signal quality depends on anesthesia, equipment, and expertise.

What should patients know and ask before surgery?

Well-informed patients and families make safer, more realistic decisions.

• Will neuromonitoring be used in my surgery—and why?
• Which modalities will be monitored?
• Does the team have experience with neuromonitoring in my condition?
• What happens if signal changes occur?
• Are there anesthesia-related considerations?