11.2 Surgical vs Systemic Signal Changes

11.2 Surgical vs Systemic Signal Changes

Quick Answer: Focal, unilateral, or pathway-specific changes that track a surgical maneuver suggest surgical risk; bilateral or multi-modality drops that track anesthetic or vital-sign shifts suggest systemic causes. Decide in real time by comparing waveforms to baseline beside MAP, temperature, EtCO₂, anesthetic events, and the step on the field.

The CNIM exam and the OR both punish one-dimensional thinking. Amplitude can fall because a screw is malpositioned—or because the vaporizer was just increased. Your job is pattern recognition plus contemporaneous correlation, then clear communication.

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Pattern Framework: Focal vs Global

Features that favor a surgical (focal) cause

• Unilateral or asymmetric change (left PTN cortical drop with stable right)
• Pathway-specific change (lower-extremity SSEPs change during lumbar correction while upper-extremity SSEPs remain stable)
• Level-localizing pattern (peripheral potential preserved; cortical lost → central problem along that pathway)
• Tight temporal link to distraction, reduction, clamping, retraction, embolization, or screw placement
• Single modality or expected-at-risk modality affected first (for example, MEP change during cord-risk deformity work with later SSEP change)

Features that favor a systemic cause

• Bilateral and roughly symmetric deterioration across homologous channels
• Multi-modality suppression (SSEP + MEP ± EEG) without a lateralized surgical explanation
• Cortical peaks hit harder than peripheral during volatile increases or deep anesthesia (peripherals may remain)
• Tracks a vital-sign or drug event: MAP fall, temperature drop, bolus, MAC increase, hypocarbia
• Occurs when the surgeon is not manipulating the neuraxis (for example, during waiting, imaging, or closing)

These are probabilistic guides, not laws. A midline aortic or bilateral vascular event can look "systemic." Severe hypotension can injure the cord. Always integrate surgery + physiology + technical quality.

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Anesthesia and Physiologic Mimics

Chapter 6 detail still applies in the moment of change:

Volatile agents potently suppress cortical SSEP amplitude and can abolish or raise thresholds for TcMEPs. A sudden bilateral cortical drop after a bolus or vaporizer increase is classic systemic pharmacology—not proof of bilateral cord transection.

Narcotics and TIVA are generally friendlier to EPs than volatiles at comparable surgical depth, but bolus effects and deep anesthesia still matter. Interpret MEPs in light of the agreed anesthetic plan.

Neuromuscular blockade selectively devastates muscle-derived MEPs and EMG while leaving SSEPs relatively intact—another "pattern" clue.

Mean arterial pressure: Hypotension reduces spinal cord and cerebral perfusion. Expect bilateral amplitude loss that improves when MAP is restored—especially during deformity correction or vascular cases where the cord is watershed-vulnerable.

Temperature: Hypothermia prolongs latencies and can reduce amplitudes; asymmetric cooling (cold irrigant on one side) can mimic focal change—check the surgical field and patient temperature together.

Hematocrit / oxygenation / EtCO₂: Significant anemia, hypoxia, and hypocapnia (excessive ventilation) can suppress cortical responses. Pair waveform trends with the anesthesia monitor, not memory.

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Using Montages and Modalities to Localize

SSEP level diagnosis

A three-level approach (peripheral → subcortical/cervical → cortical) turns a "signal change" into a localization statement:

1. Peripheral lost (Erb's / popliteal): stimulator failure, electrode displacement, limb ischemia from positioning, or severe peripheral nerve insult—not a cortical stroke pattern.
2. Peripheral present, subcortical/cortical lost: problem proximal to the peripheral recording site along plexus, root, cord, or brain pathway.
3. Subcortical present, cortical lost: more rostral (brain) or anesthetic/cortical suppression pattern—correlate with volatiles and hemispheric risk.

Multi-modality cross-check

• SSEP change with stable MEP (or the reverse) narrows which tracts are affected and how urgent the conversation is.
• EEG slowing with bilateral cortical SSEP loss supports global cerebral/anesthetic effect.
• Free-run EMG bursts during pedicle work support mechanical irritation even when SSEPs are still within percentage criteria.

Never declare "everything is fine" because one modality is unchanged if the structure at risk is better monitored by another modality.

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Contemporaneous Assessment: Do It in the Moment

Contemporaneous means you assess cause while the change is happening—or within seconds—not during a quiet charting session ten minutes later.

Practical sequence when a significant change appears:

1. Confirm technical quality — impedance, stimulator output, obvious artifact, electrode security (Section 11.3).
2. Check reproducibility — is the change present on repeat averages/trials?
3. Read the room clocks — What did surgery just do? What did anesthesia just give? What are MAP, temp, EtCO₂, inhalational agent now?
4. Localize the pattern — unilateral vs bilateral; peripheral vs cortical; one modality vs many.
5. Communicate — concise alert to surgeon and anesthesia with the pattern and your leading differential.
6. Document — time, waveforms, vitals/anesthetic note, surgical response, and recovery or persistence.

Change detected
   ├─ Technical? → fix electrodes/cables/stim
   ├─ Systemic? → anesthesia / MAP / temp / CO₂ / Hct
   └─ Surgical? → pause maneuver, reverse step, inspect field
         ↓
   Re-acquire → communicate result → annotate

Do not wait for absolute certainty before speaking. Say: "Left lower-extremity cortical SSEP amplitude has fallen more than 50% from baseline over the last two averages during rod reduction; right side and upper extremities are stable; peripheral potentials are present. Please pause while we optimize and recheck." That statement is useful. "Something might be wrong" without pattern is not.

If the change reverses with raising MAP or reducing volatiles, document the systemic correlation. If it reverses only after reducing distraction, document the surgical correlation. If it persists, escalate urgency and continue multi-modality surveillance.

Trap patterns on the exam

• Bilateral sudden loss → think anesthetic/systemic/technical first, then bilateral surgical catastrophe.
• Unilateral loss timed to ipsilateral manipulation → think surgical/focal first, still rule out a yanked unilateral electrode.
• Cortical loss with intact peripherals after volatile increase → pharmacology until proven otherwise.
• MEP loss with unchanged SSEP after paralytic → blockade, not cord transection alone.

Mastering this differential is the core of intraoperative recognition: every alert is a differential diagnosis delivered under time pressure, not a single number crossing a threshold in isolation.