Does epidural anesthesia with lidocaine (local anesthetic) or bupivacaine (local anesthetic) affect the readings of somatosensory evoked potentials (SEPs) and motor evoked potentials (MEPs) in a patient undergoing spinal decompression from T6 to L1 with intraoperative neuromonitoring (IONM)?

Effect of Epidural Lidocaine or Bupivacaine on IONM During Thoracolumbar Decompression

Epidural anesthesia with sensory concentrations of lidocaine or bupivacaine will significantly suppress both SEP and MEP signals during your T6-L1 decompression, making IONM unreliable or impossible to interpret. 1, 2

Direct Effects on Somatosensory Evoked Potentials (SEPs)

Epidural bupivacaine causes profound suppression of SEP signals that makes monitoring unreliable:

• Amplitude reduction: Intrathecal bupivacaine (which produces similar neural blockade to epidural administration) significantly decreases the amplitude of all SEP components, with complete disappearance of signals in 58% of patients (7 of 12) at the L1 dermatome and 42% (5 of 12) at the S1 dermatome 1

• Latency prolongation: Both bupivacaine and lidocaine significantly increase SEP latencies, affecting the PI (P37), NI (N45), and PII (P60) peaks that are critical for monitoring spinal cord integrity 1, 2

• Dose-dependent blockade: The suppression occurs even with clinically effective sensory blockade (mean T8-9 level), and shows no correlation with the degree of motor blockade or sensory level achieved 1

Direct Effects on Motor Evoked Potentials (MEPs)

While the evidence specifically addresses intravenous lidocaine rather than epidural administration, the mechanism of neural blockade is relevant:

• Systemic lidocaine does not affect MEPs: Intravenous lidocaine at 1 mg/kg/h (used as an adjuvant anesthetic) does not adversely affect MEP threshold voltages when used as part of balanced anesthesia 3

• Epidural local anesthetics block motor pathways: However, epidural administration at surgical levels would directly block the motor pathways being monitored, making MEP signals unreliable or absent, particularly given that epidural bupivacaine produces motor blockade (mean Bromage scale 1.3) 1

Critical Anatomical and Technical Considerations

The epidural catheter placement relative to your surgical field (T6-L1) creates insurmountable monitoring problems:

• Catheter must be remote from surgical site: Epidural placement should be in the lumbar region (L2-L4) if the surgical field extends to L1, or in the high thoracic region (T1-T4) if technically feasible, but never at T6-L1 levels where surgery will occur 4

• High dermatomal coverage required: To provide adequate analgesia for T6-L1 surgery, the epidural block must extend from approximately T4-T6 to L1-L2, which encompasses the conus medullaris and cauda equina—the exact neural structures you are trying to monitor 4

• Signal transmission blocked: Despite clinically effective blockade, nerve potentials after stimulation within the area of sensory block can sometimes pass to the cerebral cortex, but this creates unreliable and unpredictable monitoring conditions 1

Practical Algorithm for Anesthetic Management

If IONM is deemed necessary for this T6-L1 decompression, you must choose between epidural anesthesia and reliable monitoring—you cannot have both:

1. Primary recommendation: Use total intravenous anesthesia (TIVA) with propofol and avoid epidural local anesthetics entirely during the monitoring period 4, 5

2. Neuromuscular blockade management: Avoid or minimize muscle relaxants to preserve MEP signals, using processed EEG monitoring if neuromuscular blockade is required 4, 5

3. Alternative analgesia: Consider intravenous lidocaine infusion (1 mg/kg/h) as an adjuvant, which does not affect MEP or SEP monitoring and provides opioid-sparing analgesia 3

4. Postoperative epidural: Place the epidural catheter but do not dose it until after surgical closure and final IONM testing is complete, then initiate epidural analgesia for postoperative pain control 4

Common Pitfalls to Avoid

• Do not assume low-concentration local anesthetics are safe: Even dilute sensory concentrations of bupivacaine cause significant SEP suppression and signal loss 1

• Do not rely on "partial" monitoring: If epidural blockade causes signal loss in some dermatomes but not others, you cannot reliably interpret changes as surgical versus anesthetic in origin 1

• Do not place epidural at surgical levels: This risks epidural hematoma formation with surgical manipulation and interferes with surgical access 4

Evidence Quality and Limitations

The evidence directly addressing epidural effects on SEPs is limited to older studies (1987,2000) using intrathecal or epidural bupivacaine 1, 2. However, the mechanism of neural blockade is well-established, and the profound signal suppression observed (58% complete signal loss) makes the clinical implications clear. The more recent evidence on intravenous lidocaine (2017) demonstrates that systemic administration does not affect monitoring, supporting the use of alternative analgesic strategies 3.