Somatosensory Evoked Potentials (SSEP): Procedure and Interpretation
Technical Procedure
SSEPs are cerebral cortical electrical potentials recorded with scalp electrodes during electrical stimulation of peripheral nerves, most commonly the median nerve (upper extremity) or posterior tibial/peroneal nerves (lower extremity), conducted via the lateral and posterior columns of the spinal cord. 1
Stimulation Parameters
- Median nerve stimulation: Percutaneous electrical stimulation at the wrist with intensity of 110% of the movement threshold 2
- Lower extremity: Posterior tibial or peroneal nerve stimulation 1
- Stimulus rate: Typically 5 Hz for short-latency SSEPs, though 1 Hz may be used for giant SEP conditions 2
- Bandpass filter: 20 Hz-3 kHz for standard short-latency SSEPs 2
Recording Technique
- Electrode placement: Scalp electrodes at C3'/C4' for upper extremity, with recording from the primary cortical somatosensory area 1
- Reference electrode: Fz for standard short-latency SSEPs 2
- Critical technical requirement: SSEP recording requires appropriate skills and experience, with utmost care to avoid electrical interference from muscle artifacts or ICU environment 1
Neuromuscular Blockade Considerations
- Paralytic agents may be used during SSEP monitoring because SSEPs are less sensitive to anesthetic drugs compared to motor evoked potentials 1
- When baseline noise is excessive, neuromuscular junction blockade significantly improves reliability of interpretation, with inter-observer reliability increasing from kappa 0.03 pre-blockade to 0.41 post-blockade 3
Critical Interpretation Parameters
N20 Wave Analysis (Median Nerve)
The N20 waveform is the primary cortical response evaluated for prognostication, representing the short-latency cortical potential recorded approximately 20 milliseconds after median nerve stimulation. 1
Amplitude Thresholds
- Normal cortical amplitude: ≥ 0.7 μV provides better inter-observer reliability 3
- Mean + 3SD threshold: 7.8 μV in standard SSEP conditions, 10.0 μV in giant SEP conditions 2
- Critical finding: Bilateral absence of N20 wave is the key prognostic indicator 1
Prognostic Interpretation in Post-Cardiac Arrest Coma
In comatose patients after cardiac arrest (regardless of targeted temperature management), bilateral absence of the N20 SSEP wave 24-72 hours after cardiac arrest or after rewarming predicts poor outcome (death or vegetative state) with 1% false positive rate (95% CI, 0%-3%). 1
Timing Considerations
- During targeted temperature management (TTM): Absent N20 predicts poor outcome with 2% false positive rate (95% CI, 0%-4%) 1
- After rewarming: Absent N20 predicts poor outcome with 1% false positive rate (95% CI, 0%-3%) 1
- Without TTM: Bilateral absence of N20 at 24-72 hours predicts poor outcome with 0% false positive rate (95% CI, 0%-3%) 1
Intraoperative Monitoring Interpretation
During thoracic aortic or spinal surgery, alterations in monitored SSEP potentials indicate neurologic compromise is occurring and require immediate intervention. 1
Intervention Triggers
- Significant amplitude decrease or latency prolongation warrants corrective action including: segmental artery reimplantation, optimization of hemodynamics, clamp repositioning, or adjustment of distal perfusion pressure 1
- Target distal arterial pressure: Minimum 60 mm Hg to ensure adequate spinal cord blood flow 1
Critical Limitations and Pitfalls
Anatomical Limitations
SSEP monitoring is limited because it only evaluates the integrity of the lateral and posterior columns of the spinal cord; the anterior motor column is more likely to be affected by ischemic injury, making isolated anterior column injury possible without SSEP changes. 1
- False-negative rate: Patients can develop immediate or delayed paraplegia despite normal intraoperative SSEPs 1
- Sensitivity comparison: In thoracic aortic surgery, only 7% of patients showed SSEP changes compared to 29% with motor evoked potential changes 1
- Delayed ischemia detection: SSEP monitoring detects ischemia later than transcranial motor evoked potentials 1
Technical Reliability Issues
Inter-observer reliability of SSEP interpretation in comatose patients is only "moderate" (average kappa 0.60), while intra-observer reliability is "substantial" (average kappa 0.84). 3
Factors Improving Reliability
- Cortical amplitudes ≥ 0.7 μV enhance inter-observer agreement 3
- Smaller baseline noise improves interpretation accuracy 3
- Similar experience levels between reviewers increases reliability 3
- Neuromuscular blockade when baseline noise is excessive 3
Self-Fulfilling Prophecy Risk
SSEP results are more likely to influence physicians' and families' withdrawal of life-sustaining treatment decisions than clinical examination or EEG, creating potential for self-fulfilling prophecy that may artificially inflate the apparent predictive accuracy. 1
Screening Approach in Critical Care
A novel low-cost screening approach using peripheral train-of-four stimulator and standard 21-channel electroencephalograph can reliably detect SSEPs, but bilateral absence of responses requires confirmation with standard SSEP recordings before prognostic decisions. 4