SSEPs & Motor Evoked Potentials

Somatosensory evoked potentials (SSEPs) record the electrical response of central somatosensory pathways to peripheral nerve stimulation. Motor evoked potentials (MEPs) record muscle responses to transcranial magnetic or electrical stimulation of motor cortex. Together they assess the integrity of the long ascending and descending tracts of the spinal cord and brain. SSEPs are essential for multimodal coma prognostication, MS diagnostic support, and intraoperative monitoring during spine and brain surgery. MEPs complement SSEPs by testing corticospinal tract function specifically. This page covers SSEP technique, the major recording sites and waves, MEP methodology, and the integrated clinical applications.

Somatosensory Evoked Potentials (SSEPs)

Concept

• Stimulate a peripheral nerve electrically.
• Record from multiple sites along the somatosensory pathway: peripheral nerve, plexus, spinal cord, brainstem, thalamus, somatosensory cortex.
• Track the volley as it ascends, identifying delays or blocks at each level.

Stimulation

• Median or ulnar nerve at wrist (upper extremity SSEP).
• Tibial or peroneal nerve at ankle (lower extremity SSEP).
• Posterior tibial nerve at ankle most common for lower extremity.
• Stimulus intensity: just above motor threshold (visible thumb twitch for median).
• Stimulus duration: 0.1–0.3 ms.
• Repetition rate: 3–5 Hz typically.

Recording

Upper Extremity SSEP (Median Nerve)

• Erb’s point (above clavicle): N9 wave; tests proximal peripheral nerve and brachial plexus.
• Cervical spine (C7-Fz montage): N13 wave; tests dorsal column and spinal cord.
• Contralateral somatosensory cortex (e.g., C3’/C4′ contralateral to stimulated side): N20 wave; tests cortical response.
• Additional waves: P14 (brainstem), N18, P22, N30 (cortical).

Lower Extremity SSEP (Posterior Tibial)

• Popliteal fossa: N7 wave; peripheral nerve response.
• Lumbar spine (T12-iliac crest): N22 wave; spinal cord response.
• Cervical spine: P39 wave (after cord transmission).
• Contralateral cortex (Cz’-Fz): P40 wave; cortical response.

Normal SSEP Latencies (Approximate)

Interpeak Intervals

• N9–N13: peripheral nerve to cervical cord.
• N13–N20: cervical cord to cortex.
• N22–P40: lumbar to cortex (lower extremity).
• Side-to-side and within-patient consistency essential.

Abnormal SSEPs

Prolonged Peripheral Latencies

• Slow conduction in peripheral nerve.
• Polyneuropathy, focal nerve compression.
• NCS more sensitive than SSEP for peripheral disease.

Prolonged Central Latencies

• Conduction delay in spinal cord or brainstem.
• Demyelination (MS), spinal cord injury, post-cardiac arrest, hypoxic-ischemic injury.

Absent Cortical Response (N20 or P40)

• Severe injury to spinal cord, brainstem, or cortex.
• Post-cardiac arrest: bilateral absent N20 strongly predicts poor outcome.
• Brain death: absent N20 expected.

Reduced Amplitudes

• Axonal loss in peripheral or central pathways.
• Severe disease.

Clinical Applications of SSEPs

Multiple Sclerosis

• Demonstrates subclinical sensory pathway involvement.
• Supports diagnosis (McDonald criteria allow EP evidence).
• Prolonged central conduction times.
• Tracks clinical recovery and progression.

Post-Cardiac Arrest Prognostication

• Bilateral absent N20 at 24–72 hours post-arrest: strong predictor of poor neurologic outcome.
• Specificity for poor outcome: ~95–99% (very few false positives).
• Combined with EEG, exam, NSE for multimodal assessment.
• One of the most reliable single tests for prognosis.

Coma Evaluation

• Preserved SSEPs in comatose patient: better prognosis.
• Absent SSEPs: worse prognosis.
• Combined with BAEPs and clinical exam.

Brain Death Determination (Ancillary)

• Absent N20 expected in brain death.
• Used as confirmatory test in some jurisdictions.

Spinal Cord Disease

• Myelopathy: prolonged central conduction or absent cortical response.
• Subacute combined degeneration (B12 deficiency).
• HIV myelopathy.
• Spinal cord tumor.

Intraoperative Monitoring

• Spine surgery: SSEPs monitor sensory pathway integrity during corrective procedures.
• Aortic aneurysm surgery: monitor spinal cord during cross-clamping.
• Brain surgery: monitor sensory cortex.
• Real-time alerts for amplitude reduction or latency increase.

Brachial Plexus Injury

• SSEPs help localize: absent N9 with normal N13 suggests pre-ganglionic injury (root avulsion); absent N13 suggests post-ganglionic.
• Important for surgical planning.

Pediatric Neurology

• HIE in neonates: SSEPs prognostic.
• Spinal cord injury monitoring.

Motor Evoked Potentials (MEPs)

Concept

• Stimulate motor cortex (transcranially) → record motor response from peripheral muscle.
• Tests corticospinal tract function.
• Complements SSEP (sensory pathway) for complete spinal cord assessment.

Stimulation Methods

Transcranial Magnetic Stimulation (TMS)

• Magnetic field induces current in cortex.
• Painless (used clinically and for research).
• Single pulse evokes MEP.
• Standard for diagnostic MEPs and for therapeutic rTMS.

Transcranial Electrical Stimulation (TES)

• Brief high-voltage electrical pulse.
• More painful in awake patients.
• Standard for intraoperative MEP monitoring.
• Multi-pulse “trains” produce more reliable responses.

Recording

• Surface electrodes over peripheral muscle (e.g., abductor pollicis brevis, abductor hallucis).
• Standard EMG recording setup.
• Latency from stimulus to muscle response.
• Amplitude of MEP.

Measurements

• MEP latency: time from cortical stimulus to onset of muscle response.
• Central motor conduction time (CMCT): MEP latency minus peripheral conduction time (from spinal cord stim or F-wave estimate).
• CMCT: tests corticospinal tract specifically.
• Normal CMCT to upper extremity: ~6–9 ms; lower extremity: ~16–22 ms.

Abnormal MEPs

• Prolonged CMCT: corticospinal tract demyelination or slowed conduction.
• Absent MEP: severe corticospinal tract injury.
• Reduced amplitude: axonal loss or conduction abnormality.

Clinical Applications

• ALS: prolonged CMCT supports upper motor neuron involvement.
• MS: corticospinal tract demyelination.
• Spinal cord disease: complements SSEPs.
• Stroke: motor pathway integrity.
• Intraoperative monitoring: spine surgery, brain tumor surgery, vascular surgery.

Combined SSEP + MEP in Intraoperative Monitoring

• SSEPs monitor sensory pathway (dorsal column-medial lemniscus).
• MEPs monitor motor pathway (corticospinal tract).
• Together they assess both columns of the spinal cord.
• Real-time alerts during surgical maneuvers.
• Used during scoliosis correction, spinal tumor resection, aortic aneurysm surgery, neurosurgical procedures.
• Alert criteria: amplitude reduction >50% or latency increase >10% triggers surgeon notification.

Anesthesia Effects on EPs

SSEP

• Cortical SSEPs (N20, P40) attenuated by inhalational anesthetics.
• Subcortical SSEPs (N13, P22) relatively preserved.
• Total intravenous anesthesia (propofol) better preserves cortical responses.
• Muscle relaxants do not affect SSEP.

MEP

• Very sensitive to inhalational anesthetics — often abolished.
• Propofol-based total IV anesthesia required for reliable intraoperative MEP.
• Muscle relaxants attenuate or abolish MEP.
• For MEP monitoring: avoid muscle relaxants or use only partial blockade.

Pediatric Considerations

• SSEP latencies are shorter in infants (faster conduction velocities relative to limb length).
• Maturation continues through childhood.
• Used in neonatal HIE prognostication.

Reporting

An SSEP/MEP report should include:

• Indication.
• Stimulation and recording details.
• Latencies and interpeak intervals.
• Side-to-side comparison.
• Amplitudes if relevant.
• Comparison to lab norms.
• Pattern of abnormality (central, peripheral, mixed).
• Interpretation and clinical correlation.

🔍 Did You Know?

The bilateral absence of N20 SSEPs at 24–72 hours after cardiac arrest has emerged as one of the most reliable single-test predictors of poor neurologic outcome in clinical neurology. In adult patients comatose after cardiac arrest, with appropriate hypothermia/rewarming timing and no significant sedation effects, bilateral absent N20 carries 95–99% specificity for poor outcome — defined as death, persistent vegetative state, or severe disability. This finding has been validated across multiple large studies and is now incorporated into ERC/ESICM and AAN guidelines for post-cardiac arrest prognostication. The clinical implications are profound: SSEP findings can support difficult conversations with families about prognosis and end-of-life decisions, particularly when clinical examination is ambiguous due to recent sedation or hypothermia. However, the test has critical requirements: technical adequacy (must be confident that absent N20 is real, not artifact), timing (24–72 hours after rewarming), absence of significant sedation effects, and combined assessment with other prognostic markers (EEG, NSE, exam). False positive rate (predicting poor outcome in patients who could recover) is very low but not zero — making SSEPs ideal for multimodal prognostication rather than single-test decisions. The lesson is that specific neurophysiologic findings can support life-altering clinical decisions, and the rigor of test interpretation (especially confirming absent waves are real and not technical failure) is essential. For practicing neurologists and intensivists, the practical implication is that bilateral absent N20 with adequate technical recording is among the strongest objective markers we have for poor neurologic recovery — a powerful prognostic tool used judiciously.

Pitfalls and Pearls

• SSEP standard upper extremity: median nerve at wrist; record Erb’s (N9), cervical (N13), cortex (N20).
• SSEP standard lower extremity: posterior tibial; record popliteal (N7), lumbar (N22), cortex (P40).
• N20 generator: primary somatosensory cortex.
• N20 absence post-arrest: very strong predictor of poor outcome (specificity ~95–99%).
• MS: prolonged central conduction times; demonstrate dissemination.
• MEPs: stimulate motor cortex (TMS clinical, TES intraoperative); record muscle response.
• Central motor conduction time (CMCT): tests corticospinal tract specifically.
• ALS: prolonged CMCT supports UMN involvement.
• Intraoperative monitoring: SSEPs + MEPs for spinal cord; alert at 50% amplitude or 10% latency change.
• Anesthesia effects: inhalational attenuates cortical SSEP and abolishes MEP; use TIVA for MEP.
• Muscle relaxants: abolish MEP; avoid or partial blockade.
• Brachial plexus localization: absent N9 with normal N13 = pre-ganglionic (root avulsion).
• Spinal cord disease: SSEP + MEP characterize sensory and motor pathway involvement.
• Multimodal coma prognostication: SSEP + BAEP + EEG + exam + NSE.
• Brain death: SSEPs may be confirmatory in some jurisdictions.
• Pediatric SSEPs: maturation pattern; HIE prognostication.

References

1. Mauguière F, Allison T, Babiloni C, et al. Somatosensory evoked potentials. Recommendations for the Practice of Clinical Neurophysiology: Guidelines of the IFCN. 2nd ed. Elsevier; 1999:79-90.
2. Sandroni C, D’Arrigo S, Cacciola S, et al. Prediction of poor neurological outcome in comatose survivors of cardiac arrest: a systematic review. Intensive Care Med. 2020;46(10):1803-1851.
3. Nuwer MR, Aminoff M, Desmedt J, et al. IFCN recommended standards for short latency somatosensory evoked potentials. Report of an IFCN committee. Electroencephalogr Clin Neurophysiol. 1994;91(1):6-11.
4. Aminoff MJ, ed. Aminoff’s Electrodiagnosis in Clinical Neurology. 6th ed. Elsevier; 2012.
5. Walsh P, Kane N, Butler S. The clinical role of evoked potentials. J Neurol Neurosurg Psychiatry. 2005;76(suppl 2):ii16-ii22.
6. Macdonald DB. Intraoperative motor evoked potential monitoring: overview and update. J Clin Monit Comput. 2006;20(5):347-377.