Evoked potentials (EPs) record the electrical response of the central nervous system to a defined sensory or motor stimulus. By time-locking the recording to the stimulus and averaging across many trials, the stimulus-related response emerges from background EEG noise. EPs assess conduction along discrete neural pathways with millisecond precision, and they remain a core tool for diagnosing demyelinating disease, evaluating brainstem and spinal cord integrity, prognosticating coma, and monitoring during surgery. This page is a short umbrella to the EP modalities covered in detail elsewhere in this section.

Modalities Covered in This Section

• Visual Evoked Potentials (VEP) — pattern-reversal stimulation of the visual pathway; classic P100 response from occipital scalp. Sensitive to optic pathway demyelination (MS, NMOSD, MOGAD), ischemic optic neuropathy, compressive lesions, and hereditary optic neuropathies.
• Brainstem Auditory Evoked Potentials (BAEP) — click stimulation; waves I–V trace the auditory pathway from cochlear nerve to inferior colliculus. Useful for acoustic neuroma evaluation, brainstem function in coma, anesthesia-resistant intraoperative monitoring, and neonatal hearing screening.
• Somatosensory & Motor Evoked Potentials (SSEPs & MEPs) — peripheral nerve stimulation with multi-site recording (SSEPs) or transcranial cortical stimulation with peripheral muscle recording (MEPs). Central to multimodal coma prognostication after cardiac arrest, MS workup, spinal cord assessment, and intraoperative monitoring during spine and brain surgery.

Shared Principles

• Signal averaging: each EP is small relative to ongoing EEG; hundreds to thousands of trials averaged to extract the stimulus-locked response. Noise reduces as √N.
• Latency is more informative than amplitude: latency reflects conduction time and is reproducible; amplitude varies with attention, pupil size, electrode placement, and effort.
• Side-to-side comparison: an unaffected side serves as the patient’s own reference and is often more useful than absolute lab norms.
• Anesthetic sensitivity differs by modality: BAEPs are largely anesthetic-resistant; cortical SSEPs are attenuated by inhalational agents; MEPs are abolished by inhalational anesthetics and by muscle relaxants.
• EPs are sensitive but not specific: a delayed response indicates a conduction abnormality, not a specific etiology — clinical correlation is required.

When to Order Which EP

EPs in MS Diagnosis

• VEP, SSEP, and MEP can each demonstrate subclinical demyelination, contributing evidence of “dissemination in space.”
• VEP is the most clinically informative single EP modality in MS — it remains the only EP explicitly retained for MS dissemination criteria in some updates.
• BAEP abnormalities are less specific to MS but support brainstem involvement when present.

EPs in Coma Prognostication (Post-Cardiac Arrest)

• Bilateral absent N20 SSEPs at 24–72 hours post-arrest carries 95–99% specificity for poor neurologic outcome (death, persistent vegetative state, or severe disability).
• Preserved BAEPs (especially wave V) indicate retained brainstem function and contribute favorable evidence.
• EPs are part of a multimodal package alongside EEG, clinical exam, neuron-specific enolase, and imaging — no single test is decisive.

EPs in the Operating Room

• Real-time SSEP + MEP monitoring during spine surgery alerts the surgeon to impending cord injury before it becomes permanent.
• BAEP during posterior fossa surgery monitors cochlear nerve and brainstem.
• Direct cortical SSEPs and MEPs map eloquent cortex during tumor and epilepsy surgery.
• Alert criteria: typically >50% amplitude reduction or >10% latency increase triggers surgeon notification.

🔍 Did You Know?

Different evoked potential modalities have strikingly different sensitivities to anesthetic agents, and choosing the right modality for the clinical context is part of expert electrodiagnosis. BAEPs are largely anesthetic-resistant because their generators are subcortical (cochlear nucleus, superior olivary complex, lateral lemniscus, inferior colliculus) and operate at a level relatively unaffected by metabolic state. SSEP cortical responses (N20) are attenuated by inhalational anesthetics but reasonably preserved under total intravenous anesthesia (TIVA, propofol-based). MEPs are exquisitely sensitive to both inhalational anesthetics and muscle relaxants, requiring TIVA without paralysis for reliable intraoperative recording. The clinical implications: BAEPs can confirm brainstem function in a heavily sedated ICU patient when SSEPs would be unreliable; spine surgery requiring MEP monitoring mandates a specific anesthetic protocol; and the anesthesia team is part of the IONM team. For practicing neurologists and intensivists, the take-home is that “the EPs are normal” only means something if the recording conditions were appropriate, and asking which agents the patient is on, at what doses, is essential before interpreting any EP in the ICU or OR.

Pitfalls and Pearls

• VEP: optic pathway, demyelination — pattern-reversal P100.
• BAEP: cochlear nerve through inferior colliculus — waves I, III, V; anesthesia-resistant.
• SSEP: dorsal columns / medial lemniscus / sensory cortex — N20 in upper extremity, P40 in lower.
• MEP: corticospinal tract — central motor conduction time.
• Latency, not amplitude, is the most informative measurement.
• Side-to-side comparison often more useful than absolute lab norms.
• Anesthetic sensitivity: BAEP < SSEP < MEP (MEP most affected).
• Bilateral absent N20 post-arrest: strong predictor of poor outcome.
• MS diagnosis: VEP is the most clinically informative single EP modality.
• IONM: SSEP + MEP for spine; BAEP for posterior fossa; alert criteria >50% amplitude / >10% latency.
• EPs are sensitive but not specific: always correlate with clinical context.
• See dedicated pages: VEP, BAEP, SSEP+MEP for full detail.

References

1. Chiappa KH. Evoked Potentials in Clinical Medicine. 3rd ed. Lippincott-Raven; 1997.
2. Walsh P, Kane N, Butler S. The clinical role of evoked potentials. J Neurol Neurosurg Psychiatry. 2005;76(suppl 2):ii16-ii22.
3. Aminoff MJ, ed. Aminoff’s Electrodiagnosis in Clinical Neurology. 6th ed. Elsevier; 2012.
4. Mauguière F, Allison T, Babiloni C, et al. Recommendations for the Practice of Clinical Neurophysiology: Guidelines of the IFCN. 2nd ed. Elsevier; 1999.
5. 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.