General Principles

• Evoked potentials (EPs) — measure CNS conduction along specific sensory or motor pathways in response to a defined stimulus
• Signal averaging — hundreds to thousands of repetitions are averaged to extract the time-locked response from random background EEG noise
• Latency — reflects myelin integrity (prolonged in demyelination)
• Amplitude — reflects axonal integrity and number of functioning fibers (reduced in axonal loss)
• Primary clinical uses — detect subclinical lesions (especially in MS), intraoperative monitoring, prognostication
• EPs test the entire pathway from stimulus to cortex — can localize lesion to peripheral, brainstem, or cortical segments

Key Terminology

• Absolute latency — time from stimulus to a specific peak
• Interpeak latency (IPL) — time between two peaks; localizes the segment of pathway involved
• Central conduction time (CCT) — transit time through the CNS (excludes peripheral segment)
• Waveforms named by polarity and latency: N = negative, P = positive, number = approximate latency in ms (e.g., N20 = negative peak at ~20 ms)

Technique and Waveforms

• Stimulus — pattern-reversal checkerboard (alternating black/white squares on a screen); each eye tested separately
• Recording — active electrode at Oz (occipital midline), referenced to Fz (midfrontal)
• Key waveform: P100 — positive peak at ~100 ms; generated by striate cortex (V1); P100 latency 90–110 ms; interocular difference <5–10 ms is significant
• Full-field stimulation — entire visual field of one eye; detects pre-chiasmal lesions (optic nerve)
• Half-field stimulation — one hemifield at a time; localizes post-chiasmal lesions (optic tract, radiation)

Abnormalities

Technique

• Stimulus — monaural clicks delivered via earphones (alternating polarity); contralateral ear receives white noise masking; click stimulus 60–70 dB above threshold (sensation level) or ~70 dB nHL, presented monaurally at ~10–11 Hz
• Recording — vertex (Cz) referenced to ipsilateral ear (A1 or A2)
• 5 waveforms generated within 10 ms of stimulus

BAEP Waves — Generators

Memory Aid: BAEP Wave Generators

• "E-COSLIn" — Eighth nerve distal (I), proximal CN VIII / Cochlear nerve intracranial (II), Olivary complex (III), Lateral lemniscus (IV), Inferior colliculus (V)
• Wave V is the most robust and last wave to disappear — it is the most clinically important waveform

Interpeak Latencies

Clinical Applications

• Acoustic neuroma — prolonged I–III IPL or absent waves beyond Wave I; interaural Wave V latency difference >0.3–0.4 ms is the most sensitive BAEP criterion for vestibular schwannoma
• MS — prolonged III–V or I–V IPL (central demyelination)
• Intraoperative monitoring — posterior fossa surgery, CN VIII preservation
• Brain death — absent all waves, or only Wave I present (peripheral response without brainstem conduction)
• Infant hearing assessment — objective hearing threshold estimation in neonates (no patient cooperation needed)

Technique

• Stimulus — electrical stimulation of peripheral nerve (square-wave pulse at motor threshold)
• Upper extremity — median nerve at wrist (most common)
• Lower extremity — posterior tibial nerve at ankle (most common)
• Pathway tested — peripheral nerve → dorsal columns → medial lemniscus → thalamus (VPL) → primary sensory cortex

Upper Extremity SSEP Waveforms (Median Nerve)

Central conduction time (CCT) for upper extremity SSEP is typically computed as N13–N20, with P14–N20 as an alternate CCT measure.

Lower Extremity SSEP Waveforms (Posterior Tibial Nerve)

Clinical Applications

• MS — central conduction delay (prolonged N13–N20 or N22–P37 interpeak latency); subclinical spinal cord/brainstem lesions
• Intraoperative spinal cord monitoring — monitors posterior column (dorsal column) function during scoliosis correction, spinal tumor surgery
• Post-cardiac arrest prognostication — bilateral absent cortical N20 with preserved peripheral/cervical responses, assessed ≥48 h after ROSC = highly reliable poor-outcome predictor; use as part of multimodal neuroprognostication, not in isolation
• Myelopathy evaluation — detect subclinical dorsal column dysfunction in cervical spondylotic myelopathy, B12 deficiency

Technique

• Stimulus — transcranial magnetic stimulation (TMS) or transcranial electrical stimulation (TES) over motor cortex
• Recording — compound muscle action potential (CMAP) from target muscle (e.g., hand intrinsics, tibialis anterior)
• Pathway tested — motor cortex → corticospinal tract → anterior horn cell → peripheral nerve → muscle

Central Motor Conduction Time (CMCT)

• CMCT = cortical MEP latency − spinal (root) MEP latency
• Isolates the central segment (cortex to anterior horn cell)
• Normal CMCT — ~6–8 ms (upper extremity), ~12–16 ms (lower extremity)
• Prolonged CMCT → corticospinal tract demyelination (MS, hereditary spastic paraplegia) or degeneration (ALS)

Clinical Applications

Master Comparison of All EP Modalities

Sensitivity by Modality

Role in MS Diagnosis

• EPs detect subclinical lesions — objective evidence of demyelination in a pathway without clinical symptoms
• VEP showing prolonged P100 in a clinically unaffected eye supports dissemination in space
• Under the 2017 McDonald criteria, VEP is no longer required or included as supportive evidence (it was used in earlier 2001/2005 criteria). VEP remains clinically useful and the 2024 McDonald revision adds optic nerve as a topographic location with OCT/VEP support. Note: older board questions and review books may still reference VEP as evidence for dissemination in space (DIS) — that wording is based on pre-2017 criteria and is now outdated.
• Multimodal EPs (VEP + SSEP + BAEP) increase overall sensitivity for detecting MS lesions
• EPs are complementary to MRI — they detect functional demyelination that may not be visible on imaging

EP Modalities for Intraoperative Use

Alarm Criteria

• For SSEP: ≥50% amplitude drop or ≥10% latency increase → significant (alert surgeon)
• For MEP: all-or-none / ≥50–80% amplitude drop or stimulation threshold increase ≥100 V; ⚠️ LATENCY CRITERIA DO NOT APPLY TO MEP — a common board trap; SSEP alarm rules (≥10% latency increase) are not transferable to MEP because MEP latency is highly variable shot-to-shot
• Complete loss of waveform → critical alert (immediate surgical response)

Anesthesia & EPs

• TIVA (propofol + ketamine or remifentanil) preferred for intraoperative monitoring
• Volatile anesthetics (sevoflurane, desflurane, isoflurane) suppress cortical SSEP and especially MEPs
• Subcortical SSEP components relatively resistant
• N₂O suppresses MEPs
• Muscle relaxants abolish MEPs (avoid or use light blockade)

Alert Protocol

• Verify technical factors (electrode displacement, anesthetic changes, blood pressure, temperature)
• Notify surgeon immediately
• Consider reversible causes: hypotension, hypothermia, anesthetic depth change
• If changes persist after technical check → surgical cause likely → consider reversing recent surgical maneuver

Post-Cardiac Arrest

• Bilateral absence of cortical N20 SSEPs with preserved peripheral/cervical responses, assessed ≥48 h after ROSC, is a highly reliable poor-outcome predictor (specific marker per ERC/ESICM 2021 and Neurocritical Care Society 2023 post-arrest neuroprognostication guidelines) — use as part of multimodal assessment, not stand-alone
• False positive rate <1% — this is among the most specific and reliable prognostic tools after cardiac arrest
• SSEP is relatively resistant to sedation and mild hypothermia; severe hypothermia and high-dose anesthetics can suppress cortical N20, but SSEP is generally more reliable than clinical exam or EEG in the ICU setting
• Preserved N20 does NOT guarantee good outcome (low positive predictive value for recovery)
• Should be interpreted in conjunction with other prognostic tools (clinical exam, EEG, NSE, brain MRI)

Brain Death

• BAEP — absent all waves, or only Wave I present (cochlear/peripheral response without brainstem conduction)
• Presence of Wave I with absent Waves II–V confirms absent brainstem function with intact cochlea
• BAEP findings (absent waves II–V with Wave I preserved) support absent brainstem function but are NOT among recommended ancillary tests per the 2023 AAN/AAP/CNS/SCCM brain death/death by neurologic criteria consensus guideline. Acceptable ancillary tests are radionuclide cerebral blood-flow scan, 4-vessel catheter angiography, and TCD (adults only). EEG and evoked potentials (BAEPs, SSEPs) are no longer endorsed.

Prognostication Summary Table