Evoked Potentials
Core concepts
Know the three main modalities: visual evoked potentials (VEP), brainstem auditory evoked potentials (BAEP), and somatosensory evoked potentials (SSEP). For VEP, the P100 wave reflects the visual pathway and is delayed in demyelinating optic neuropathy; check reproducibility and monocular stimulation. For BAEP, waves I–V correspond to peripheral auditory nerve through brainstem structures; wave V latency and interpeak intervals (I–III, III–V, I–V) assess brainstem conduction. For SSEP, stimulation of a peripheral nerve (e.g., median) produces cortical and subcortical waveforms (N9, N13, N20) that assess the dorsal column pathway. Understand that EPs assess function, not structure, and are sensitive to demyelination.
Traps
A frequent trap is misassigning waveform generators (e.g., wave I = auditory nerve, not cochlear nucleus). Another is ignoring technical factors: BAEP is affected by hearing loss and stimulus polarity; VEP depends on check size and luminance; SSEP depends on limb temperature and stimulation rate. Questions may ask whether a prolonged latency with preserved amplitude suggests demyelination (yes). Do not overinterpret absent waveforms without confirming technical adequacy.
Worked example / rule of thumb
A patient with acute unilateral optic neuritis may show a delayed P100 on the affected eye even when acuity is recovering - latency prolongation outlasts clinical recovery. Rule of thumb: demyelination prolongs latency; axonal loss reduces amplitude. For BAEP, compare interpeak intervals rather than absolute latencies to control for peripheral hearing.
Key points
- VEP P100 is the key waveform; delayed in demyelinating optic neuropathy.
- BAEP waves I–V map to auditory nerve through brainstem; use interpeak intervals.
- SSEP (N9, N13, N20) assesses dorsal column conduction.
- Demyelination prolongs latency; axonal loss lowers amplitude.
- Technical factors (hearing, luminance, temperature, rate) alter results.
- Confirm technical adequacy before calling a waveform absent.