Evoked Potentials (Visual, Brainstem Auditory, Somatosensory)
Domain 5 - Evoked Potentials (Visual, Brainstem Auditory, Somatosensory)
The exam tests stimulus parameters, recording sites, waveform identification, and interpretation of latency and amplitude changes.
Core rules
- VEP. Pattern-reversal checkerboard at about 1–2 reversals/s, full-field, each eye separately with the other patched. Record occipital (Oz) referred to Fz or linked ears. Identify N75, P100, N145; P100 latency is the key measure (about 100 ms, upper normal near 115 ms, lab-specific). Check acuity, pupil size, and fixation.
- BAEP. Clicks at about 10–11/s, 60–70 dB nHL, rarefaction or alternating, recorded Cz to ipsilateral ear. Waves I–V; wave V around 5.5–6.0 ms with I–V interpeak near 4 ms. Sedation has little effect.
- SSEP. Median nerve at the wrist, supramaximal stimulation at 3–5/s; record Erb's point (N9), cervical spine (N13), and contralateral scalp (N20). Posterior tibial nerve yields P37. Keep limbs warm; repeat for reproducibility.
- Interpretation. Prolonged latency suggests demyelination; reduced amplitude suggests axonal loss. Compare sides and use age-matched norms.
Worked example
A patient has unilateral hearing loss and a suspected acoustic neuroma. BAEP shows a normal wave I but prolonged wave V and I–V interpeak latency on the affected side, with normal contralateral responses. Reasoning: the normal wave I places the lesion after the distal nerve, and the prolonged central conduction fits retrocochlear compression; a cochlear lesion would typically reduce wave I amplitude as well.
Traps
Testing both eyes together; misidentifying the wave IV/V complex; not documenting stimulus polarity and rate; ignoring temperature in SSEP; overinterpreting a single unreplicated waveform.