Evoked Potentials

Visual evoked potentials VEP:

  • P100 latency ~100ms
  • Prolonged P100 latency:
    • Optic neuritis, retrobulbar neuritis, anterior ischemic optic neuropathy, toxic amblyopia, vitamin B12 deficiency, Leber optic atrophy, and tumors compressing the optic nerve
    • Optic nerve compression (eg, orbital tumor) may distort the P100 morphology and reduce the amplitude relative to the contralateral eye

Somatosensory evoked potentials SSEP:
Upper extremity SSEP:

  • Stimulating at the median nerve:
    • N5 potential is recorded in the median or ulnar nerve
    • N9 potential is recorded at the Erb’s point
    • N11 potential is recorded at the dorsal root entry
    • N13 potential is created by dorsal column of the cervical cord
    • N14 at the cervicomedullary junction
    • N20 potentials is at the cortical area

Lower extremity SSEP:

  • Stimulation at the posterior tibial nerve:
    • N22 at the lumbar cord grey matter
    • P38 at the cortex

Clinical correlation:
In brain death:

  • N13 may be present

Brain stem auditory evoked potentials BAEP:
Brain stem auditory evoked responses BAERs:

  • Five distinct electrical potentials from different points in the pathway are measured:
  • Latency between potential I & II: cochlea & auditory nerve
  • Latency between potential II & III: first relay in brain stem. Superior olive and trapezoid body.
  • Latency between potential III & IV: further relays prior to inferior colliculus. i.e. pons and midbrain.
  • Use:
    • For brain death, intraoperative monitoring & auditory pathway


  • Prolongation of the absolute latencies of waves I-V with a normal I-V interpeak latency are found in peripheral auditory dysfunction
  • Prolonged I-V interpeak latency and a reduced V/I amplitude ratio are seen in central auditory conduction abnormalities