Evoked Potentials Interpretation and Pitfalls

1
Evoked PotentialsInterpretation and Pitfalls

• Dr. T P Chan
• March 2007

2
Assumptions

• Methodology
• Equipment
• Electrode application
• Channel
• Filter
• Specific diseases

3
PSVEP
4
PSVEP

• Q Where is P100 generator?
• In striate and prestriate occipital cortex not
  only as a result of primary activation but also
  from subsequent thalamocortical volleys
• Exact generators and temporal sequence of their
  activation not well defined

5
PSVEP

• Sensitive extension of neuro exam but
  non-specific
• When PSVEP normal, never abnormal in clinical
  exam
• When PSVEP abnormal, various clinical exam could
  still be normal
• Brooks and Chiappa 1982

6
PSVEP

• Hence, abnormal PSVEP must be carefully
  integrated into clinical situations
• Other procedures usually needed to differentiate
  the possible causes

7
PSVEP

• Q Full field stimulation posterior midline
  recording can pick up postchiasmastic lesion.
  Correct?
• No
• Half field stimulation or full field stimulation
  LO/RO recordings needed
• But seldom done by most lab
• Great normal variability difficult in
  interpretation

8
PSVEP

• Q Full field stimulation monocular abn VEP ?
• A conduction defect in the left/right visual
  pathways anterior to the chiasm
• To find out specific etiology needs further
  investigations

9
PSVEP

• Q Full field stimulation binocular abn VEP ?
• Conduction defects in the visual pathway
  bilaterally
• Exact lesion location cannot be determined
• Marked asymmetry at least one lesion anterior
  to chiasm??

10
PSVEP

• Amplitude
• Related to visual acuity, attention, and fixation
• Great variability, much less useful

11
PSVEP

• W pattern
• Rarely in normal usually suggest abn
• Possibilities
• Retrochiasmatic conduction defect, confirmed by
  hemifield/imaging
• Different conduction between upper and lower
  fields due to altitudinal field defect
• Different conduction between macular and
  peripheral retinal response if using large field
  of stimulation

12
PSVEP

• Technical consideration
• TV screen vs projection system
• Small field small check vs large field large
  check
• Dark and light check contrast

13
PSVEP

• References
• Normal P100 latency (mean 3SD) 117 ms
• Normal intereye latency difference (mean 3SD)
  even more sensitive 8 ms
• Shahrokhi et al 1978

14
BAEP
15
BAEP

• Q I V generator sources?
• Distal eighth nerve
• Proximal eighth nerve/cochlear nucleus
• Lower pons (superior olivary complex?)
• Mid/upper pons (lateral lemniscus tracts and
  nuclei?)
• Upper pons/inferior colliculus

16
BAEP

• Q Which wave to identify first? Why and how?
• V, because most prominent
• Always after 5ms, seldom before 5.2ms
• Followed by prolonged downward deflection
• 1/3 as IV/V complex with IV as small notch
• Reduce click intensity only V persists

17
BAEP

• Q Then identify which wave? How?
• I. Polarity always negative, independent of
  polarity of click
• At least 1.2ms following the click before 1.2ms,
  either artefacts, cochlear microphonics (receptor
  potentials of hairy cells of the spiral organ) or
  summation potentials (presynaptic potentials)

18
BAEP

• Q How to differentiate CM/SP from wave I?
• CM and SP polarity changes with polarity of click

19
BAEP

• Q Abn III V suggests contralateral mid/upper
  pontine lesion?
• Issue of laterality
• No.
• Ipsilateral for clinical interpretation purpose
• ? BAEP generators distinct from pathway
  subserving normal hearing

20
BAEP and hearing

• Q What is the clinical effect of peripheral
  hearing disorder on central IPL?
• No
• Q What is the effect of cortical deafness on
  central IPL?
• No

21
BAEP and hearing

• Q What is the correlation between II-V abn and
  hearing?
• No.
• Yes only for II-V and sound source localization
  performance
• Q What if I absent and V delayed?
• Correct with degree of hearing loss before
  concluding central conduction defect
• Selters and Brackmann 1977

22
BAEP

• Q I-III abn?
• A conduction defect in the brainstem auditory
  system between eighth nerve close to cochlea and
  the lower pons
• Q III-V abn?
• A conduction defect in the brainstem auditory
  system between lower pons and midbrain

23
BAEP

• Q I absent and normal III-V?
• Wave I could not be recorded. This is usually due
  to peripheral hearing disorder. Hence the state
  of conduction in the segment of brainstem
  auditory system between peripheral eighth nerve
  and lower pons could not be determined
• Lower pons to midbrain conduction was normal

24
BAEP

• Q How useful is I/V amplitude ratio?
• Limited because great variation
• Q If wave V is markedly reduced in amplitude or
  poor defined, interpretation?
• This abnormality suggests the presence of a
  conduction defect in the brainstem auditory
  system rostral to the lower pons

25
BAEP

• Remark
• BAEPs are very resistant to alternation by
  anything other than structural pathology in the
  brainstem auditory tracts
• This specificity allows localization of
  conduction defects in the brainstem to within a
  centimeter or so

26
BAEP

• Hence, features are
• Anatomic specificity
• Physiologic and metabolic immuntability
• Etiologic non-specificity

27
BAEP

• References
• I (average latency) 1.4ms, then 1ms interval
  each wave
• Mean 3SD
• I 1.8ms III 4ms V 6ms
• I-III 2.6ms III-V 2.3ms I-V 4.6ms

28
Thank you