Neurophys - EP

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Neurophys - EP
2012-09-03 08:24:52
Evoked Potentials

Evoked Potentials
Show Answers:

  1. What is an Evoked Potential?
    • Time-locked response to a stimulus related to background noise
    • By convention, they are relative positive at the recording electrode (downward deflection)
  2. What are examples of Evoked Potentials?
    • Visual Evoked Response (VEPs)
    • Brainstem Auditory Evoked Responses (BAERs) 
    • Somatosensory Potentials (SPs) 
  3. Describe how Visual Evoked Potentials (VEPs) are assessed ...
    • Monocular stimulation with a high-contrast checkerboard pattern or flashing lights given repetatively hundreds of times with the responses averaged and subtracted from background noise
    • Tests the visual pathway from nerve to cortex
    • The VEP is triphasic (neg/pos/neg)
    • Major peak is P100 (V1 - Area 17/18)
  4. What EP phenomenon is demonstrated here?
    P100 of the VEP
  5. How are VEP's interpretted?
    • Abnormal if:
    • Latency delayed (> 110 msec)
    • Latency differs between sides (>8 msec)
    • Response Absent

    • Absent Response is normally prechiasmatic
    • Mildly delayed latencies are seen in AION, Compression of the ON, Ocular d/s like glaucoma
    • Severely delayed laterncies are seen in MS / Optic Neuritis, Leukodystrophies, Spinocerebellar Degen, SCID, Vitamin E d/f
  6. What EP phenomenon is demonstrated here?
    Unilateral delay in P100 indicative of a prechiasmatic lesion.
  7. Describe how Brainstem Auditory Evoked Responses (BAERs) are assessed ...
    • mall signals requiring thousands of simuli to each ear (loud clicks)
    • These are far-field potentials and the polarities are upgoing (positive)
    • Recorded from A1/A2 (Areas 41, 42) 
  8. What are the origins of the waveforms in BAER's?
    • Wave 1 - Ipisilateral CN VIII
    • Wave 2 - Ipisilateral Cochlear Nerve
    • Wave 3 - Bilateral SON (lower pons)
    • Wave 4 - Bilateral LL (mid-upper pons)
    • Wave 5 - Bilateral IC (midbrain)
    • Wave 6 - Bilateral MG (thalamus)
    • Wave 7 - Bilateral Auditory Radiation (cortex)   
  10. How are BAERs evaluated?
    • Prolongation of the interpeak latencies
    • Side to side differences
    • Absence of waves
    • Amplitude   
  11. How are BAER abnormalities localized?
    Absence of all waveforms - abnormal peripheral auditory function

    Interpeak latency prolongation - abnormal conduction along that segment

    Absent conduction past a specific wave - structural lesion at that location
  12. BAER's may be helpful in diagnosing ...
    • Demyelinating diseases
    • Acoustic Neuromas
    • Toxic / Metabolic Coma (normal)
    • Brain Death (1 and 2 may be preserved while all others absent)
  13. Somatosensory Evoked Potentials (SSEPs)
    These are predominantly generated from large sensory afferents (proprioception and vibration) using the dorsal column / medial lemniscal system
  14. What are the majors waveforms observed in an SSEP and from where do they originate?
    • N9 - brachial plexus at Erb's Point (nerve AP)
    • N14 - Postsynaptic activity in the cervical cord
    • N20 - Primary Somatosensory Cortex
    • P22 - Cortical positivity following N20
  15. How are SSEP's interpretted?
    • All waves absent / abnl - peripheral sensory conduction
    • Prolonged interpeak latency - lesion between points