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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)
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What are examples of Evoked Potentials?
- Visual Evoked Response (VEPs)
- Brainstem Auditory Evoked Responses (BAERs)
- Somatosensory Potentials (SPs)
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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)
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What EP phenomenon is demonstrated here?
P100 of the VEP
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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
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What EP phenomenon is demonstrated here?
Unilateral delay in P100 indicative of a prechiasmatic lesion.
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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)
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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)
NCSLIMA
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How are BAERs evaluated?
- Prolongation of the interpeak latencies
- Side to side differences
- Absence of waves
- Amplitude
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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
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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)
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Somatosensory Evoked Potentials (SSEPs)
These are predominantly generated from large sensory afferents (proprioception and vibration) using the dorsal column / medial lemniscal system
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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
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How are SSEP's interpretted?
- All waves absent / abnl - peripheral sensory conduction
- Prolonged interpeak latency - lesion between points
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