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18 Cards in this Set
- Front
- Back
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Role of evoked potentials? |
Noninvasive, sensitive, and quantitative way to assess the functional integrity of the somatosensory, auditory, and visual pathways |
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What is the basic principle of evoked potentials? |
1. Apply a stimulus in a controlled manner to create a volley of depolarization and subsequent repolarization 2. The stimulus ascends through the pathways and can be recorded as the signals pass underneath recording electrodes 3. Generated evoked potential waveforms represent a transmitting volley of signals and not synaptic activity at a sensory nucleus or at the cortex |
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Use of evoked potentials has declined in the past decades. However, their use has had a resurgence in which settings? |
Electrophysiologic monitoring of surgical cases of the spine and posterior fossa, for which they are now part of the standard care |
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Role of somatosensory evoked potentials (SSEP)? |
Assess the functional integrity of the peripheral and central components of the proprioceptive (posterior column, medial-lemniscal) sensory pathways |
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What are SSEPs? |
Short-latency response recorded 30 to 50 ms after a nonpainful stimulation of a mixed sensorimotor nerve in the periphery, most commonly the tibial, median, or ulnar nerve |
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Why are mixed nerves used for SSEP? |
Signals and waveforms generated outside of these nerves (tibial, median, or ulnar) are much lower in amplitude and often do not have well-defined normal values, relying instead on side-to-side comparison |
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What is the most important factor for causing an SSEP abnormality? |
Location of a lesion is more important than its size, its etiology, or the severity of associated neurologic defects |
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Use of SSEP? |
1. Assess functional impairment or preservation of the proprioceptive pathways in cases of known CNS structural abnormality (spinal stenosis or tumor) 2. Provide objective identification and localization of abnormalities in imaging-negative myelopathies |
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How are SSEPs named? |
1. Based on whether they are negative (N) or positive (P) , where P is a downward deflection and N and upward deflection 2. Followed by a number that refers to the average time in milliseconds 3. A negative waveform is defined as one in which the G1 electrode records a negative charge relative to the G2 electrode |
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What does it mean if all the absolute latencies of central waveforms are prolonged in an SSEP? |
Indicates slowing of the signal volley reaching the central proprioceptive pathways |
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What does it mean if interpeak latencies are normal? |
Indicates normal central conduction times |
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Name a limitation of SSEPs? |
They can localize slowing over only a large segment (e.g., between the cervical spine and scalp), which may not seem discriminating |
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Despite a notable limitation of SSEPs, when can it be especially useful? |
When other data have shown a potential cause for a patient's symptoms (e.g., cervical stenosis) but questions remain about the pertinence of that finding to the symptom (.e.g, unsteadiness), the SSEP may be able to provide objective evidence of functional impairment of the proprioceptive pathway (slowing in the lumbar-to-cervical interpeak potential) that can be correlated with the ancillary tersting to make more specific clinical conclusions |
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When can SSEPs be used for prognosticating? |
SSEPs are less influenced by drugs and metabolic derangements than are EEGs and have been validate to be useful for determining the prognosis in postanoxic coma; when the bilateral median SSEP scalp responses are absent in the setting of preserved Erb's point (N9) and cervical spine (N13) responses, this finding invariably predicts a dire prognosis; the converse is not true |
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When are brainstem auditory evoked responses (BAERs) used? |
Functional assessment of central auditory pathways to objectively confirm brainstem dysfunction if clinical signs are equivocal or to screen for brainstem dysfunction with symptoms that are more commonly presumed to be peripheral (vertigo, diplopia, or hearing loss) |
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When are visual evoked potentials (VEPs) still used? |
Work-up of suspected MS or neuromyelitis optica because they are sensitive to lesions of the optic nerve anterior to the optic chiasm and often remain abnormal for years after clinical resolution of symptoms of optic neuritis |
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Does visual acuity affect VEPs? |
Only if it severely impaired (worse than 20/200) |
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Name of the VEP waveform? |
P100 becuase it usually occurs 100 ms after the stimulus |
