Evoked Potential Studies
These tests evaluate the integrity of visual, somatosensory, and auditory nerve pathways by measuring evoked potentials-the brain's electrical response to stimulation of the sensory organs or peripheral nerves. Evoked potentials are recorded as electronic impulses by surface electrodes attached to the scalp and skin over various peripheral sensory nerves. A computer extracts these low-amplitude impulses from background brain wave activity and averages the signals from repeated stimuli.
Three types of responses are measured:
Procedure And Posttest Care
Visual evoked potentials
On the waveform, the most significant wave is P100, a positive wave appearing about 100 msec after the patternshift stimulus is applied. The most clinically significant measurements are absolute P100 latency (the time between stimulus application and peaking of the P100 wave) and the difference between the P100 latencies of each eye. Because many physical and technical factors affect P100 latency, normal results vary greatly among laboratories and patients.
Somatosensory evolred potentials
Waveforms obtained vary, depending on locations of the stimulating and recording electrodes. The positive and negative peaks are labeled in sequence, based on normal time of appearance. For example, N19 is a negative peak normally recorded 19 msec after application of the stimulus. Each wave peak arises from a discrete location: N19 is generated mainly from the thalamus, P22 from the parietal sensory cortex, and so on. Interwave latencies (time between waves), rather than absolute latencies, are used as a basis for clinical interpretation. Latency differences between sides are significant.
Information from evoked potential studies is useful but insufficient to confirm a specific diagnosis. Test data must be interpreted in light of clinical information.
Visual evolred potentials
Generally, abnormal (extended) P100 latencies confined to one eye indicate a visual pathway lesion anterior to the optic chiasm. A lesion posterior to the optic chiasm usually doesn't produce abnormal P100 latencies. Because each eye projects to both occipital lobes, the unaffected pathway transmits sufficient impulses to produce a normal latency response. Bilateral abnormal P100 latencies have been found in patients with multiple sclerosis, optic neuritis,retinopathies, amblyopia (although abnormal latencies don't correlate well with impaired visual acuity), spinocerebellar degeneration, adrenoleukodystrophy, sarcoidosis, Parkinson's disease, and Huntington's disease.
Somatosensory evoked potentials
Because somatosensory evoked potential components are assumed to be linked in series, an abnormal interwave latency indicates a conduction defect between the generators of the two peaks involved. This often allows precise location of a neurologic lesion. Abnormal upperlimb interwave latencies may indicate cervical spondylosis, intracerebral lesions, or sensorimotor neuropathies. Abnormalities in the lower limb demonstrate peripheral nerve and root lesions, such as those in Guillain-Barre syndrome, compressive myelopathies, multiple sclerosis, transverse myelitis, and traumatic spinal cord injury.
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