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Measurement of Evoked Responses (Potentials)

By Michael C. Levin, MD, Saskatchewan Multiple Sclerosis Clinical Research Chair and Professor of Neurology and Anatomy-Cell Biology; Adjunct Professor of Neurology, College of Medicine, University of Saskatchewan; University of Tennessee Health Science Center

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Visual, auditory, or tactile stimuli are used to activate corresponding areas of the cerebral cortex, resulting in focal cortical electrical activity. Ordinarily, these small potentials are lost in EEG background noise, but computer processing cancels out the noise to reveal a waveform. Latency, duration, and amplitude of the evoked responses indicate whether the tested sensory pathway is intact.

Evoked responses are particularly useful for the following:

  • Detecting clinically inapparent deficits in a demyelinating disorder

  • Appraising sensory systems in infants

  • Substantiating deficits suspected to be histrionic

  • Following the subclinical course of disease

For example, visual evoked responses may detect unsuspected optic nerve damage caused by multiple sclerosis.

When integrity of the brain stem is in question, brain stem auditory evoked responses is an objective test.

Somatosensory evoked responses may pinpoint the physiologic disturbance when a structural disorder (eg, metastatic carcinoma that invades the plexus and spinal cord) affects multiple levels of the neuraxis.

Somatosensory evoked responses can also help predict the prognosis of patients in a coma, particularly those with hypothermia, when the usual bedside indicators are unclear.