Measurement of Evoked Responses (Potentials)
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.