Evoked potential
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In neurophysiology, an evoked potential (or "evoked response") is an electrical potential recorded from a human or animal subject following presentation of a stimulus, as distinct from spontaneous potentials such as electroencephalograms or electromyograms. Evoked potential amplitudes tend to be low, ranging from less than a microvolt to several microvolts, compared to tens of microvolts for EEG, millivolts for EMG, and often close to a volt for EKG. In order to resolve these low-amplitude potentials against the background of ongoing EEG, EKG, EMG and other biological signals and ambient noise, signal averaging is usually required. The signal is time-locked to the stimulus and most of the noise occurs randomly, allowing the noise to be averaged out with averaging of repeated responses.
Signals can be recorded from cerebral cortex, brainstem, spinal cord and peripheral nerves. Usually the term "evoked potential" is reserved for responses involving either recording from, or stimulation of, central nervous system structures. Thus evoked CMAP (compound motor action potentials) or SNAP (sensory nerve action potentials) as used in NCV (nerve conduction studies) are generally not thought of as evoked potentials, though they do meet the above definition.
Sensory evoked potentials have been widely used in clinical medicine since the 1970s, including SSEP (somatosensory), VEP (visual) and BSEP (brainstem auditory) evoked potentials. These are also abbreviated as SSER, etc., and the BSEP is known as BSER, BAER, ABR, etc. SSEP are elicited by an electrical shock to a peripheral nerve as with NCV; VEP by an alternating checkerboard stimulus (PSVEP, pattern shift visual evoked potential) or a flash (strobe light or LED), and auditory evoked potentials by a click or tone, usually presented through earphones. Gustatory, olfactory, and even nociceptive (pain-evoked) potentials also exist, as do cognitive (event-related) potentials, though none of these have found widespread clinical application. In addition, transcranial motor evoked potentials, recorded either epidurally or from muscles following electrical or magnetic transcranial stimulation of motor cortex, have become increasingly important in recent years.
BSEP originally were very useful for detection of brainstem tumors and acoustic neuromas; when clinical averaging systems were first developed, the BSEP was more sensitive than early CT scanners. After several generations of progress in clinical imaging, however, this application is of marginal usefulness. BSEP is still used as a method of screening hearing in neonates, who can not be effectively tested by behavioral audiometry (though evoked oto-acoustic emissions recordings may eventually supplant BSEP in this application). BSEP is also used for intraoperative neurophysiologic monitoring in skull base surgery such as acoustic neuroma resection, where the auditory nerve is at risk.
All three of the commonly used sensory modalities are useful in clinical diagnosis and monitoring of MS (multiple sclerosis); this is the main application for PSVEP. SSEP is also useful in coma.
SSEP and MEP (motor evoked potentials), along with EEG and EMG, are extremely useful in intraoperative neurophysiologic monitoring for a wide variety of neurosurgical, otologic, spinal and other surgical procedures. See IONM (intraoperative neurophysiologic monitoring), or surgical neurophysiology.
Evoked potentials are usually done on one sensory mode at a time. Visual evoked potentials may be induced by a flashing light, auditory evoked potentials by the onset of a sound, and somatosensory evoked potentials by a solenoid pulsing against a person's skin.