Evoked potentials

Evoked potentials, also known as evoked responses, are electrical signals elicited by stimulation of sensory organs or nervous tissue, as opposed to spontaneous potentials such as EEG or EMG. Due to their low amplitude, evoked potentials usually require signal averaging to improve the signal-to-noise ratio.

In theory, or in research applications, evoked potentials can also be elicited by thermal cutaneous stimulation, olfactory and gustatory stimuli, and vestibular stimulation, among other things.

Sensory evoked potentials
Sensory evoked potentials (SEP or SSEP) are recorded from the central nervous system following stimulation of sense organs (for example, visual evoked potentials elicited by a flashing light or changing pattern on a monitor; auditory evoked potentials by a click or tone stimulus presented through earphones) or by electrical stimulation of a sensory or mixed nerve. They have been widely used in clinical diagnostic medicine since the 1970's, and also in intraoperative neurophysiology monitoring (IONM), also known as surgical neurophysiology.

There are three kinds of evoked potentials in widespread clinical use since the 1970's: auditory evoked potentials, usually recorded from the scalp but originating at brainstem level (ABR, BAER, BSER, BAEP, BSEP); visual evoked potentials, and somatosensory evoked potentials, which are elicited by electrical stimulation of peripheral nerve. See the articles on each of these modalities.

Intraoperative monitoring
Somatosensory evoked potentials provide monitoring for the dorsal columns, MEP for the ventral cord, specifically the lateral corticospinal tract. Since the ventral and dorsal spinal cord have separate blood supply with very limited collateral flow, an anterior cord syndrome (paralysis or paresis with some preserved sensory function) is a possible surgical sequela, so it is important to have monitoring specific to the motor tracts as well as dorsal column monitoring.

Magnetic stimulation is generally regarded as unsuitable for intraoperative monitoring because it is more sensitive to anesthesia. Electrical stimulation is too painful for clinical use in awake patients. The two modalities are thus complementary, electrical stimulation being the choice for intraoperative monitoring, and magnetic for clinical applications. bindi.

Motor evoked potentials
Motor evoked potentials (MEP) are recorded from muscles following direct stimulation of exposed motor cortex, or transcranial stimulation of motor cortex, either magnetic or electrical. Transcranial magnetic MEP (TCmMEP) potentially offer clinical diagnostic applications, but remain investigational as of 2005; transcranial electrical MEP (TCeMEP) has been in widespread use for several years for intraoperative monitoring of pyramidal tract functional integrity.

During the 1990's there were attempts to monitor "motor evoked potentials", including "neurogenic motor evoked potentials" recorded from peripheral nerves, following direct electrical stimulation of the spinal cord. It has become clear that these "motor" potentials were almost entirely elicited by antidromic stimulation of sensory tracts-- even when the recording was from muscles (antidromic sensory tract stimulation triggers myogenic responses through synapses at the root entry level). TCMEP, whether electrical or magnetic, is the most practical way to ensure pure motor responses, since stimulation of sensory cortex cannot result in descending impulses beyond the first synapse (synapses cannot be backfired).

Potentiel évoqué