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Evoked potentials (EPs) are electrical signals generated by the central nervous system in response to a stimulus. EP tests are used to diagnose neurological diseases and monitor surgeries. Somatosensory evoked potentials (SEPs) are used to test the somatosensory pathways, and clinical trials use electrical stimulation of peripheral nerves. Signal averaging is required due to low-amplitude potentials. SEPs are still valuable diagnostic tests and increasingly important in the operating room.
An evoked potential (EP) is an electrical signal that is recorded or generated by the central nervous system in response to the presentation of a stimulus. Evoked potential tests are performed on patients who may have sensory deficits and are unable to recognize a stimulus. Evoked potential signals can be recorded from a variety of areas, including the cerebral cortex, spinal cord, and peripheral nerves. This is distinct from spontaneous potentials which are detected by electroencephalography (EEG) or electromyography (EMG). Visual, auditory, and somatosensory stimuli are commonly used for evoked potential studies.
The individual performing a PE test locates specific areas on the patient’s head to place the electrodes. For a visual evoked potential, the patient focuses on a screen that displays a checkerboard pattern, with one eye covered by a patch. Auditory evoked potentials involve headphones being used to deliver a series of clicks to one ear, while a static sound is played in the other ear. For somatosensory PEs, electric shocks are delivered to the arm or leg, causing some tingling.
Somatosensory evoked potentials (SEPs) deal with waves that are reflected by the activation of neural structures along somatosensory pathways. Clinical trials generally use electrical stimulation of peripheral nerves to elicit stronger responses. Areas commonly tested in clinical trials for somatosensory evoked potentials include the posterior tibial nerve at the ankle, the median nerve at the wrist, and the common peroneal nerve at the knee.
SEPs are used in the diagnosis of neurological diseases in patients. They are also used to monitor surgeries involving risks to the somatosensory pathways. Although abnormal SEP results can result from some form of peripheral nerve dysfunction, patients cannot force abnormal results. Abnormal SEP results suggest a dysfunction within the somatosensory pathways.
Evoked potential amplitudes tend to be much smaller than on EEGs or EMGs. Evoked potential amplitudes tend to be less than a few microvolts, compared to thousands of microvolts for EEG and close to one volt for ECG. Signal averaging is then required to resolve these low-amplitude potentials against EEG, ECG, EMG, and surrounding sounds. Because background noise is random, it can be averaged by the mathematical mean of repeated responses.
Clinical settings involving somatosensory evoked potentials have been greatly influenced by the development of sophisticated neuroradiological imaging. Although fewer diagnostic SEP studies have been performed recently, SEPs are still valuable diagnostic tests in many clinical situations. SEPs are widely used as research tools for sensory physiology. Furthermore, somatosensory evoked potentials are becoming increasingly important in the operating room.
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