Reliability of Median Nerve Somatosensory Evoked Potentials Using the Nicolet Pathfinder II

摘要

Evoked potentials (EPs) are electrical signals generated by the central nervous system (CNS) in response to a specific type of auditory, visual, or somatosensory stimulus (5). These electrical potentials are much smaller than electroencephalographic (EEG) records, which record all brain electrical activity, and as such, they are embedded within EEGs. The use of signal averaging systems such as the Nicolet Pathfinder II (Nicolet Biomedical Inc., Madison, WI) allows for EPs to be isolated from the background EEG. This method reduces the amplitude of the EEG component that is not related to the stimulus and thereby enhances the features of the response that is time-locked to the stimulus (23). Somatosensory evoked potentials (SEPs) are obtained by placing receiving electrodes (to measure electrical activity of the brain and spinal cord) on specific anatomical locations to calculate responsiveness of the CNS to an applied electrical stimulus. The CNS EPs can originate in the spinal cord, brainstem, or cerebral areas of the brain (10). One of the most often used SEPs is obtained by stimulating the median nerve. Median nerve SEPs have a high signal-to-noise ratio and reliable latency responses, which allows for their use in clinical evaluations of ONS integrity (23). For the median nerve SEP, there are several commonly labeled peak latencies (Ng, N13, N19 or N20, and P22). With electrode montages that are commonly used for SEPs, convention has that upward deflections in the electrical potentials are regarded as negative and downward defections are regarded as positive (20). The general time from stimulus to peak deflection is referred to as the expected latency. For example, N9 means that an expected negative (up) deflection should occur somewhere around 9 msec after the stimulus to the nerve as been applied.