The optimal method to determine the stimulus intensity for median nerve somatosensory evoked potentials

In order to find the best method to determine the stimulus intensity for median nerve somatosensory evoked potentials (SEPs), we tried to activate muscle or cutaneous branch as preferentially as possible using ordinary surface stimulation. We minutely moved the stimulating electrode at the wrist in normal subjects, and changed the stimulus intensity stepwise at each site. We evaluated the correlation between the amplitudes of the SEP components and peripheral parameters such as the relative intensities to the motor threshold (rMT) or the sensory threshold (rST), and the amplitude of the sensory nerve action potential recorded over the index finger (SNAP2) or that of the compound muscle action potential (CMAP). The sensory parameters (rST and SNAP2) showed better correlation with SEP amplitude than the motor parameters (rMT and CMAP). In an extreme case, stimulation 40% over the motor threshold elicited no N9 response and only a small N20. Adjusting the stimulus intensity at slightly above the motor threshold, as recommended by most guidelines, in such a case would cause an erroneous result. We propose the stimulus intensity resulting in SNAP2 amplitude of 80% of its maximum as the optimal method because it gave almost saturated SEP responses consistently.