Purpose: Median nerve somatosensory evoked responses (MnSSER) are frequently used clinically and scientifically. However, the stimulus-response interrelationship has not been documented for the midlatency components. Therefore we investigated the spinal and cortical midlatency MnSSER, to document the effect of different stimulus intensities.
Methods: Spinal and midlatency MnSSER were recorded following percutaneous electrical stimulation of the median nerve at the wrist in 32 neurological healthy females. Three different stimulation intensities were applied: threefold sensory threshold (INT 1), motor plus sensory threshold (INT 2), individual level of tolerance (INT 3).
Statistics: Multivariate analyses of variance (independent variable: stimulation intensity).
Results: Three negative and two positive midlatency components were identified after median nerve stimulation. At INT 1 the early waves N20, P25 and N35 were identified in all of the patients. In contrast, the component P45 was identifiable in 23% of the sample at the lowest intensity level and N55, respectively, in 32%. At INT 2 the spinal and the midlatency cortical amplitudes (except amplitude N35P45) were significantly increased (p < 0.014). At INT 3 the amplitudes of the primary cortical complex did not increase further, but the components > 35 ms became identifiable in the whole sample except in one subject. In contrast, there was no change in the MnSSER latencies. The intensity was comfortable for all subjects.
Conclusion: For monitoring the midlatency MnSSER components > 35 ms the authors suggest a stimulation intensity close to individual tolerance level for optimal cortical responses.