Short Latency Somatosensory Evoked Potentials Following Median Nerve Stimulation in Man

In 20 normal subjects, short latency components of the somatosensory evoked potentials (SEPs) were recorded simultaneously from the scalp and cervical electrodes after stimulation of the median nerve at the wrist. Using a knee reference, 4 positive peaks, P9, P11, P13 and P14, were identified in the scalp recorded potentials, whereas the cervical potentials consisted of the initial positive peak, Pc9, and 3 subsequent negative peaks, N11, N13 and N14. The initial component was negative over the shoulder but otherwise distributed widely as a positive field from the neck to the scalp. Its latency was slightly less than that of the nerve potential recorded at Erb\u27s point. The second component was best recorded at the low cervical area as a negative peak, increasing in amplitude and decreasing in latency laterally towards the site of the stimulus. Its latency was nearly the same as an estimated nerve conduction time from the wrist to the spinal cord. The third component was most consistently recorded as a negative peak at mid and low cervical electrodes, progressively decreasing in amplitude toward the high cervical region. It was often difficult to detect over the scalp. The last component was best recorded as a positive peak at the scalp and nasopharyngeal electrode. This peak was equivocal when recorded from the cervical region unless an ear reference was used. The latencies and field distributions suggest that the cervical potentials represent a negative field near the generator site and that the scalp recorded potentials are primarily attributable to an approaching field of positivity from the same source. We propose that the possible generator sites of the short latency SEPs are: (1) a distal portion of the brachial plexus (Pc9 and P9), (2) entry to the spinal cord (N11 and P11), (3) cervical cord (N13 and P13), and (4) brain stem or cerebellum (N14 and P14)

Distal Slowing of Motor Nerve Conduction Velocity in Diabetic Polyneuropathy

Using the latencies of M response and F wave, motor nerve conduction was assessed along the entire course of the nerve from the spinal cord to the muscle in 102 diabetics and 74 control patients. In diabetics, latencies were increased and conduction velocity decreased over both proximal and distal segments. However, the latency ratio of the proximal to distal segment (F ratio) was slightly but significantly smaller in diabetics (mean ± SD:0.93 ± 0.14, 1.35 ± 0.20, 1.09 ± 0.19 and 1.02 ± 0.19 for median, ulnar, tibial and peroneal nerves, respectively) than in control patients (1.05 ± 0.09, 1.41 ± 0.12, 1.17 ± 0.13 and 1.08 ± 0.12). These findings suggest that motor conduction abnormalities in diabetic polyneuropathy are diffuse over the total length of the nerve, but more intense in the distal than proximal segment. An additional finding in diabetics was that both proximal and distal segments were more frequently affected in the lower than in the upper extremities