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Transcranial magnetic stimulation of the cerebral cortex was used to study motor system function in 31 cirrhotics (29 post-necrotic and 2 cryptogenic) with and without hepatic encephalopathy (HE). The results were compared with those of 14 healthy subjects matched for age. A significant increase of central motor conduction time, a significant raising of the motor evoked potential (MEP) threshold at rest and a significant reduction of the MEP/muscle action potential (MAP) amplitude ratio were found only in patients with chronic stable (12 patients) and recurrent (9 patients) HE. Vice versa, a significant shortening of the central silent period was observed in all 31 cirrhotic patients. The peripheral silent period was normal in all instances. These results indicate that the damage to the cortico-spinal pathways is related to the progression of cirrhosis to HE, and that cirrhotic patients present a dysfunction of the inhibitory motor mechanisms before HE is clinically manifest.

We studied orthodromic sensory conduction velocity along the distal and proximal segments of the median and ulnar nerves by tactile stimulation of the distal phalanx of the 3rd and 5th digits in 44 healthy subjects divided into 2 age groups: from 16 to 35 years and from 63 to 81 years. In the same nerves, we used selective electrical stimulation of the corresponding digital nerves to obtain sensory potentials. In both groups, responses to tactile stimuli had a longer latency and smaller amplitude than those to electrical stimulation, and they were distributed in a series of 6-7 main deflections, apparently regardless of whether the recording site was distal or proximal. Moreover, irrespective of the nerve and of subject age, conduction velocity along both the digit-wrist and the wrist-elbow nerve segments was significantly slower with tactile stimuli than with electrical stimuli. However, independently of the stimulus used, conduction velocity along the proximal nerve segment was significantly faster than that measured along the digit-wrist nerve segment. In both the median and ulnar nerves, maximum potential amplitude, cumulative area and conduction velocity were significantly reduced in the older age group. This finding could reflect the smaller number of Meissner's corpuscles in older subjects, and the loss of large nerve fibres in individuals over 60.

Magnetic stimulation has not been routinely used for studies of peripheral nerve conduction primarily because of uncertainty about the location of the stimulation site. We performed several experiments to locate the site of nerve stimulation. Uniform latency shifts, similar to those that can be obtained during electrical stimulation, were observed when a magnetic coil was moved along the median nerve in the region of the elbow, thereby ensuring that the properties of the nerve and surrounding volume conductor were uniform. By evoking muscle responses both electrically and magnetically and matching their latencies, amplitudes and shapes, the site of stimulation was determined to be 3.0 +/- 0.5 cm from the center of an 8-shaped coil toward the coil handle. When the polarity of the current was reversed by rotating the coil, the latency of the evoked response shifted by 0.65 +/- 0.05 msec, which implies that the site of stimulation was displaced 4.1 +/- 0.5 cm. Additional evidence of cathode- and anode-like behavior during magnetic stimulation comes from observations of preferential activation of motor responses over H-reflexes with stimulation of a distal site, and of preferential activation of H-reflexes over motor responses with stimulation of a proximal site. Analogous behavior is observed with electrical stimulation. These experiments were motivated by, and are qualitatively consistent with, a mathematical model of magnetic stimulation of an axon.