Muscle & nerve
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We report a case of tarsal tunnel syndrome (TTS) with focal conduction block across the tarsal tunnel (TT). A 46-year-old woman had pain in the left foot, sensory loss on the plantar surface, and positive Tinel sign over the TT. ⋯ Conduction block has rarely been reported in TTS. In this case, conduction block provides evidence for focal demyelination as the primary pathological process in TTS.
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High-frequency alternating current (HFAC) applied to a peripheral nerve can reversibly block skeletal muscle contractions. We evaluated the ability of HFAC delivered via intrafascicular electrodes to selectively block activation of targeted muscles without affecting activation of other muscles. Utah slanted electrode arrays (USEAs) were implanted into the sciatic nerves of five cats, and HFAC was delivered to individual USEA electrodes. ⋯ In each animal, activity evoked in targeted muscles by nerve cuff stimulation could be selectively abolished by HFAC delivered via individual USEA electrodes. Two mechanisms of blockade were evoked: selective neuromuscular blocks were achieved with 500-8000-HZ HFAC, and selective nerve conduction block was achieved in one animal using 16-kHZ HFAC. These results show that intrafascicular HFAC can be used to block selected muscles independent of activation of other muscles.
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To determine the conversion factor (CF) of the sural nerve the correlation between the maximum nerve conduction velocity (NCV) and the diameter of the largest fibers was studied in 30 patients suspected of having neuropathy. Sensory nerve action potentials were obtained by on-nerve needle nerve conduction study using needle electrodes placed on the exposed sural nerve during biopsy. ⋯ The CF was helpful in differentiating between demyelinating and axonal neuropathies. We propose that a 36% decrease from the mean value of NCV is a reasonable criterion for demyelination of the nerve.