Abnormalities of sympathetic vasomotor tone in distal axonal neuropathy.

Microneurographic studies have shown that sympathetic vasomotor nerve activity is observed less frequently in patients with peripheral neuropathy than in controls, but when detected its pattern and the baroreflex latencies appear to be normal. Vasomotor nerve function was examined in chloralose-urethane anaesthetised dogs by comparing the discharge of renal sympathetic nerves in control animals and animals with acrylamide neuropathy. There was a normal pattern of pulse related inhibition of sympathetic nerve activity as well as normal amplitude spontaneous compound nerve action potentials in the animals with neuropathy. When vasomotor tone was altered abruptly by raising pressure in a bilateral isolated carotid sinus preparation, the baroreflex latencies were normal in the affected animals. However, when carotid sinus pressure was kept constant, pulse-related sympathetic inhibition, normally mediated by vagal cardiopulmonary baroreceptors, was absent in animals with neuropathy. It is likely that the vagal nerve fibres to cardiopulmonary baroreceptors as well as the receptors themselves are damaged while shorter carotid sinus nerve fibres are relatively spared in axonal neuropathies. As long as these shorter nerves are intact patients with axonal neuropathy should have relatively normal baroreflexes and normal sympathetic vasomotor tone when measured microneurographically.