Proximal motor conduction evaluated by transcranial magnetic stimulation in acquired inflammatory demyelinating neuropathies

Objectives: To evaluate conduction abnormalities in the proximal motor nerv e in patients with acquired inflammatory demyelinating neuropathies by tran scranial magnetic stimulation (TMS). Methods: TMS intensity and background voluntary contraction (BVC) to evoke maximal size of motor evoked potential (MEP) in hand muscle were investigat ed in 24 normal subjects. Effect of experimentally induced conduction block by injecting local anesthetics in the peripheral nerve on MEP size was als o studied in two normal subjects. In 22 patients with inflammatory demyelin ating neuropathies, maximal MEPs were recorded in the deteriorating and rec overy stages of the illness. Results: In normal subjects, the MEP became maximal with 30-50% of maximal BVC and at more than 80% the maximal stimulator output of the 2.0 T circula r coil. The change in MEP size well reflected the degree of conduction bloc k induced by local anesthetics. Findings for patients suggested conduction abnormalities proximal to axilla in 9 patients, and that the abnormal reduc tion of Erb CMAP was the result of submaximal stimulation, not true conduct ion block, in 3 patients. The increase in MEP/wrist CMAP ratio was better c orrelated with improvement in muscle strength than with change in the axill a or Erb CMAP/wrist CMAP ratio, Conclusions: Problems such as conduction abnormalities in the motor tract o f the central nervous system could not fully be excluded, but we consider t hat maximal MEP size can be used to predict proximal motor nerve conduction abnormalities. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.