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

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Objectives: To evaluate conduction abnormalities in the proximal motor nerve in patients with acquired inflammatory demyelinating neuropathies by transcranial magnetic stimulation (TMS). Methods: TMS intensity and background voluntary contraction (BVC) to evoke maximal size of motor evoked potential (MEP) in hand muscle were investigated in 24 normal subjects. Effect of experimentally induced conduction block by injecting local anesthetics in the peripheral nerve on MEP size was also studied in two normal subjects. In 22 patients with inflammatory demyelinating neuropathies, maximal MEPs were recorded in the deteriorating and recovery 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 circular coil. The change in MEP size well reflected the degree of conduction block induced by local anesthetics. Findings for patients suggested conduction abnormalities proximal to axilla in 9 patients, and that the abnormal reduction of Erb CMAP was the result of submaximal stimulation, not true conduction block, in 3 patients. The increase in MEP/wrist CMAP ratio was better correlated with improvement in muscle strength than with change in the axilla or Erb CMAP/wrist CMAP ratio. Conclusions: Problems such as conduction abnormalities in the motor tract of the central nervous system could not fully be excluded, but we consider that maximal MEP size can be used to predict proximal motor nerve conduction abnormalities.