CORTICOMOTOR THRESHOLD TO MAGNETIC STIMULATION - NORMAL VALUES AND REPEATABILITY

The threshold for corticomotor excitation by magnetic stimuli is a cli nically important parameter in the assessment of central motor conduct ion. Hitherto, arbitrary criteria have been adopted to define threshol d and there are few published data using a figure of eight coil. A new method is described in which the highest stimulus intensity evoking r esponses in the relaxed first dorsal interosseous muscle with a probab ility of zero [lower threshold (LT)] and the lowest intensity evoking responses with a probability of one [upper threshold (UT)] have been d etermined. The method, which utilizes a coil located and optimally ori ented over hand motor cortex, minimizes the number of stimuli needed. Both LT and UT are normally distributed and are independent of age, ge nder, and hemisphere. In 102 healthy hands of 55 control subjects 23-8 4 years of age, mean +/- SD UT was 46.6 +/- 9.4% and LT was 38.0 +/- 8 .6% maximum stimulator output. Repeatability estimates indicated an ab solute change of more than 13% or 11% in UT or LT respectively in the same individual over 1-3 months was significant at the 5% level. (C) 1 997 John Wiley & Sons, Inc.