CORTICAL INHIBITION IN PARKINSONS-DISEASE - A STUDY WITH PAIRED MAGNETIC STIMULATION

The activity of motor cortical inhibitory circuits was studied with pa ired transcranial magnetic stimuli in 16 patients with Parkinson's dis ease 'off' therapy, jive patients 'off' and 'on' therapy, and 11 norma l subjects. paired stimuli were delivered at short (3-20 ms) as well a s long (100-250 ms) intervals during slight voluntary contraction. The intensity of the conditioning stimulus was subthreshold (80%) at shor t, and suprathreshold (150%) at long intervals. In addition, the silen t period following a single magnetic shock given at 150% of threshold was measured With short interstimulus intervals, no significant differ ence between patients and normal subjects could be detected With long interstimulus intervals, the test response was significantly move inhi bited in patients than in normal subjects. Although the cortical silen t period was found to be slightly shorter the recovery of motor evoked potentials was incomplete in patients with Parkinson's disease. This alteration could be partially reverted by dopaminergic therapy. In con clusion, the responsiveness of motor cortices to suprathreshold magnet ic stimuli delivered after the end of the silent period is impaired in patients with Parkinson's disease, possibly due to prolonged activity in intracortical inhibitory circuits. The positive effect of L-dopa s uggests that dopaminergic modulation of cortical activity, most probab ly at basal ganglia level, is involved in the pathogenesis of this phe nomenon.