Effects of botulinum toxin type A on intracortical inhibition in patients with dystonia

To find out whether botulinum toxin alters the excitability of cortical mot or areas, we studied intracortical inhibition with transcranial magnetic st imulation in patients with upper limb dystonia before, 1 month after, and 3 months after the injection of botulinum toxin type A in the affected muscl es. Eleven normal subjects and 12 patients with dystonia involving the uppe r limbs (7 with generalized dystonia, 2 with segmental dystonia, and 3 with focal dystonia) were studied. Patients were assessed clinically with the D ystonia Movement Scale. Paired magnetic stimuli were delivered by two Magst im 200 magnetic stimulators connected through a Bistim module to a figure-o f-eight coil placed over the motor area of the forearm muscles. Paired stim ulation was given at rest. A subthreshold (80% of motor threshold) conditio ning stimulus was delivered 3 and 5 msec before the suprathreshold (120% of motor threshold) test stimulus. Electromyographic signals were recorded ov er the flexor or extensor muscles of the forearm on the affected side. We m easured the amplitude of the test motor evoked potential (expressed as a pe rcentage of the unconditioned motor evoked potential). All results were com pared using ANOVA. In all patients, a botulinum toxin type A injection (50- 100 mouse units) reduced dystonic movements in the arm. In normal subjects, electromyographic recordings showed significant inhibition of the test res ponse. Before botulinum toxin injection, patients had less test response in hibition than normal subjects. One month after injection, patients had test response inhibition similar to that of normal subjects. At 3 months after injection, they again had less inhibition than normal subjects or patients at 1 month after injection. In conclusion, our data suggest that botulinum toxin can transiently alter the excitability of the cortical motor areas by reorganizing the inhibitory and excitatory intracortical circuits. The cor tical changes probably originate through peripheral mechanisms.