title: Pathways mediating abnormal intracortical inhibition in Parkinson's disease.
abstract: Previous studies have used paired-pulse transcranial magnetic stimulation to show that short-interval intracortical inhibition (SICI) is reduced in patients with Parkinson's disease (PD). This study examined whether reduced SICI in PD is caused by an increase in the threshold of inhibitory pathways or a reduction in the threshold of excitatory pathways. Motor-evoked potentials were recorded from a hand muscle in 12 patients with PD (7 patients were tested off and on antiparkinsonian medications) and 12 control subjects. SICI was tested at seven conditioning stimulus intensities (CSIs; 40-100% of resting motor threshold) and at interstimulus intervals (ISIs) of 2, 3, and 4 milliseconds. No differences were found between groups in resting or active motor threshold, SICI threshold, or the extent of SICI at CSIs at or below 80% of resting motor threshold. Significant differences between groups were observed at CSIs of 90% and 100% with an ISI of 3 milliseconds. Antiparkinsonian medication had no effect on SICI. These findings show that the low threshold inhibitory pathways mediating SICI are normal in PD. The suppression of SICI observed at higher CSIs suggests that the threshold of intracortical facilitatory pathways is decreased in PD.
authors.affiliation: Department of Physical Therapy and Human Movement Sciences, Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611, USA. c-mackinnon@northwestern.edu
authors.forename: Colum D
authors.initials: CD
authors.lastname: MacKinnon
authors.forename: Emily A
authors.initials: EA
authors.lastname: Gilley
authors.forename: Annette
authors.initials: A
authors.lastname: Weis-McNulty
authors.forename: Tanya
authors.initials: T
authors.lastname: Simuni
mesh_terms: D000368:Aged; D016022:Case-Control Studies; D004056:Differential Threshold / Q000528:radiation effects; D004558:Electric Stimulation / Q000379:methods*; D019054:Evoked Potentials, Motor / Q000502:physiology / Q000528:radiation effects; D005260:Female; D006801:Humans; D008280:Magnetics; D008297:Male; D008875:Middle Aged; D009433:Neural Inhibition / Q000502:physiology* / Q000528:radiation effects; D009434:Neural Pathways / Q000503:physiopathology* / Q000528:radiation effects; D010300:Parkinson Disease / Q000503:physiopathology / Q000628:therapy*; D011930:Reaction Time / Q000502:physiology / Q000528:radiation effects; D013997:Time Factors