Triad-conditioning transcranial magnetic stimulation in Parkinson's disease

Background: Transcranial magnetic stimulation (TMS) has been used to reveal excitability changes of the primary motor cortex (M1) in Parkinson's disease (PD). Abnormal rhythmic neural activities are considered to play pathophysiological roles in the motor symptoms of PD. The cortical responses to external rhythmic stimulation have not been studied in PD. We recently reported a new method of triad-conditioning TMS to detect the excitability changes after rhythmic conditioning stimuli, which induce facilitation by 40-Hz stimulation in healthy volunteers.

Objective: We applied a triad-conditioning TMS to PD patients to reveal the motor cortical response characteristics to rhythmic TMS.

Methods: The subjects included 13 PD patients and 14 healthy volunteers. Three conditioning stimuli over M1 at an intensity of 110% active motor threshold preceded the test TMS at various inter-stimulus intervals corresponding to 10-200 Hz.

Results: The triad-conditioning TMS at 40 Hz induced no MEP enhancement in PD patients in either the On or Off state, in contrast to the facilitation observed in the normal subjects. Triad-conditioning TMS at 20-33 Hz in the beta frequency elicited significant MEP suppression in PD patients. The amount of suppression at 20 Hz positively correlated with the UPDRS III score.

Conclusion: We observed abnormal M1 responses to rhythmic TMS in PD. The suppression induced by beta frequency stimulation and no facilitation by 40-Hz stimulation may be related to abnormal beta and gamma band activities within the cortical-basal ganglia network in PD patients. The motor cortical response to rhythmic TMS may be an additional method to detect physiological changes in humans.