Objective: To evaluate if the intrinsic oscillations within the primary motor cortex (M1) at rest are entrained at the same frequencies of trains of transcranial magnetic stimulation (TMS) in focal hand dystonia (FHD) patients as in healthy controls.

Background: Trains of triple magnetic pulses delivered at sub-threshold intensities can entrain cortical oscillations in the gamma range and enhance the excitability of the cortico-spinal pathways tested with a fourth supra-threshold pulse (Hanajima et al, 2009). This phenomenon was not found in cortical myoclonus or in Parkinson’s disease (Hanajima et al, 2011, 2014).

Methods: We recruited only right-handed FHD patients and matched controls; recruitment is not yet complete for the whole protocol. We recorded the patients at least 3months after their last botulinum toxin injection. All subjects had the M1 excitability profile quantified with an input-output curve, and the resting and active (AMT) motor thresholds (Awiszus, 2003). Four magnetic pulses were delivered over each M1 in trains with different inter-stimulus intervals: 10, 15, 20, 25, 30, 40, 50, 100ms (corresponding to 10–100 Hz). The first 3 conditioning pulses had an intensity of 110% AMT; the fourth pulse had the intensity of the S50 point of the respective input-output curve. The amplitudes of conditioned motor evoked potentials were compared to those evoked by the test stimulus alone.

Results: We found facilitations (p<0.01) of the cortical excitability at 10 and 15ms in controls, but not the previously described facilitation at 25ms (40Hz). There were no differences between the right and left M1 or between the patients and controls in this interim analysis. There was a trend for higher excitability increase in patients in the affected M1, when the conditioning triad was delivered in the beta range (20Hz). Also, there was a trend for lower excitability increase in the non-affected M1 of patients, when the conditioning triad was delivered at 66 and 100Hz.

Conclusions: Our results suggest that in FHD, the mechanisms responsible for cortical oscillation entrainment are not different from normal. A tendency for higher responsiveness of the affected M1 to the beta-band stimulation might be a sign of dysfunction of the inhibitory interneurons in layers II and III unmasking the beta rhythm specific of layer V neurons. Also, the triad conditioning appears to be highly sensitive to the amplitude of the test motor-evoked potential.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/triad-conditioning-transcranial-magnetic-stimulation-in-focal-hand-dystonia/