Does transcranial direct current stimulation during writing alleviate upper limb freezing in people with Parkinson's disease? A pilot study

Transcranial direct current stimulation (tDCS) over the primary motor cortex (M1) can boost motor performance in Parkinson's disease (PD) when it is applied at rest. However, the potential supplementary therapeutic effect of the concurrent application of tDCS during the training of motor tasks is largely unknown. The present study examined the effects of tDCS on upper limb motor blocks during a freezing-provoking writing task (the funnel task) requiring up- and downstroke movements at alternating amplitudes. Ten PD patients and 10 age-matched controls underwent two sessions of writing combined with 20 min of anodal or sham tDCS on the left M1 in a randomized cross-over design. The primary outcome was the number of upper limb freezing episodes during five trials of the funnel task on a touch-sensitive tablet. PD patients showed a significant reduction in freezing episodes during tDCS compared to sham. No effects of tDCS were found for the amplitude, variability and speed of the strokes outside the freezing episodes. However, patients who reported freezing episodes in daily life (N = 6) showed a beneficial effect of tDCS on stroke characteristics. These results indicate a subgroup-dependent variability in response to non-invasive brain stimulation applied during the performance of motor tasks in PD. This warrants future studies to examine tDCS as an adjuvant tool for training programs aimed to reduce motor deficits related to freezing.