Transcranial direct current stimulation combined with physical or cognitive training in people with Parkinson’s disease: a systematic review

Abstract

Background: Pharmacologic therapy is the primary treatment used to manage Parkinson's disease (PD) symptoms. However, it becomes less effective with time and some symptoms do not respond to medication. Complementary interventions are therefore required for PD. Recent studies have implemented transcranial direct current stimulation (tDCS) in combination with other modalities of interventions, such as physical and cognitive training. Although the combination of tDCS with physical and cognitive training seems promising, the existing studies present mixed results. Therefore, a systematic review of the literature is necessary. Aims: This systematic review aims to (i) assess the clinical effects of tDCS when applied in combination with physical or cognitive therapies in people with PD and; (ii) analyze how specific details of the intervention protocols may relate to findings. Methods: The search strategy detailed the technique of stimulation, population and combined interventions (i.e. cognitive and/or physical training). Only controlled studies were included. Results: Seventeen of an initial yield of 408 studies satisfied the criteria. Studies involved small sample sizes. tDCS protocols and characteristics of combined interventions varied. The reviewed studies suggest that synergistic effects may be obtained for cognition, upper limb function, gait/mobility and posture when tDCS is combined with cognitive and/or motor interventions in PD. Conclusion: The reported results encourage further research to better understand the therapeutic utility of tDCS and to inform optimal clinical use in PD. Future studies in this field should focus on determining optimal stimulation parameters and intervention characteristics for maximal benefits in people with PD.

An Acute Application of Cerebellar Transcranial Direct Current Stimulation Does Not Improve Motor Performance in Parkinson’s Disease

“…Most importantly, most of the effects of c-tDCS on motor skill were evident within the first block of 10 practice trials, but they continued at a decreasing rate until reaching the lowest point over the course of 20 further practice trials. The current results also differ from the augmentations in motor performance attained in the preponderance of M1-tDCS studies in PD [ 11 , 49 , 50 ]. Conversely, Steiner et al found that c-tDCS did not increase the magnitude of motor learning attained by young adults in a whole-body dynamic balance task [ 51 ].…”

“…However, only one of these investigations found that the effects of tDCS on motor performance were not also significant on the first day of practice and stimulation. In addition, the overwhelming balance of tDCS studies in both healthy adults and in PD have only involved a single stimulation session and review articles [ 10 , 11 , 49 , 50 ] appear to show that tDCS yielded positive results in 75–80% of these acute studies. Accordingly, it is not obligatory that multiple daily stimulation sessions are required to observe enhancements in motor function due to c-tDCS.…”

Individual Cerebral Blood Flow Responses to Transcranial Direct Current Stimulation at Various Intensities

“…Similarly, these results support the high variability evident in several reviews and meta-analyses [ 8 , 9 , 10 , 11 , 12 , 13 ] and continue to raise questions regarding the ability of sufficient residual current to reach the cortex and reliably affect brain function. Indeed, an animal and cadaver study concluded that higher intensities (≥4 mA) might be required to adequately overcome skin/skull electrical current shunting [ 17 ].…”

“…This neuromodulation technique has several desirable characteristics, including ease of use, low cost, and potential for in-home use [ 7 ]. However, as more results from tDCS studies continue to be published, it is becoming increasingly evident that this neuromodulatory tool is hampered by high inter- and intra-subject variability [ 8 , 9 , 10 ]. Indeed, the results of some meta-analyses challenge the efficacy of tDCS [ 11 , 12 , 13 ], although a group of tDCS experts has argued against some of these null meta-findings [ 14 ].…”

Effects of Transcranial Direct Current Stimulation Combined With Physical Training on the Excitability of the Motor Cortex, Physical Performance, and Motor Learning: A Systematic Review

“…More recently, researchers have started to combine tDCS with other types of interventions, such as physical (e.g., exercise, physiotherapy, etc.) and cognitive training (Beretta et al, 2020), and explore the effects of this mixed type of intervention. Studies have shown that both tDCS and exercise (e.g., strength training) can increase the excitability of cortical (e.g., primary motor cortex, M1) as measured by the amplitude of motor-evoked potential (MEP) (Kidgell et al, 2010;Mazzoleni et al, 2019).…”