Transcranial Direct Current Stimulation for Poststroke Motor Recovery: Challenges and Opportunities

Abstract

There has been a renewed research interest in transcranial direct current stimulation (tDCS) as an adjunctive tool for poststroke motor recovery as it has a neuro-modulatory effect on the human cortex. However, there are barriers towards its successful application in motor recovery as several scientific issues remain unresolved, including device-related issues (ie, dose-response relationship, safety and tolerability concerns, interhemispheric imbalance model, and choice of montage) and clinical trial-related issues (ie, patient selection, timing of study, and choice of outcomes). This narrative review examines and discusses the existing challenges in using tDCS as a brain modulation tool in facilitating recovery after stroke. Potential solutions pertinent to using tDCS with the goal of harnessing the brains plasticity are proposed.

Figure 1.. Comparison of Safety Between Animal and Human Studies based on tDCS Dose Levels.

Typical human studies involve charge density (current amplitude × duration of stimulation ÷ pad size) of ~1 kC/m² or less. A recent tDCS dose escalation study demonstrated the safety of ~2 kC/m² in stroke subjects. Stimulation using 10 mA tDCS for 30 minutes on a standard 35 cm² pad size offers ~5 kC/m² charge density, which is still an order of magnitude lower than >50 kC/m² as required in animal studies to cause brain injury.

Figure 2.. Spheric Head Model Simulations Suggest that tDCS current in the order of 10 mA in humans generate electric fields that are experimentally shown to cause no damage in rodent brain.

A-D, A human spherical head model of radius 8 cm with bihemispheric C3/C4 montage is used to simulate electric fields using 1, 2, 5 and 10 mA currents. E-F, Rat spherical head model of radius 0.8 cm is used to simulate electric fields using 0.1 mA current with F3 montage (E) and C3/C4 montage (F). 0.1 mA current was shown to be safe in rodents for tDCS duration of 4.5 hours (270 minutes). Note close similarity of generated electric field intensity between 10 mA human model (D) and 0.1 mA rat model (F) and compare it with conventional 1 mA (A) and 2 mA (B) currents in human tDCS applications. Figure panels are generated with SPHERES and adapted.