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The neural, behavioural, and clinical effects of transcranial direct current stimulation in patients with a prolonged disorder of consciousness; feasibility study

As part of an on-going grant (Medical Research Council [MRC]; 2017-2020), the Chief Investigator (CI) is conducting a series of concurrent fMRI-tDCS experiments to characterise the mechanisms that underlie the effects of tDCS and passive mobilisation in the healthy brain. The main aim of RAINDROP is to establish the infrastructure to translate this work into the PDOC clinical environment, thus supporting the world’s first study of tDCS in PDOC from a specific mechanistic motor hypothesis. The infrastructure and proof of concept data will be used as evidence to funding agencies that team’s methods for restoring motor responsiveness in PDOC are feasible and effective, and are therefore a priority for subsequent multi-centre trial across England. If the team’s methods are successful, the increased behavioural responsiveness will allow some patients to use assistive devices to regain a level of autonomy for the first time since their injury. Importantly, even if only transient improvements are achieved, these will open the door for patients to actively participate in some motor/cognitive rehabilitation programmes (physical therapy, neuropsychology, speech and language therapy), which are not currently indicated for PDOC.1 Indeed, active rehabilitation is associated with accelerated recovery and shorter hospital stays.31 Furthermore, increased patient autonomy and reduced hospitalisation time will greatly reduce the healthcare costs of this challenging population.
Importantly, the team’s work to date suggests that motor impairments also interfere with patients’ ability to demonstrate their ‘true’ cognitive functioning. While PDOC patients are considered to be entirely or partially unconscious (VS and MCS, respectively), some patients retain a significantly higher level of cognitive function than is evident from their behaviour. Specifically, functional neuroimaging has revealed that 15-20% of VS patients are in fact aware but simply unable to exhibit overt purposeful responses. For example, in one patient who had been repeatedly diagnosed as VS for 12 years, we successfully used fMRI to demonstrate that he was not only aware, but also able to form memories and communicate answers to clinically relevant questions. In this context, providing patients with a degree of control over their external behaviour will have profound implications not only for quality of life in those who are aware, but also for accuracy of diagnoses across all patients.