The primary objective of this study is to determine, in a healthcare setting, the effectiveness of actual tDCS in reducing pain compared with placebo tDCS. This is a multicenter randomized controlled trial with parallel groups (real tDCS vs. placebo) blinded to participants, assessors and tDCS providers. The real tDCS group will receive a daily 20-minute session of tDCS (current intensity = 2 mA), for 5 consecutive days, while the placebo tDCS group will receive an equivalent treatment, but the current will be stopped after the first 30 seconds. The study will take place in 5 rehabilitation clinics in 3 Quebec regions. One hundred and fifty (150) seniors aged 65 years or older with chronic (> 6 months), moderate to severe musculoskeletal pain will be recruited (50 participants/region). Follow-ups will take place at 1 week and 3 months post-treatment. The primary dependent variable is pain intensity (numerical scale from 0 to 10). Secondary variables will be measured using standardized and validated questionnaires: 1) pain-related interferences (physical function, mood, quality of life) and 2) perception of post-TDCS changes. Neurophysiological measures (pain control pathways).

Rationale : Chronic pain is the leading cause of disability and affects a large number of seniors. Transcranial direct current stimulation (tDCS), which provides non-invasive stimulation of the brain, is a promising avenue for relieving pain that is refractory to traditional treatments. To date, however, its clinical efficacy has yet to be confirmed in the elderly.

Objectives : The primary objective of this study is to determine, in a healthcare setting, the effectiveness of actual tDCS in reducing pain compared with placebo tDCS. Secondary objectives are to assess the effect of tDCS on pain-related interference and the relationship between the response to tDCS and the integrity of pain control systems (descending pathways controlling pain circuits).

Methods : This is a multicenter randomized controlled trial with parallel groups (real tDCS vs. placebo) blinded to participants, assessors and tDCS providers. The real tDCS group will receive a daily 20-minute session of tDCS (current intensity = 2 mA), for 5 consecutive days, while the placebo tDCS group will receive an equivalent treatment, but the current will be stopped after the first 30 seconds. The study will take place in 5 rehabilitation clinics in 3 Quebec regions: Sherbrooke (n=1), Quebec City (n=2), Montreal (n=2). One hundred and fifty (150) seniors aged 65 years or older with chronic (> 6 months), moderate to severe musculoskeletal pain will be recruited (50 participants/region). Follow-ups will take place at 1 week and 3 months post-treatment. The primary dependent variable is pain intensity (numerical scale from 0 to 10). Secondary variables will be measured using standardized and validated questionnaires: 1) pain-related interferences (physical function, mood, quality of life) and 2) perception of post-TDCS changes. Neurophysiological measures (pain control pathways), i.e., the integrity of corticothalamic and corticospinal projections will be tested by diffusion MRI and transcranial magnetic stimulation.

Anticipated results: Our hypotheses are that (i) real tDCS will be more effective in reducing pain intensity than placebo tDCS at 1 week post-treatment. This reduction in pain will be maintained at the 3-month post-treatment follow-up of real tDCS. (ii) Real tDCS will be more effective than placebo treatment in reducing pain interference with physical function, mood and quality of life. Perceived improvement with the treatment received will also be greater for real tDCS. (iii) Individuals with stronger cortico-thalamic and corticospinal projections in pre-tDCS will be more relieved by actual tDCS than those with less strong projections.

Ultimately, this study will allow us to evaluate the effectiveness of tDCS in the care setting for the relief of chronic musculoskeletal pain in the elderly and to identify those individuals most likely to respond to this type of treatment based on potential biomarkers related to the integrity of the pain control system.

Inclusion Criteria:

• Be aged ≥ 65 years
• Have chronic (≥ 6 months) musculoskeletal pain of moderate to severe intensity (≥4 out of 10 on a numerical scale of 0 to 10)

Exclusion Criteria:

• Those with contraindications to tDCS
• Those with contraindications to TMS
• Those with contraindications to IRM
• Individuals for whom the procedure could cause impairment of well-being, or has another medical condition that could put them at risk in the judgment of a health care professional.
• Patients taking medications that act on the GABAergic and glutamatergic systems (modulating tDCS effects) will be excluded
• Individuals taking other types of medications or receiving rehabilitation are not excluded but will be asked, in the absence of clinical contraindication, to avoid any modification (e.g., new treatment, discontinuation, or change in dose) during the study.
• People with epilepsy and seizures will not be excluded from the study. The research team will simply be asked to pay closer attention to these participants during the administration of MST.

Participants and team members, also referred to as evaluator members above (researchers, analysts, physical therapists, and research assistants) will all be blinded to the intervention, except for those not involved in data collection and analysis: a) the statistician who will generate the randomization list, b) the computer team who will program the randomization system, and c) the interventionist who will apply the tDCS power.