Effects of Transcranial Direct Current Stimulation on Knee Osteoarthritis Pain in Elderly Subjects With Defective Endogenous Pain-Inhibitory Systems: Protocol for a Randomized Controlled Trial

BACKGROUND: Knee osteoarthritis (OA) has been the main cause behind chronic pain and disabilities in the elderly population. The traditional treatment for knee OA pain currently concerns a number of combinations of pharmacological and nonpharmacological therapies. However, such combinations have displayed little effects on a significant group of subjects. In addition to this, pharmacological treatments often cause adverse effects, which limits their use on this population. Previous studies showed that chronic knee OA pain may be associated with maladaptive compensatory plasticity in pain-related neural central circuits indexed by a defective descending pain-inhibitory system. Transcranial direct current stimulation (tDCS) can revert some of these maladaptive changes, thus decreasing chronic pain sensation. Numerous studies have demonstrated that the use of anodal tDCS stimulation over the primary motor cortex (M1) has positive effects on chronic neuropathic pain. Yet, data on OA pain in elderly patients, including its effects on the endogenous pain-inhibitory system, remain limited. OBJECTIVE: The objective of this study is to evaluate the efficacy of tDCS in reducing pain intensity caused by knee OA in elderly subjects with defective endogenous pain-inhibitory systems. METHODS: We designed a randomized, sham-controlled, single-center, double-blinded clinical trial. Patients with knee OA who have maintained a chronic pain level during the previous 6 months and report a pain score of 4 or more on a 0-10 numeric rating scale (NRS) for pain in that period will undergo a conditioned pain modulation (CPM) task. Participants who present a reduced CPM response, defined as a decrease in NRS during the CPM task of less than 10%, and meet all of the inclusion criteria will be randomly assigned to receive 15 sessions of 2 mA active or sham tDCS for 20 minutes. A sample size of 94 subjects was calculated. The Brief Pain Inventory pain items will be used to assess pain intensity as our primary outcome. Secondary outcomes will include pain impact on functioning, mobility performance, quality of life, CPM, pressure pain threshold, touch-test sensory evaluation, and safety. Follow-up visits will be performed 2, 4, and 8 weeks following intervention. The data will be analyzed using the principle of intention-to-treat. RESULTS: This study was approved by the institutional review board with the protocol number 1685/2016. The enrollment started in April 2018; at the time of publication of this protocol, 25 subjects have been enrolled. We estimate we will complete the enrollment process within 2 years. CONCLUSIONS: This clinical trial will provide relevant data to evaluate if anodal tDCS stimulation over M1 can decrease chronic knee OA pain in elderly subjects with defective CPM. In addition, this trial will advance the investigation of the role of central sensitization in knee OA and evaluate how tDCS stimulation may affect it. TRIAL REGISTRATION: ClinicalTrials.gov NCT03117231; https://clinicaltrials.gov/ct2/show/NCT03117231 (Archived by WebCite at http://webcitation.org/73WM1LCdJ) INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID): RR1-10.2196/11660