Transcranial Direct Current Stimulation to Lower Neuropathic Pain in People With Multiple Sclerosis

The investigators aim is to identify changes in brain activation following transcranial direct current stimulation (tDCS), and determine whether these changes are associated with reduced ratings of pain. The investigators hypothesize that the analgesic effects of tDCS are associated with altered glucose metabolism of key regions of the top-down pain modulatory system, such as the dorsolateral prefrontal cortex (DLPFC) and medulla. This study will follow an interventional protocol with two groups. Upon enrollment into the study, all participants will undergo testing for the outcome measures described above. Each participant will then be randomly assigned into the SHAM or tDCS group for the study intervention. This study will be completed over the course of 6 consecutive days and 1 follow-up phone call 1 week after the final testing for each participant (total of 13 days). Two age and sex matched groups, each n =8, will receive either tDCS or SHAM stimulation. All of the following measures will be performed before and after the tDCS or SHAM intervention. Fluorodeoxyglucose (\[18F\] FDG) - Whole and regional brain FDG uptake will be measured to determine the mechanistic effect of tDCS on brain activity. Visual Analog Scale (VAS) - The VAS is a self-evaluation scale where the participant is asked to mark a segment that ranges from 0-100 as visually described in millimeter units where 0 mm indicates no pain and 100 mm indicates the worst possible pain. This scale has been widely used in studies that evaluated pain as an outcome measure: both validity and reproducibility have been demonstrated. VAS for Anxiety - This is a self-evaluation scale that ranges from 0 to 100 mm: 0 mm indicates no anxiety and 100 mm indicates the worst possible anxiety. Prior to beginning the 2nd-5th tDCS sessions, the effectiveness of tDCS will be assessed using the following procedures: Visual Analog Scale (VAS) - Each participant will be asked how effective the previous day's tDCS session was at reducing their pain by marking a segment on the range of 0-100 as visually described in millimeter units where 0 mm indicates no reduction in pain and 100 mm indicates complete alleviation of pain. Duration of Relief - If the participant indicates any reduction in pain following the previous day's tDCS session, the participant will be asked to estimate how long the participant's pain was reduced following the session. One week following the post-intervention testing, participants will be contacted via telephone and asked the following questions: 1. Was tDCS effective at reducing the participant's pain? 2. If so, how long did the participant notice a reduction in pain following the participant's final tDCS session? 3. Have the participants reduced their use of pain relieving medications since the last tDCS session?