Can transcranial direct current stimulation at the motor cortex provide effective relief for severe menstrual pain? a preliminary behavioral and functional connectivity study

Abstract Introduction: Severe primary dysmenorrhea (PDM) is a distressing pain affecting 10% to 25% of menstruating women. Little is known about the analgesic efficacy and neural mechanisms of transcranial direct current stimulation (tDCS) at the primary motor area (M1) for menstrual pain control. Objectives: We examined the analgesic efficacy of anodal tDCS over the left M1 of patients with severe PDM and studied the functional connectivity (FC) changes between the periaqueductal gray matter (PAG) and the medial motor area (MMA) to elucidate the possible central mechanisms. Methods: Twenty-eight patients with severe PDM participated in this randomized and sham-controlled study. The participants received daily M1-tDCS for 5 to 7 days, beginning 2 to 3 days before menstruation and continuing until their pain severity decreased to a mild level. We evaluated the menstrual pain and PAG-seeded FC with MMA using resting-state functional magnetic resonance imaging right after and 1 month later after M1-tDCS modulation. Results: The neuromodulation by active M1-tDCS led to a significant decrease in the FC between the PAG and MMA. This reduction in FC correlated with a decrease in menstrual pain experienced in the subsequent menstrual cycle. Notably, only the FC modulated by active M1-tDCS could predict the pain intensity in the following menstrual period. Conclusion: Our preliminary data suggest that M1-tDCS may have a potential analgesic impact on responders, particularly in the month after the intervention. M1-tDCS could potentially normalize the maladaptive hyperconnectivity between the PAG and MMA, thereby contributing to the regulation of the descending pain modulation system and providing analgesia for severe PDMs.