Hypoalgesia and parasympathetic effects of millimeter waves on experimentally induced pain in healthy volunteers.

In humans, exposure to electromagnetic millimeter waves (MMW) has a hypoalgesic effect. In animals, this effect has been shown to depend on innervation density of the area exposed. This study aims to assess hypoalgesic and parasympathetic effects of MMW applied on the palmar side of the wrist in healthy participants. In a within-subject design, 10 healthy participants had the palmar side of their wrist exposed to MMW (61.25 GHz, 17 mW/cm²) for 30 minutes, 1 h, & 1 h30, and 30 minutes of sham exposure. Experimental pain was induced after the exposure sessions with the Cold Pressor Test, and pain threshold and pain tolerance values were compared to that of the sham condition. Participants' heart rate and blood pressure were measured before and after exposures. Finally, innocuity of the exposure system was controlled with a pre-post exposure visual examination scale and skin temperature measured by a thermal camera. Exposure to 30 minutes, but not 1 h or 1 h30, of MMW led to significant increases in pain thresholds compared to the sham condition, but no increase of pain tolerance. All conditions led to decreased heart rate, while no change in blood pressure was observed. No change in skin state or temperature was observed for any of the conditions. MMW applied on the inner part of the wrist diminish pain sensations more effectively than placebo, and seem to increase parasympathetic activities, while remaining innocuous. Building a miniaturized MMW emission system to be worn on the wrist would provide access to ambulatory MMW therapy for pain management.