Asymmetry of Pain-Induced Facial Grimacing

Pain has two main components: the sensory-discriminative (the quality, intensity and location of pain) and the motivational-affective (the emotional aversiveness of pain reflective of suffering). A plethora of translational preclinical and clinical measures for the sensory-discriminative component exist (e.g., von Frey, cold pressor). However, few existing measures capture the more elusive motivational-affective component, and those that do are hampered as they are not translatable across species. Post-lesion evaluation of facial grimacing of emotion-related areas of the brain suggests that the Mouse Grimace Scale is reflective of the motivational-affective component. Facial expressions of emotion (e.g., fear, anger) are lateralized such that the left side of the face exhibits facial expressions more strongly than the right side. Comparing pain-induced facial grimacing to facial expressions of emotion is one way to determine which component of the pain experience is most captured by the Mouse Grimace Scale. We hypothesized that grimacing would be lateralized to the left side of the face. Examining lateralization of pain-induced facial grimacing is novel to pain research. We examined the asymmetry of pain-induced facial grimacing in CD-1 mice using inflammatory, neuropathic, and reflexive pain. And we found that pain is expressed predominantly on the right side of the face, contrary to other emotions. Our findings have important implications for the measurement of pain, as characterized by suffering, in non-verbal populations and for application in veterinary care settings.