The mouse grimace scale, a standardized way to measure pain from facial expression in mice, is published online this week in Nature Methods. This facial coding system will allow for accurate pain assessment and advances in pain research, while eliminating unnecessary pain of laboratory mice.
Humans express pain through facial expressions, and these have been coded and used for pain assessment in individuals who cannot communicate in any other way, for example, infants. Whether the same can be done for mice was an open question. Pain research heavily depends on rodent models but at the moment researchers are largely limited to measuring withdrawal responses to pressure and thermal stimuli, which model only minor aspects of the chronic pain experience.
Jeffrey Mogil and colleagues analyzed hundreds of images of mice before and during a moderate pain stimulus. They established five facial features ― orbital tightening, nose and check bulge, and changes in ear and whisker carriage ― that change in accordance with the severity of the stimulus and allow the assigning of pain scores. One of the goals is to detect insufficient postoperative analgesia by mere visual inspection.