We are used to the idea that facial expressions can tell us a lot about what our fellow humans are experiencing and researchers in Canada and the Netherlands have now found that the same is true for mice. In human volunteers, grimacing has been shown to match self-reported pain intensity and facial expression is also used to evaluate pain in patients who are unable to communicate orally.
Existing pain remedies do not work well for all patients and, in the search for more effective pain relief, mice and other rodents are used to develop new treatments. The present study has shown that mice, like humans, express pain through facial expressions and should provide a more accurate way to ensure that laboratory animals do not suffer unnecessarily and could also lead to better drugs for humans. The team have developed a ‘Mouse Grimace Scale’ by analysing high-resolution video images of mice before and during moderately painful stimuli – pain that is suggested to be comparable to a headache or that associated with an inflamed and swollen finger. The researchers, who are leaders in developing standards for facial expression to assess pain in human infants and others unable to communicate verbally, have proposed that five facial features in the mice should be scored. Several of these criteria – which are orbital tightening (eye closing), nose and cheek bulges, and ear and whisker positions – are similar to those used to assess pain in humans.
The study, which is the first to propose such a standardised and accurate scoring system linking facial expression to pain in animals, is published in the journal Nature Methods. Based on facial expression, trained researchers were able to correctly and reliably assess pain levels in real time. Previously, assessment of pain was limited to measuring withdrawal responses to pressure and heat and the ‘Mouse Grimace Scale’ may provide a more accurate way of predicting whether new treatments will eventually work in humans as well as reducing unnecessary pain in laboratory animals.