The Mouse Grimace Scale: A clinically useful tool?

Medical research has a heavy and continuing demand for rodent models across a range of
disciplines. Behavioural assessment of pain in such models is highly time consuming, thus
limiting the number of models and analgesics that can be studied. Facial expressions are
widely used to assess pain in human infants. Recently the mouse grimace scale (MGS) has
been developed and shown to be accurate and reliable, requiring only a short amount of
training for the observer. This system therefore has the potential to become a highly useful
tool both in pain research and clinical assessment of mouse pain. To date, the MGS has
only been used as a research tool, however there is increasing interest in its use in cageside clinical assessment. It is often wrongly assumed that MGS scores of animals not in
pain (i.e. at baseline) are zero. Here, we aimed to assess the variability in baseline MGS
scores between cohorts, sexes and strains of mice. Establishing the presence of a consistent baseline MGS score could lead to a valuable clinical pain assessment tool for mice
when baseline information from the individual mouse may not be available as a comparator.
Results demonstrated a significant difference in baseline MGS scores between both sexes
(males > females) and strains of mice. The method used to score the facial action units
(Live vs. retrospectively from still images) demonstrated significant differences in scores
with live scores being significantly lower than retrospective scoring from images. The level
of variation shown demonstrates the need for further research to be undertaken with regard
to establishing baseline MGS scores for specific strains and sexes of mice, taking into
account the method of scoring, prior to considering clinical implementation of this method in
pain assessment.