The mouse is the most used animal in research. Their pain may be assessed by the Mouse Grimace Scale (MGS).
Mouse Grimace Scale (MGS)
The MGS presents five characteristics:
1) Orbital tightening
2) Nose Bulge
3) Cheek Bulge
4) Ear position
5) Whisker changes
Each characteristic has two levels scored from 0 (normality or absence of pain) to 2 (the greatest possible score).
CODING PROCEDURES
Before beginning the coding process the coder should view and familiarize oneself with baseline photos (score 0) to determine the specific mouse features present.
A. PAIN INTENSITY RATINGS
Score
Intensity ratings are coded for each facial action unit:
Not present
0
Moderately visible
1
Severe
2
An MGS score for each photograph is calculated by averaging intensity ratings for each action unit.
B. ACTION UNITS
1. Orbital Tightening*
Mouse must display a narrowing of the orbital area, a tightly closed eyelid, or an eye squeeze. An eye squeeze is defined as the orbital muscles around the eyes being contracted. A wrinkle may be visible around the eye. As a guideline, any eye closure that reduces the eye size by more than half should be coded as a “2”.
* Note that sleeping mice display closed eyes, and this may be mistaken for a tightly closed eyelid. Sleeping mice should therefore not be evaluated.
2. Nose Bulge*
Mouse must display a bulge on top of the nose. The skin and muscles around the nose will be contracted creating a rounded extension of skin visible on the bridge of the nose. A nose bulge may also be coded if a coder sees vertical wrinkles extending down the side of the nose from the bridge. In frontal headshots, a bulge may be seen as a widening of the nose area (i.e., V-shape connecting eyes to nose appears broader).
* Note that a nose bulge may also appear when mice are actively exploring (i.e., sniffing). Ideally, these images should not be evaluated.
3. Cheek Bulge*
The cheek muscle is contracted and extended relative to the baseline condition; it will appear to be convex from its neutral position.
* Note: The cheek is considered to be the area directly below the eye and extending to the beginning of the whiskers on the nose (in humans, the infraorbital triangle). The distance from eye to whisker pad may appear shortened relative to baseline.
4. Ear Position*
Ears may be pulled back from their baseline position, or may be seen as laid flat against the head. In a typical baseline position ears are roughly perpendicular to the head and are directed forward. In pain, the ears tend to rotate outwards and/or back, away from the face. As a result, the space between the ears may appear wider relative to baseline.
* Note that mice engaged in exploration or grooming may also pull ears back, but distance between ears tends to narrow rather than widen. In any case, these may cause confusion, and it is advised that photographs of mice actively exploring or grooming not be taken and/or coded.
5. Whisker Changes
Whiskers must have moved from the baseline position. They could either be pulled back to lay flat against the cheek or pulled forward as if to be “standing on end”. Whiskers may also clump together compared to baseline whiskers, which tend to be fairly evenly spaced.
The sum of the points of each evaluated action unit of the animal reflects the pain intensity, which varies from 0 (no pain) to 10 points (maximum pain). Calculate the mean of your results (total score/5). The mean will range from 0 (no pain) to 2 (maximum pain). Regardless of the score, it is up to the observer to decide whether or not to use analgesics, according to the clinical evaluation.
After reading and training the previous items, click below to assess pain in your animal.