The use of facial movement to express pain and suffering as well as internal emotional states and the ability to recognize this expression in conspecifics is a well-known part of the human heritage and experience. Facial expressions of pain in other species are often much more inscrutable to human caregivers. Therefore, grimace scales have been developed for the evaluation of pain based on facial expression in rats, mice, ferrets, horses, sheep, lambs, piglets, rabbits, and also human infants. Cats are masters at concealing their pain and discomfort and often do not receive the pain mangement they need and deserve; only recently have validated behavior-based feline pain assessment tools become available.
The purpose of this prospective, case-control study of 35 client-owned and 20 control cats was to develop and validate a feline grimace scale (FGS) to detect naturally occurring acute pain such as would occur with diseases causing somatic or visceral pain. Facial expressions can be objectively evaluated using a facial action coding system (FACS) that measures individual movements of the face, called action units (AUs) involved in producing a facial expression. Although orbital tightening and ear flattening are common AUs in all species, other AUs are different in each species. Therefore, the development of species-specific grimace scales is essential.
Cats enrolled in the study were video-recorded in their cages. The painful cats, who had been admitted to the emergency and critical care unit of a veterinary teaching hospital, received analgesic treatment after being video-recorded undisturbed and then all animals were video-recorded an hour after the painful cats received the analgesic. Feline patients who had diseases that could affect their facial expressions, such as head trauma or ophthalmic conditions, as well as those requiring emergent care and those who appeared very shy or feral, were excluded from the study. Most of the case cats had diseases that caused them to present with abdominal pain, such as cholangitis, hepatic lipidosis, constipation, urethral obstruction, lymphoma, pancreatitis, inflammatory bowel disease, urolithiasis, idiopathic cystitis, and suspected foreign body.
All of the case cats received a full physical examination as well as evaluation by one observer with the Glasgow composite measure pain scale for acute pain in cats. Those cats with a score of >4/16 on this scale were considered painful. A video camera was placed between the cage bars at the level of the eyes, and video recordings of the cats’ facial expressions were made without the presence of human beings in the room. The control cats also received a full physical examination and were video-recorded twice with a one hour interval between recordings; these cats received no analgesic therapy.
Most of the cats included in the study were domestic shorthairs. Two brachycephalic cats were initially recruited into the study, but were excluded due to poor image quality. Black cats were also excluded due to poor image quality and difficulty in identification of facial landmarks.
Four observers, two PhD candidates and two board-certified anesthesiologists, independently scored all the images of the case and control cats. The AUs indicating pain were defined as follows: (1) ear position–tips of the ears pulled apart and rotated outwards; (2) orbital tightening—narrowing of the orbital area, or tightly closed eyelids (squinted eyes); (3) muzzle tension—flattening and stretching of the muzzle from a round to an elliptical shape; (4) whiskers position—forward movement of whiskers rostrally and away from the face; (5) head position in relation to the shoulders—head below the shoulder line or tilted down (chin pointed towards the chest). Linear distance ratios and angles were measured on several facial features: ears from tip to tip and base to base; width and height of palpebral fissure; muzzle height and width, as well as the angles between the medial border of the ear and the top of the head (medial ear angle) and the lateral border of the ear (lateral ear angle) and a line connecting both lateral cutaneous pouches of the ear pinnae.
Each patient image received a single total pain score based on the sum of scores (0-2) from each AU divided by the maximum possible score (10). The final FGS score was reported on a scale from 0 to 1.
Calculated ratios of ear tips distance/ear base distance, eye height/width, muzzle height/width, medial ear angles, and lateral ear angles differed significantly between case and control groups. The FGS scores were significantly higher (p < 0.001) in the painful group prior to analgesia than in the control group, and there was also a very strong correlation between results of the Glasgow composite measure pain scale-feline and the FGS scores. Sex of the animal did not have a significant effect on FGS scores. The FGS also had good inter-rater reliability and excellent intra-rater reliability. The authors recommend that an FGS score of >0.39/1.0 requires intervention with analgesia, as this represents an optimal balance between sensitivity (90.7%) and specificity (86.6%). This criterion provides the FGS with substantial clinical utility. [PJS]