We all know how difficult it can be to assess pain or discomfort in cats. These investigators examined the usefulness of facial expressions to recognize acute pain in cats. They used anatomic markers and measurable distances on two dimensional digital images of the feline face to discriminate pain-free states from acute pain.
Fifty-nine 2D facial images of healthy, pain-free cats were collected from a variety of sources such as veterinary clinics, cat breeders and cat owners recruited from the general public. Seventy-eight landmarks (points) were chosen on the feline face based on anatomical knowledge and ease of identification on 2D images and between cats with different hair lengths Following identification of landmarks, 80 distances between pairs of landmarks were developed based on the accuracy of measurement and where changes might be expected between painful and pain-free cats incorporating knowledge of facial changes described during pain in other species. The 80 distances were measured and analysed. Subsequently, a separate group of cats undergoing postoperative care or hospitalised for traumatic or medical conditions were recruited If analgesia was required, a 2D portrait facial image was obtained before analgesia administration. Twenty-eight painful cat portrait images were obtained and each was landmarked with the anatomical points identified from the pain-free cats (controls). Observers comprised five veterinary nurses, one animal care assistant, five veterinary students, nine interns, 12 residents of varying disciplines, 10 senior university clinicians and 26 general practice veterinarians.
Observers (n=68) had difficulty in identifying pain-free from painful cats, with only 13% of observers being able to discriminate more than 80% of painful cats. Analysis of 78 facial landmarks and 80 distances identified six significant factors differentiating pain-free and painful faces including ear position and areas around the mouth/muzzle. Standardised mouth and ear distances when combined showed excellent discrimination properties, correctly differentiating pain-free and painful cats in 98% of cases. a cartoon-type picture scale was developed.
