The Rat Grimace Scale: A Partially Automated Method for Quantifying Pain in the Laboratory Rat via Facial Expressions

Abstract

We recently demonstrated the utility of quantifying spontaneous pain in mice via the blinded coding of facial expressions. As the majority of preclinical pain research is in fact performed in the laboratory rat, we attempted to modify the scale for use in this species. We present herein the Rat Grimace Scale, and show its reliability, accuracy, and ability to quantify the time course of spontaneous pain in the intraplantar complete Freund's adjuvant, intraarticular kaolin-carrageenan, and laparotomy (post-operative pain) assays. The scale's ability to demonstrate the dose-dependent analgesic efficacy of morphine is also shown. In addition, we have developed software, Rodent Face Finder®, which successfully automates the most labor-intensive step in the process. Given the known mechanistic dissociations between spontaneous and evoked pain, and the primacy of the former as a clinical problem, we believe that widespread adoption of spontaneous pain measures such as the Rat Grimace Scale might lead to more successful translation of basic science findings into clinical application.

Development of a facial expression scale using footrot and mastitis as models of pain in sheep

“…Facial expression scoring systems for pain assessment have been recently developed for use in rodents, rabbits and horses (Dalla Costa et al, 2014;Langford et al, 2010;Leach et al, 2012). Facial expression scoring has shown to be successful in identifying and assessing the severity of pain in animals, with minimal time and training required for observers (Langford et al, 2010;Leach et al, 2012;Sotocinal et al, 2011). Changes in facial expression are likely to be an involuntarily response by an animal in response to the fluctuating level of pain experienced (Langford et al, 2010) leading to higher sensitivity in the assessment.…”

Irinotecan-Induced Gastrointestinal Dysfunction and Pain Are Mediated by Common TLR4-Dependent Mechanisms

“…Chemotherapy-induced pain was measured by two independent assessors, in a blinded manner, 4 times daily in all mice using the validated rodent facial grimace criteria (20), as previously published by our group (17). Briefly, the scoring method consists of five distinct criteria: orbital tightening, cheek bulge, nose bulge, ear position and whisker position.…”

Why Pain Is Still a Welfare Issue for Farm Animals, and How Facial Expression Could Be the Answer

“…Having a scale that can objectively identify and assess pain and provide guidance on when to give pain relief will improve the chances of that animal receiving treatment. Pain relief should change the facial expression of animals in pain [113,114] demonstrating if it is working or not. This can be particularly useful when giving pain relief to animals that do not have a licensed drug, or where there is limited research into the effectiveness of the drug.…”

“…It has only been in the last decade that facial expression has been developed for use in animals as a pain assessment tool (mice [113], rats [114], rabbits [73], horses [71]), including some farm species (sheep [32,115], lambs [116] and piglets [117]). Across the different species, there are similar facial movements and action units expressed in the presence of pain, demonstrating an evolutionary stability in pain expression across mammalian species [118].…”