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

TLDR

Spontaneous pain, which is mechanistically distinct from evoked pain and clinically more relevant, may be better translated to human outcomes using measures like the Rat Grimace Scale. The authors adapted the Grimace Scale for laboratory rats and demonstrated its reliability, accuracy, and capacity to quantify spontaneous pain across several experimental models. They developed Rodent Face Finder® software to automate the labor‑intensive coding of facial expressions. The Rat Grimace Scale reliably tracks spontaneous pain over time, distinguishes dose‑dependent analgesic effects of morphine, and shows strong inter‑rater reliability.

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.