*Review* **The Utility of Grimace Scales for Practical Pain Assessment in Laboratory Animals**

### **Daniel Mota-Rojas 1, Adriana Olmos-Hernández 2, Antonio Verduzco-Mendoza 2, Elein Hernández 3, Julio Martínez-Burnes 4 and Alexandra L. Whittaker 5,\***


Received: 4 September 2020; Accepted: 22 September 2020; Published: 9 October 2020

**Simple Summary:**Grimace scales forlaboratory animalswere first reported ten years ago. Yet, despite their promise as pain assessment tools it appears that they have not been implemented widely in animal research establishments for clinical pain assessment. We discuss potential reasons for this based on the knowledge gained to date on their use and sugges<sup>t</sup> avenues for further research, which might improve uptake of their use in laboratory animal medicine.

**Abstract:** Animals' facial expressions are widely used as a readout for emotion. Scientific interest in the facial expressions of laboratory animals has centered primarily on negative experiences, such as pain, experienced as a result of scientific research procedures. Recent attempts to standardize evaluation of facial expressions associated with pain in laboratory animals has culminated in the development of "grimace scales". The prevention or relief of pain in laboratory animals is a fundamental requirement for in vivo research to satisfy community expectations. However, to date it appears that the grimace scales have not seen widespread implementation as clinical pain assessment techniques in biomedical research. In this review, we discuss some of the barriers to implementation of the scales in clinical laboratory animal medicine, progress made in automation of collection, and sugges<sup>t</sup> avenues for future research.

**Keywords:** facial expressions; pain; grimace scales; mice; rat; rabbit
