Measurement properties of grimace scales for pain assessment in nonhuman mammals: a systematic review

Facial expressions of pain have been identified in several animal species. The aim of this systematic review was to provide evidence on the measurement properties of grimace scales for pain assessment. The protocol was registered (SyRF#21-November-2019), and the study is reported according to the PRISMA guidelines. Studies reporting the development, validation, and the assessment of measurement properties of grimace scales were included. Data extraction and assessment were performed by 2 investigators, following the COnsensus-based Standards for the Selection of Health Measurement INstruments guidelines. Six categories of measurement properties were assessed: internal consistency, reliability, measurement error, criterion and construct validity, and responsiveness. Overall strength of evidence (high, moderate, and low) of each instrument was based on methodological quality, number of studies, and studies' findings. Twelve scales for 9 species were included (mice, rats, rabbits, horses, piglets, sheep or lamb, ferrets, cats, and donkeys). Considerable variability regarding their development and measurement properties was observed. The Mouse, Rat, Horse and Feline Grimace Scales exhibited high level of evidence. The Rabbit, Lamb, Piglet and Ferret Grimace Scales and Sheep Pain Facial Expression Scale exhibited moderate level of evidence. The Sheep Grimace Scale, EQUUS-FAP, and EQUUS-Donkey-FAP exhibited low level of evidence for measurement properties. Construct validity was the most reported measurement property. Reliability and other forms of validity have been understudied. This systematic review identified gaps in knowledge on the measurement properties of grimace scales. Further studies should focus on improving psychometric testing, instrument refinement, and the use of grimace scales for pain assessment in nonhuman mammals.

Automated summary

This systematic review aimed to assess the measurement properties of grimace scales for pain assessment in animals. The review found significant variability in the development and measurement properties of these scales across different species. The Mouse, Rat, Horse, and Feline Grimace Scales showed strong evidence, while the Rabbit, Lamb, Piglet, and Ferret Grimace Scales, as well as the Sheep Pain Facial Expression Scale, had moderate evidence. The Sheep Grimace Scale, EQUUS-FAP, and EQUUS-Donkey-FAP had low evidence for measurement properties. The most reported measurement property was construct validity, while reliability and other forms of validity were understudied.