Enhancing the Horse Grimace Scale (HGS): Proposed updates and anatomical descriptors for pain assessment

The use of grimace scales enables the clinical identification of changes in the facial expressions of animals caused by pain. The Horse Grimace Scale (HGS) is one such tool, comprising a pain coding system based on facial expressions and assessing six Facial Action Units (FAUs). Each FAU is accompanied by descriptions and anatomical details to assist the evaluator. However, the morphological descriptions for certain FAUs in the HGS are not sufficiently detailed, potentially hindering accurate interpretation. This study is an analytical investigation aimed at enhancing the morphoanatomical details in the HGS and providing raters with more comprehensive materials for pain evaluation in horses using this scale. To achieve this, detailed anatomical analyses were conducted using established references in veterinary anatomy. Initially, we propose substituting the term 'ear' with 'auricle' or 'pinna' and replacing 'area above the eye' with 'supraorbital region' for anatomical accuracy. Additionally, we introduce detailed morphoanatomical descriptions that identify specific landmarks, with the goal of ensuring more consistent application of the HGS and reducing interpretation variability. Furthermore, this study provides an explanation of the muscles involved in the investigated FAUs. These adjustments on the descriptions and evaluations remain unverified, however it is anticipated that the descriptive enhancements lead us to understand that higher interobserver reliability can be achieved for each of the FAUs.