Client-specific outcome measure for chronic osteoarthritis pain assessment in horses

Introduction: Osteoarthritis (OA) is a prevalent cause of chronic pain and lameness in horses. Whereas lameness can be quantified using objective measures, the assessment of OA-associated pain remains challenging. This study aimed to evaluate the applicability of the Client-Specific Outcome Measure (CSOM), a tool widely used in small animals, for the assessment of chronic OA pain in horses through caretaker evaluation. Methods: Seventeen privately owned horses with confirmed OA were enrolled in a 20-week randomized, double-blind, placebo-controlled study. For each horse, three individual pain-related indicators (CSOM items) were identified through a veterinarian-caretaker consultation. The selected items were regularly scored (0-4), and the scores were summed to obtain a total CSOM score (CSOM-sum). The CSOM-sum was compared with other pain assessment measures, including a caretaker-assigned visual analogue scale (VAS-own), a veterinarian-assigned visual analogue scale (VAS-exp), subjective lameness scores, and a gait asymmetry index. Results: The CSOM-sum showed moderate but significant correlations with all other pain and lameness measures (rs = 0.49-0.60, p < 0.05). In particular, CSOM-sum correlated with gait asymmetry (rs = 0.434, p < 0.0001); subgroup analysis revealed a moderate correlation in treated horses (rs = 0.4539, p = 0.0025) and a higher correlation in controls (rs = 0.5536, p = 0.0006). The VAS-own and VAS-exp scores showed good overall agreement (bias = -4.76 mm; ICC = 0.727; ICC_T = 0.625; ICC_C = 0.838), although with relatively wide limits of agreement. The internal consistency of the CSOM items was high (Cronbach's alpha = 0.81 overall; alpha_T = 0.787; alpha_C = 0.890). Discussion: These findings indicate that, provided relevant items are carefully identified, the CSOM may represent a valuable complementary tool for assessing and monitoring pain severity in horses under field conditions.