Application of an equine composite pain scale and its association with plasma adrenocorticotropic hormone concentrations and serum cortisol concentrations in horses with colic

This study assessed the application of a modified equine composite pain scale (CPS) and identified the inter-observer reliability. Associations between CPS scores and the measured concentrations of serum cortisol ([cortisol]) and plasma adrenocorticotrophic hormone ([ACTH]) in horses presenting with colic were determined. The study design was prospective, uni-centred and observational. The inter-observer reliability of the adapted CPS was determined for 59 horses hospitalised for a variety of conditions. The associations between CPS, ACTH and cortisol were assessed in a further 49 horses admitted for medical or surgical colic. During hospitalisation, blood samples were obtained each morning and analysed for serum [cortisol] and plasma [ACTH]. Horses were pain scored using the adapted CPS score. Data from the most painful time point (n = 48 horses; n = 48 [cortisol]; n = 44 [ACTH]) and all data time points (n = 49 horses and n = 133 time points) were used for analysis of association between [cortisol], [ACTH] and CPS score. The CPS score inter-observer reliability was excellent (n = 59 horses; n = 102 pain scores; weighted kappa 0.863). CPS score and [cortisol] were positively associated at the most painful time point (P < 0.001) and at all data time points (P < 0.001). No significant association was found between CPS score and [ACTH]. [ACTH] was associated with [cortisol] (P = 0.034) when all time points were analysed but not when only the most painful point was analysed. The significant correlation identified between CPS score and [cortisol] in medical and surgical colic cases provides physiological validation of pain scores as a marker of underlying stress in horses with colic.