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indicated (Tremaine and Casey 2012; Henry and Rice 2017). The effect of oral and dental disorders on the horse’s performance and rideability can be convincingly established if the treatment proves to be successful in reducing signs, bearing in mind that correction of dental abnormalities may not immediately influence the movement or rideability of the horse (Moine et al. 2017). The potential presence of bit- induced pain might be relevant to investigate further by comparing the ridden horse’s behaviour with and without a bit (Cook and Kibler 2019). In some instances, it could also be valuable to perform diagnostic nerve blocks on the tissues of the oral cavity prior to a ridden examination to test its results on the horse’s performance and rideability (Foster 2013). However, this could potentially cause the rider to have less control of the horse and should therefore be reserved as a diagnostic procedure of last resort and only be performed with extreme caution (Foster 2013). Since EGUS is a highly prevalent disease in adult horses,
performing a gastroscopy is often worthy of inclusion in the standard work-up if history and/or clinical examination does not negate its relevance to the problem. The presence, severity or location of gastric ulcers in horses may, however, not necessarily correlate with the clinical presentation (Sykes et al. 2015), wherefore the likely effect of EGUS on performance and rideability first can be fully appreciated after an assessment of response to treatment (Murray 1991; Franklin et al. 2008). A minor fraction of horses with poor performance or
rideability issues remain undiagnosed after a standard work- up. These horses merit further examination focusing on the potential presence of visceral pain associated with various other types of internal medical issues, for example disorders involving the gastrointestinal or the urogenital system causing primary and/or referred pain (Lindegaard et al. 2009). Such an investigation must be tailored to the individual horse based on clues obtained from the history and the clinical examination. Visceral pain is often diffuse, poorly localised and can be referred to distant somatic sites (Dunckley et al. 2005; Christoffersen et al. 2007; Lindegaard et al. 2009), which may complicate the establishment of a diagnosis. However, certain patterns of referred pain could prove useful in a diagnostic setting as it might be indicative of a specific organ system as the originating site of pain (Kaufman and Jones 2018). For instance, hypersensitivity and/or hyperalgesia in the lower back, flanks and/or hindquarters have been linked to referred urogenital pain in horses (Christoffersen et al. 2007; Lindegaard et al. 2009). It is reasonable to assume that other visceral pain conditions in horses could also cause referred pain manifested as hypersensitivity/hyperalgesia in specific somatic areas, for example gastric ulcers causing girthiness (Lindegaard et al. 2017; Millares-Ramirez and Le Jeune 2019). Thus, referred pain should be considered in horses exhibiting hypersensitivity/hyperalgesia, and specific referral patterns could potentially help point to the site of visceral pain. Initial diagnostic testing for visceral pain conditions often
includes routine haematologic and biochemical blood tests with measurement of inflammatory markers, electrolytes and organ-specific enzyme activities, which may, however, prove unrewarding. The diagnostic work-up of horses with suspected pain-related chronic gastrointestinal disorders may additionally include faecal worm egg count, faecal sand sedimentation test, transrectal palpation, abdominal
ultrasonography and/or enteral biopsies (Archer 2008). In cases of suspected urogenital disorders, the basic diagnostic work-up may generally include the examination of the external genitalia, transrectal palpation and ultrasonography of the internal urogenital system, and a urinalysis (Christoffersen et al. 2007; Wilson 2007). In geldings exhibiting worsened performance or rideability issues after castration, it may also be relevant to include an evaluation of the potential presence of painful spermatic cord adhesions or neuromas by assessing response to palpation and local analgesia of the spermatic cord stump or inguinal region (Echte et al. 2006; Bengtsdotter et al. 2019). The diagnostic work-up of some particularly difficult cases
may require alternative diagnostic methods and tools. Even when the most common diagnoses have been ruled out during standard work-up, it does not assure that the presenting problem is not associated with pain. As a means of determining whether the signs are related to pain, caudal epidural analgesia could potentially be used as a valuable complementary diagnostic tool; if the horse’s signs are reduced or eliminated by the epidural analgesia, it confirms an underlying pain-related condition localised to the structures analgised by the epidural analgesia. A comparison of the ridden horse’s behaviour, performance and rideability before, during and after the epidural treatment should be assessed as objectively as possible, for example by comparing pain scores from an applied ridden horse ethogram (Dyson et al. 2018a, 2018b). To function as a diagnostic tool, it is required that the epidural drugs do not affect mentation or locomotor function; such side effects will complicate the assessment of analgesia- related improvements of performance and rideability and cause a potential hazard for the rider, thereby dismissing it as a safe and valuable diagnostic tool. Epidural co-administration of 0.1 mg/kg methadone and 0.1 mg/kg morphine diluted with saline to a total volume of 4.4 mL/100 kg has shown great potential for diagnostic use as it provides efficient analgesia at the coccygeal, perineal, sacral, lumbar and thoracic regions without accompanying adverse effects on mentation and locomotor function (Kjærulff et al. 2021). Due to the extensive regional analgesia obtained, many internal medical issues causing visceral pain, as well as some causes of musculoskeletal pain, are likely amenable to epidural administration of methadone and morphine. Thus, epidural analgesia could prove useful in confirming the presence of an underlying pain-related disorder in difficult cases of poor performance or rideability issues, where a comprehensive investigation has led no further to a diagnosis or where diagnostic analgesia cannot be performed, either because the horse’s temperament does not allow it or because the source of pain is inaccessible. The diagnostic potential of epidural analgesia in equine medicine is intriguing but is still an area that requires further research into its role in the work-up of horses presented with performance or rideability issues before implementation.
Discussion and conclusion
As described, changes in horses’ ridden performance or behaviour are more often caused by various pain-related conditions than attributed to true behavioural problems. Failing to recognise and investigate performance or rideability issues as potential manifestations of pain thus constitutes a major threat to ridden horses’ health and
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