Four horse pain scales put to the test by researchers

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The Composite Orthopedic Pain Scale emerged as the most accurate and reliable among four tested scoring systems in horses in which hind-limb lameness was temporarily induced.

The study, by researchers at the Swedish University of Agricultural Sciences, set out to investigate the relationship between orthopedic pain experienced at rest, and the degree of movement asymmetry during the trot in horses with induced lameness.

Katrina Ask and her fellow researchers, writing in the journal Frontiers in Veterinary Science, noted that changes in behavior and facial expressions have been recognized and assessed with an ethogram in ridden horses with clinical orthopedic pain. However, they have not yet been associated to different degrees of movement asymmetry detected by objective gait analysis.

“Different pain pathologies may generate different pain displays and a pain assessment tool may therefore only be valid for the pain types specified in the validation study.”

Pain, they said, is associated with several general features, but the anatomical location of the pain will induce different compensatory body behaviors, such as increased movement asymmetry due to decreased weight bearing during orthopedic pain.

Facial displays of pain, on the other hand, are thought to be general for acute pain or acute exacerbations of chronic pain in most mammals, including horses.

Indeed, grimace-based pain scales developed for horses experiencing post-surgical castration pain and acute visceral pain seem to successfully identify laminitis, post-surgical orthopedic pain, and head-related pain.

“Whether a behavior or grimace-based pain scale performs better on the same type of orthopedic pain has not been evaluated, but assessment of behaviors and facial expressions together has been recommended to optimize pain detection.”

In their study, orthopedic pain was assessed using the Horse Grimace Scale, the Equine Utrecht University Scale of Facial Assessment of Pain, the Equine Pain Scale, and the Composite Orthopedic Pain Scale.

Eight healthy horses, including seven Standardbreds and one Warmblood, were enrolled in the study, with each horse acting as its own control.

Temporary orthopedic pain was induced in a hind leg joint in each horse by injecting lipopolysaccharide, which triggered inflammation.

A series of pain assessments were performed before induction and during pain progression and regression, where three observers, all with professional equine experience, independently and simultaneously assessed pain at rest with the four scales.

Movement asymmetry was measured once before induction and a minimum of four times after induction, using objective gait analysis based on motion-capture technology and the tracking of standardized markers on the horses.

On average, six or seven objective gait analyzes and 12 pain assessments were performed per horse.

The researchers found that the total pain scores from all scales were significantly associated with an increase in movement asymmetry, with the Composite Orthopedic Pain Scale scores being most closely associated with movement asymmetry. It was the only scale where all observers had a good diagnostic accuracy, they said, and it was assessed as the most accurate and reliable pain scale in their experiment.

“This study identified significant associations between pain experienced at rest and degree of movement asymmetry for all scales,” they said.

“All scales had varying linear and non-linear relationships between total pain scores and movement asymmetry, illustrating challenges with orthopedic pain assessment during rest in subtly lame horses, since movement asymmetry needs to be rather high before total pain score increases.”

Discussing their findings, the authors said their study showed varying performance of the four pain scales when assessing low-degree orthopedic pain, but significant linear and non-linear relationships were identified between increases in movement asymmetry and total pain scores given at rest for all scales .

Varying degrees of pain behavior were observed at rest, with the majority of pain scores at the low end of each scale’s score range.

“Total pain scores of zero were sometimes seen post-induction,” they said, “which may indicate that the horses in our study did not constantly experience pain at rest.”

Both non-linear and linear relationships were seen, varying between both scales and observers. Hence, an increase in pain score of 20% did not necessarily imply an increase in pain intensity of 20%.

“Therefore, more research is needed on the clinical meaning of a numerical pain score, especially during pain progression and regression.

“Furthermore, the relationship between movement asymmetry and lipopolysaccharide-induced pain identified in this study may be very different in horses with chronic lameness, such as osteoarthritis.

“Lipopolysaccharide-induced pain is an acute pain experience not previously encountered by the horse, while most lameness types involve more long-lasting pain experiences where the horse has time to develop a coping behavior.

“Different degrees of pain may also be present depending on the pain process. For instance, osteoarthritic bone processes may only be painful during motion, whereas lipopolysaccharide-induced synovitis is painful during loading at rest and in motion. This will affect the outcome of pain assessment during rest.”

The order of pain assessment and objective gait analysis seemed important for the results obtained using the Horse Grimace Scale, the Equine Utrecht University Scale of Facial Assessment of Pain and the Equine Pain Scale, they noted.

“We tested the hypothesis that movement increases pain scores, but found that pain scores were significantly higher before objective gait analysis. This finding may be interpreted in different ways. Movement may decrease joint distension and result in transient pain relief. Alternatively, movement may contribute to concealment of facial or other cues, due to external input, tiredness, or stress during measurements.”

The Horse Grimace Scale had significantly higher scores before movement for all except one of the five observers used in the experiment, indicating that pain-related grimaces detected with that tool may decrease or be concealed after movement.

They said the possible influence, especially of stress, on tool performance should be investigated further before pain assessment tools are incorporated into lameness evaluations.

While the Composite Orthopedic Pain Scale proved most reliable, the Equine Pain Scale proved moderately reliable in the experiment, they said. The poor and moderate agreement seen for the Equine Utrecht University Scale of Facial Assessment of Pain and the Horse Grimace Scale is inconsistent with previous results showing good or excellent reliability, they noted.

“These scales only assess facial expressions, which may affect the reliability since facial configuration seems to be more difficult to assess than body movements.”

The more ambiguity there is in descriptions of a category and its scoring, the more training of observers is needed, they said. “It may be argued that scales should be designed in such a way that any observer can use them correctly. It has been suggested that before assuming that a pain scale is generalizable, it should be tested with untrained observers unfamiliar with the scale.

“Nonetheless, observers are often trained prior to reliability testing, but standardized training protocols are rarely published.”

The researchers said movement asymmetry explained only a minor part of the variation in pain scores at rest.

“Increases in pain score and movement asymmetry did not occur simultaneously and a horse may have rather high movement asymmetry before total pain scores increase.

“This is an important challenge when assessing orthopedic pain during rest in subtly lame horses, and underlines the relevance of identifying painful orthopedic lesions by other means, for example local or systemic analgesic testing,” they said.

The study team comprised Ask, Pia Haubro Andersen, Lena-Mari Tamminen, Marie Rhodin and Elin Hernlund, all with the Swedish University of Agricultural Sciences in Uppsala.

Ask K, Andersen PH, Tamminen LM, Rhodin M and Hernlund E (2022) Performance of four equine pain scales and their association to movement asymmetry in horses with induced orthopedic pain. Front. Vet. skiing 9:938022. two: 10.3389/fvets.2022.938022

The study, published under a Creative Commons Licensecan be read here.

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