Pain management is both, an ethical imperative to assure horse welfare, as well as essential to achieve an optimal outcome in surgical cases. In 2020, the International Association for the Study of Pain (IASP) updated the definition of pain for the first time since 1979. The definition states that pain is: “An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage,” (Raja et al. 2020). In addition, the IASP complemented the definition with the addition of six key notes for further valuable context.
One of them states that pain and nociception are different phenomena; pain cannot be inferred solely from the activity of sensory neurons. Nociception is the neural process of encoding noxious stimuli, and involves the stages transduction, transmission, modulation, projection and perception of the stimulus. For pain to be perceived, consciousness is required, however nociception can still occur under general anaesthesia. And nociception is a highly plastic and dynamic system (Woolf & Salter 2000), hence acute physiological pain can progress to chronic pain, constituting in itself a pathological entity.
The last key note on the pain definition adds: “verbal description is only one of several behaviours to express pain; inability to communicate does not negate the possibility that a human or a nonhuman animal experiences pain.” This note implies progress and recognition in our field, since for the first time it explicitly mentions animal pain. Horses are susceptible to suffer pain, and inadequate pain management has relevant implications at multiple levels. Firstly, the loss of good welfare, but besides the ethical arguments, there are a number of adverse behavioural and pathophysiological consequences of untreated pain. These include poor immune function, delayed wound healing, impaired gastrointestinal function or decreased range of movement, among others. In consequence, pain leads to poorer recoveries and longer hospital stays, negatively affecting the clinical outcome. Therefore, pain management is vital, and in order to adequately treat pain, it must first be recognized.
Pain recognition is one of the greatest challenges for veterinarians, inherent to the individuality of the experience and the inability of verbal communication. The horse poses a particular challenge, since, being a prey species, it has evolved to hide pain from potential predators. Variations in how horses express pain are considerable, influenced by the environment and other factors such as breed or personality (Ijichi et al. 2014). Therefore, reliable clinical methods for pain assessment are essential in recognizing horses in pain.
In surgical cases, pain assessment should ideally be performed preoperatively, to establish a baseline, and postoperatively, to tailor analgesic treatment and evaluate the progression of the horse condition. It should be conducted in an environment free from stress and distractors, as both are known to affect pain expression. Traditional clinical methods to recognized pain include behavioural and physiological parameters. Physiological parameters (heart rate, respiratory rate) are highly unspecific; therefore, they are more useful when combined with other type of assessment. Observation of the horse, its position in the box stall, posture, gait, attention to surroundings and activity levels can provide valuable information, although highly subjective. Besides, signs of pain may not always be evident, especially in the case of non-severe pain or in stoic horses. Therefore, the systematic assessment of pain by scoring systems is recommended.
Pain scales have been introduced with the aim of adding objectivity to pain evaluation. In the last decade, numerous scales have been developed for the assessment of pain in horses and have been reviewed in depth by de Grauw & van Loon 2016, Gleerup & Lindegaard 2016, and van Loon & Van Dierendonck 2018. Possible advantages of using pain scales include: improving the observer’s ability to recognize and quantify pain, improving interobserver agreement, raising awareness of animal pain, encouraging pain treatment, and documenting the progress in animal's condition (Wagner 2010).
Ideally, the pain scale to be used must be reliable, this means reproducible (having high inter- and intra-observer reliability), showing high specificity and sensitivity to the type of pain being assessed, being able to identify a response to intervention, and being validated. Furthermore, it is also important for the pain scale to have practical applicability in order to be incorporated into a routine clinical assessment. That implies being easy to follow, understandable, no time-consuming and having instant results.
Currently available pain scoring systems can be broadly classified as unidimensional pain scales, composite pain scales and facial expression scales. Unidimensional pain scales include the visual analogue scale (VAS), the numerical rating scale (NRS) and the simple descriptive scale (SDS). These are generally neither species-specific scales, nor pain-type specific scales, therefore their reliability and validity tend to be suboptimal. Composite pains scales (CPS) evaluate several pain-related parameters, which can include physiological, behavioural and interactive, these are then combined to an overall score. CPS generally increase objectivity of the evaluation and represent an improvement in reliability. However, training and experience on their use is required, and they tend to be more time consuming. Facial expression scales have been developed in humans and animal species (Descovich et al. 2017), including equids. They have gained popularity due to the practical advantages, being less time consuming, requiring less training, and allowing assessment from distance, while generally maintaining reliability. A list of currently developed equine pain scales is available on table 1.
The recognition of pain in equids has advanced considerably in recent years due to the development of pain scoring systems, improving our ability to identify and quantify pain, and allowing for more conscious, tailored and safe management of equine pain.
References