Dr. Justin High, DVM, is a veterinarian and partner in Reata Equine Hospital in Weatherford, Texas.

Health Matters: Understanding Pain

I have always thought that people and horses are greatly alike in many aspects.  In the simple terms of work ethic and pain tolerance we can see much more variance within a species than there is between species.  The most pain tolerant of horses and people are much more alike than the difference between all horses or every person.  

While pain tolerance may be an admirable quality alone, it does not specifically lend itself to a better prognosis or more rapid recovery.  Defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, pain is a multifaceted adversary. 

So, as you and I strive to better care for our horses we must be more diligent in the accurate recognition and quantification of pain so as to best manage the inciting cause.  This is no small feat as we are dealing with a nonverbal animal that minimizes pain expression as a means to avoid predation. Additionally, the equine world – like it or not – lags behind on the recognition and quantification of pain as compared to similar work in small animals, humans, and farm animals. Fortunately, within the last few years there has been a concerted effort to develop repeatable systems that account for the physiological, emotional and behavioral changes that result from pain of all types.  

The problem with pain is that it comes in an endless variety of forms, types, and severity.  The component of time acts as a force multiplier. Therefore, when it comes to pain’s limitless expression we must know full well what the baseline value in our horse/patient is to best identify a deviation from normal.  Without question, the simple physical exam parameters of heart rate and respiratory rate are still valid in pain assessment, but ultimately need to be rolled into a composite assessment. 

 In general, there are quantitative and qualitative parameters for evaluating pain.  Quantitative things are those we can count – heart rate, breathing, levels of circulating hormones, and serologic measurement of mediators of inflammation. These recordings produce numbers we can develop a scale to grade the severity of the body’s response. Qualitative things are those we appreciate through visual assessment – changes in behavior, posture and demeanor.  Gait and willingness to interact with humans are also qualitatively affected by pain. As we all know, horses vary greatly in their response and tolerance to pain, thereby reinforcing the point to know your horse prior to onset as they truly do serve as their own best study control.

Combining the parameters above with observations of behavior, posture and socialization several pain assessment scales have been developed.  They can be as simple as a number assigned to a linear graph based on the observer’s impression of pain (similar to what a nurse asks you in the emergency room).  Others are descriptive scales like that of the American Association of Equine Practitioners (AAEP) lameness grading scale commonly used in practice.  As in a grade 4 of 5 lame horse is painful at the walk. Clearly, pain affects behavior and will lead to deviations in terms of time spent doing certain things other than those behaviors considered normal.  This study is what’s called a time budget analysis.  In other words, how much time does a horse spend/budget on a specific behavior, such as time eating, ears pointed forward, or head above the withers? The observed time the horse spent exhibiting painful behaviors such as head lowered, ears back, grinding teeth, etc. will be compared against standard non-painful behavior to render a pain score. Again, knowing normal is crucial here. 

Interestingly enough, the tool that shows as much promise as any is the facial expression pain/horse grimace scale.  This scale have been in use for several years now for humans and other species. It is best suited to detect mild rather than severe pain.  The Horse Grimace Scale (HGS) evaluates six criteria dealing with the expressive features of a horse’s face such as the ears, eyes, nostrils and mouth to develop a total pain score.  The promise of this test lies in its ease of use, low cost of training and materials, and most importantly the high reliability within and between observers.    

Currently there are at least seven different pain scales in use.  So, when there isn’t one best way there must be multiple good ways, and that is what we have now.  In defense of the authors of those scales think for a moment about the most substantial causes of pain in the horse.  Laminitis, joint pain (synovitis), acute colic, abdominal surgery (colic surgery), and castration are the most common/severe. Because these diseases can have such varied physiologic, behavioral, and socializing effect within the same horse no one best scale exists to accurately represent them all. 

The prompt and accurate recognition of pain in our horses cannot be understated.  The equitarian effort is served in the alleviation of pain for that fact alone. The quicker we identify pain and eliminate the causative process responsible for altering the normal, healthy state of our horses the better prognosis they will have.  Survivability of serious conditions is related to lower pain scores.  The ultimate development of a system or scale which uses the least amount of parameters so as not to cloud the issues or overwhelm the observer so consistency can be maintained among horses regardless of observer is a goal we should all work towards.    

Presented by Kemin: kemin.com