Pain is a complex, multi-dimensional experience involving sensory and affective (emotional) components. The sensory component tells us when and where the pain occurs and whether its cause is mechanical or thermal. The affective component refers to the unpleasant feelings that cause the suffering we associate with pain; hence the expression “pain is not just about how it feels, but how it makes you feel”. It is a uniquely personal experience – we cannot appreciate how it is perceived by another person or animal – but most people now believe we should assume animals suffer pain in a similar way to ourselves.
The subjectivity of the pain experience makes its measurement an enormous challenge – how can you measure another’s feelings? According to McCaffery in 1968, “Pain is whatever the experiencing person says it is, existing whenever he/she says it does”, so highlighting the importance of self-report if that is possible, which of course is not the case in our patients.
At its simplest, pain is classified as either acute or chronic. Acute pain may be considered a symptom of disease or injury, but chronic pain is regarded as a disease in its own right. As well as having an effect on physical well-being in humans, chronic pain tends to have a significant impact on the emotional well-being of the sufferer, and people with chronic pain have reported that most, if not all, aspects of their lives are significantly affected.
It is not surprising then that many of the instruments now used to measure human chronic pain are concerned primarily with measuring not the pain per se, but rather its effect on the patient’s quality of life (QOL). Health-related quality of life (HRQL) is concerned with those aspects of QOL that change as a result of ill health and medical interventions.
Measuring the unmeasurable
Over the past three decades our medical colleagues have perfected the development of structured questionnaires to measure HRQL in people using well established psychometric methods that ensure their validity and reliability. Generally, these are designed for self-report, but where that is impossible, for example in the case of infants or the cognitively impaired, an observer who knows the patient best acts as a proxy. It is important to note that, according to the FDA (US Food and Drug Administration), “Observers cannot validly report an infant’s pain intensity but can report infant behavior thought to be caused by pain” and this makes sense for animals too.
Behavioural disturbances have long been recognised as potential indicators of the presence of pain in animals and it has been suggested that non-verbal behaviour is a form of self-report. Chronic pain behaviours are often subtle deviations from normal and may only be recognised by the owner who knows the animal best, making that person the best proxy.
With increasing companion animal longevity, the incidence of chronic painful disease is rising, with osteoarthritis (OA) at the top of that list in the dog. Various clinical metrology scales have been published for its measurement, for example, canine brief pain inventory (CBPI), Helsinki chronic
pain index (HCPI), Liverpool LOAD, canine orthopaedic index (COA). These scales tend to be primarily concerned with physical limitation rather than the effect on HRQL – a much broader concept that includes emotional as well as physical well-being.
Using Vetmetrica health-related quality of life instruments
These are valid, reliable and responsive web-based generic HRQL questionnaire instruments – one for the dog and one for the cat – designed for completion in around five minutes by the owner at home, using a computer or mobile platform. Figure 1 shows how the system works in practice. The dog questionnaire has 22 questions for the owner (20 for the cat) and these are based on easy-to-understand words or phrases. The owner rates each of these on a 0 to 6 scale where 0 equals “not at all” and 6 equals “couldn’t be more” (Figure 2). On completion of the questionnaire, results are calculated instantaneously and delivered automatically to the vet and, at the vet’s discretion, to the owner. Results comprise an HRQL profile with scores in domains of QOL, four for the dog (energetic/enthusiastic, happy/content, active/comfortable, calm/relaxed) and three for the cat (vitality, comfort, emotional well-being).
To be useful, a health measurement instrument must be easy to interpret in a clinical context. Vetmetrica instru ment scores are norm-based, such that 50 represents the score for the average healthy animal, which in the case of the dog is also age related (0 to 7 vs 8 years or more). Results can also be interpreted in the light of the minimum important difference (MID). This is defined as “the smallest difference in score in the outcome of interest that informed patients or informed proxies perceive as important, either beneficial or harmful, and which would lead the patient or clinician to consider a change”.
Figure 3 shows the vet output for a dog clinical case recorded over an 11-month period. Scores for each domain are colour coded and a dotted threshold line at a score of 44.8 indicates that 70 percent of healthy dogs will score above this line. It is easy to see how scores improved over time with treatment and to relate these to healthy dogs of the same age group. The MID for the dog is 7 so, where scores indicate an improvement of more than 7, this represents a clinically important change. However, this is not an exact science so all interpretation should be made in line with clinical judgement allowing a degree of flexibility.
Because of its prevalence in dogs and cats, osteoarthritis tends to dominate our thoughts of chronic pain, but many other prevalent chronic diseases such as cancer, cardiovascular disease, neurological and dermatological conditions may also be associated with pain and shouldn’t be forgotten. Unfortunately, chronic disease is generally not curable and so our task as veterinary surgeons is to alleviate the symptoms as best we can and make our patients feel better by improving their QOL.