The difficulty of assessing chronic pain in animals continues to be a challenge despite advances in knowledge about pain mechanisms and treatment. Common conditions known to cause chronic pain include osteoarthritis and other musculoskeletal problems, chronic inflammatory skin and ear disease. As veterinary medicine advances with new treatments for cancer, for example, our patients are living longer and as a result we are faced with more “opportunities” to manage their pain. As vets, we have a limited selection of analgesics available with a marketing authorisation for veterinary species. In patients with refractory pain we frequently reach for analgesic adjuncts licensed for use in humans to use via the cascade.
Such medications are backed by weak or no evidence of clinical efficacy or safety in the species we are treating, thus we rely heavily on anecdotal reports. An example of a medication that has been used extensively by vets in small animal practice is tramadol. Evidence of the lack of efficacy and extremely low oral bioavailability of tramadol in dogs has been published in recent years. It is likely that any benefit seen in dogs is not via an opioidergic effect as is shown in cats and humans, but rather through descending inhibitory pathways involving serotonin and noradrenalin, effects which are also responsible for the deleterious dysphoric effects frequently reported (Lascelles and Epstein, 2016).
Concerns over abuse and diversion of prescribed opioids, anxiolytics, anticonvulsants and antidepressants mean vets need to be much more careful about prescribing these in future. Clearly more research is needed to explore the clinical usefulness of some of these unlicensed medications but in the current situation the best advice that may be given to vets would be to ensure you are assessing your patient regularly to judge efficacy of the treatment you administer. So here lies the challenge!
Making accurate assessments
Assessment tools for acute traumatic and post-surgical pain as treated in a veterinary clinic environment are available and it is encouraging to see these being used increasingly in small animal practice. Examples used by the author include the Glasgow composite measure pain scale – short form (CMPS) for dogs and more recently, the combined composite measure pain scale for cats (CMPS-Feline). However, chronic pain can be much more difficult to assess and tools designed for acute pain are not useful in this context. Many animals are stoic by nature and do not communicate their pain obviously. Anxiety in a clinic environment or in the presence of a stranger will usually inhibit an animal’s response to a painful examination and this is enhanced in prey species such as small mammals, horses and farm animals. Sarah Heath, of Behavioural Referrals, goes as far as to say that it is virtually impossible to detect chronic pain in a veterinary clinic consult room (personal communication).
Animals are expert adaptors. Behavioural adaptations are subtle and occur gradually as chronic pain persists beyond injury so that many owners are completely unaware that there is a problem. This is not so different for us as humans – consider how many friends and colleagues you know who experience chronic pain. If you were to take away their verbal communication, how would you know they were in pain? You might notice them moving around with difficulty or absent-mindedly rubbing a painful area. So it is with animals. It is far more useful to look for behavioural indicators of pain as they are exhibited in the animal’s normal environment in the place where they are least stressed.
There are some behaviours that can be used to assess and score in dogs and cats; these include:
- Vitality and mobility (eg levels of energy and alertness; ease and fluidity of movement, including lying, getting up, sitting, jumping, turning, using litter box)
- Mood and demeanour (anxious, withdrawn, “sad”, dull, confident, playful, social interactions with people and other pets, temperament)
- Self-care (eating, drinking, grooming – including over-grooming a painful area, toileting posture)
- Resting, observing, relaxing (both restlessness and sleeping more can be indicators of discomfort)
- Indicators of pain (signs of stiffness at different times of the day and relating to exercise, postural changes, lameness, avoiding certain activities eg walking across slippery surfaces, refusing stairs or jumping into the car, avoiding being jostled by other pets)
Questionnaire-based measurements of health, also called clinical metrology instruments (CMIs), are widely used to assess health and well-being in people. Those designed to assess chronic pain use a combination of measures of stiffness, pain, gait, function, quality of life and pain severity. There are a limited number of instruments designed for assessment of chronic pain in dogs and cats and even fewer for small mammals, horses and farm animals. Advantages of a CMI include the fact that it assesses natural behaviours in a routine environment during extended periods. They are patient centred and require no specialised equipment.
Overcoming the caregiver placebo effect
One significant problem in non-verbal species is scoring pain severity. In human medicine it is said that no one can feel another’s pain. Only the patient can rate their pain severity.
This means that it is difficult for carers to complete pain questionnaires for non-verbal humans including very young children, people with reduced consciousness or people with advanced dementia. This clearly also applies to animals and yet several respected validated CMIs include similar questions about pain severity. Furthermore, the strength of relationship between carer and patient means results are likely to be influenced by the carer’s perception and desired outcome for the patient. The caregiver placebo effect is strong, making even use of CMIs challenging. An innovative method to overcome this problem for analgesia research uses “masked” baseline, treatment/placebo and wash-out periods. In a study of meloxicam for cats with degenerative joint disease using both a client-specific and general CMI, both the treatment and control groups appeared to improve during the testing phase. However, when treatment was withdrawn (replaced by placebo), owners of cats which had received meloxicam were able to detect a recurrence of clinical signs compared to those that had received placebo (Gruen et al., 2014).
Objective measures include the use of kinematic gait analysis, pressure sensitive walkways, and force platform devices and activity monitors. However, results of these are influenced by factors such as multiple limb lameness and differences between animal handlers. There is a weak correlation between change in peak vertical force and change in results of two CMIs – the Liverpool osteoarthritis in dogs (LOAD) and canine brief pain inventory (CBPI) (Walton et al., 2013). A health-related quality of life (QOL) assessment tool (Vetmetrica) is designed to minimise respondent bias. This validated tool is applicable to any chronic disease and not just osteoarthritis. Measuring chronic pain through its impact on QOL means that you are evaluating the whole pain experience – emotional as well as physical.
Both owners and vets express frustration and helplessness in the face of their pets suffering with long-term conditions such as arthritis (Belshaw et al., 2018). Coaching owners to evaluate behavioural indicators of pain at home takes a great deal of time, a commodity which is in short supply in general practice.
A useful educational website aimed at owners of dogs with osteoarthritis has recently been developed (caninearthritis.co.uk). Vets can direct dog owners to the website where they will find advice in an easy to understand format with free resources available to download such as the “good-day bad-day diary” and “the osteoarthritis booklet” which is aimed to assist vets and owners to keep track of a dog’s treatment over time. Their Facebook community and discussion forum receives feedback from members reporting the benefit of being in a “community” of people with similarly affected pets.
Another organisation that offers opportunities for vets to explore the multimodal management of pain is the British Veterinary Sports Medicine and Rehabilitation Association. In their recent “journal club”, the subject of pain assessment was discussed. Their autumn meeting addressing pain related behaviours will be held jointly with the British Veterinary Behaviour Association.