THIS article is the first in a series of three discussing key points about chronic pain in dogs. Part 1 covers the recognition and quantification of chronic pain; part 2 covers the pharmacotherapy of chronic pain in dogs with a focus on non-steroidal anti-inflammatory drugs (NSAIDs), particularly in animals with concurrent disease; and part 3 will discuss the monitoring of animals receiving NSAIDs, particularly with respect to safety aspects associated with NSAID administration.
What is chronic pain?
Within the veterinary profession there is widespread agreement that recognition of pain is a highly subjective and complex field that is continually evolving. Whilst most clinicians believe that their ability to identify and quantify acute pain in dogs is good, many would admit they lack the tools to accurately recognise and quantify chronic pain. Indeed, what is the definition of chronic pain? Previously, chronic pain was described as pain lasting more than three or six months, but now many clinicians are diagnosing chronic pain as pain that extends beyond the period of tissue healing. It should be emphasised that chronic pain doesn’t have a purpose and offers the animal no advantage in terms of survival. Chronic pain is not just a prolonged extension of acute pain, it signals that wide-ranging physiochemical changes have occurred in the central and peripheral nervous systems, from the level of the peripheral nociceptors, to the cerebral cortex, causing a persistent hypersensitivity to pain.
Identification and quantification
So chronic pain is a complex experience with emotional and cognitive components that can have a substantial impact on the patient’s quality of life; but how does the veterinary surgeon, nurse or owner actually identify and measure the degree of pain in their patients or pets? When trying to recognise and quantify chronic pain it is important to remember that there are three different aspects that contribute and determine the experience of pain in animals, as in man. These are broadly classified as: (1) the intensity and location of pain (i.e. where does it hurt and how much does it hurt?); (2) the emotional component of pain, the element that causes the suffering associated with chronic pain; and (3) the cognitive aspect of pain. It is difficult to conceptualise the cognitive aspect of pain in animals, but in humans it refers to the interaction between sensory and cognitive factors that determine the pain experience, for example distraction techniques to reduce pain severity. Changes in activity and behaviour are most commonly used to identify chronic pain in dogs. A list of characteristics commonly associated with pain in dogs is shown in Table 1, but in actual fact any change in behaviour, or an absence of a behaviour, may be suggestive of pain! It is important to consider the potential for pain in all dogs presented with chronic disease and evaluate these animals carefully in conjunction with the owner. In order to incorporate the emotional and cognitive aspects of pain into quantification of chronic pain in dogs, recent assessment tools focus on measurement of quality of life (QoL). There has been most emphasis on the development of QoL instruments for quantification of pain associated with osteoarthritis, reflecting the prevalence of the condition in the dog population. However, despite numerous attempts to develop, validate and utilise QoL questionnaires in dogs, we still lack a “gold standard” for criterion validity. At the end of this article references to some of the questionnaires are provided for both osteoarthritis and cancer pain in dogs. It can be useful to incorporate a QoL questionnaire into your clinical practice to facilitate management of chronic pain in dogs with different disease conditions.
Veterinary surgeons are well placed to assess the intensity and location of pain, but it is imperative to involve the owner in decision making about requirement for, and response to, pain management strategies, including drug therapy. Studies in dogs with osteoarthritis have shown that owners are very adept at quantifying pain in their own animals (Hielm-Bjorkman et al, 2003), and due to the close interaction between owners and dogs, they are best placed to evaluate the emotional impact of pain on their pet. However, some owner education may be required. Although careful questioning usually prompts the owner to recall changes in the behaviour of their pet that are likely to be indicative of pain, in geriatric patients these changes in behaviour are commonly wrongly interpreted as normal changes associated with ageing. Many behavioural changes associated with pain and an accompanying decreased QoL are very subtle, such that it is important to emphasise the language and words that are required to allow the owner to effectively communicate these changes to the veterinary surgeon. Changes in activity are a useful outcome measure to evaluate the success of therapeutic interventions in dogs with osteoarthritis and Lascelles and colleagues (2008) recommend using “client specific
outcome measures” (CSOM) as a sensitive tool to evaluate analgesia in dogs with osteoarthritis. CSOM require the owner to
identify time and place specific behaviours (usually four or five of them) that they consider to be altered in their own pet and to grade the degree of impairment compared to a precise age when they considered their dog to be normal. For example, the owner might identify that their dog no longer jumps on the bed at night. The owner is then required to rate the degree of impairment in this activity on a scale (for example, no problem, a little problematic, quite problematic, severely problematic, impossible).
At follow-up appointments the owner is asked to re-grade the degree of impairment in this activity (and the others) in order to evaluate the successfulness of the analgesic intervention and whether further strategies to improve pain management are required. In referral centres, objective measures of pain such as force plate and gait analysis may be used to quantify chronic pain associated with osteoarthritis. It is important to remember that changes in gait do not relate to the dog’s own emotional and cognitive experience of pain but reflect pain intensity and location as well as mechanical dysfunction associated with osteoarthritis.
Brown, D. C., Boston, R. C., Coyne, J. C. et al (2008) Ability of the Canine Brief Inventory to detect response to treatment in dogs with osteoarthritis. J Am Vet Med Assoc 233 (8): 1,278- 1,283.
Fox (2010) Chronic pain in small animal medicine. Manson publishing Ltd, London.
Hielm-Björkman, A. K., Kuusela, E., Liman, A. et al (2003) Evaluation of methods for assessment of pain associated with chronic osteoarthritis in dogs. J Am Vet Med Assoc 222:
Lascelles, B. D. X., Gaynor, J. S., Smith, E. S. et al (2008) Amantadine in a Multimodal Analgesic Regimen for
Alleviation of Refractory Osteoarthritis Pain in Dogs. J Vet Intern Med 22: 53-59.
Wiseman-Orr, L. M., Nolan, A. M., Reid, J. et al (2004) Development of a questionnaire to measure the effects of chronic pain on health-related quality of life in dogs. Am J Vet Res 65 (8): 1,077-1,084.
Yazbek, K. V. and Fantoni, D. T. (2005). Validity of a health-related quality-of-life scale for dogs with signs of pain secondary to cancer. J Am Vet Med Assoc 226 (8): 1,354-1,358.