Chronic pain is defined as pain that lasts longer or is expected to last longer than 30 days. This definition is rather arbitrary and does not help us define chronic pain. We know that acute pain can lead to chronic pain and that untreated chronic pain can lead to neuropathic pain. The two most common conditions with chronic pain that we manage are osteoarthritis and cancer. Both these conditions can be progressive and cannot always be cured. However, if the primary cause can treated, it should be, as it can result in a cure.
Chronic pain represents a significant part of the management of geriatric patients but can present in any patient. Patients with chronic pain seldom present as being painful or having pain as the single symptom at presentation.
The process of recognising pain in dogs and cats starts with a thorough history and clinical examination. Chronic pain often presents as a change in behaviour or activity level in the patient. The owner should be questioned carefully. The owner is in the best position to evaluate the pain and response to therapy. This requires an objective scientific method to assess pain.
Science requires the objective measurement of physiological parameters. Ideally, a number will be accurate and repeatable to allow for statical analysis without any bias. A number of pain assessment scales have been created with this philosophy in mind. Simple descriptive scales that rate pain as either mild, moderate or severe have been used. The problem encountered with this scale is that it is subjective and based on the observer's own bias. Physiological parameters (heart rate, respiration, food and water intake) and behavioural parameters (posture, activity, social interaction, etc.) scores have been developed. Evidence indicates that these scales are neither consistent nor repeatable. Endocrine parameters have also been shown not to be useful in the evaluation of pain. Visual analog scales are commonly used in man where they have been shown to be very useful. With a visual analog scale, a cross is made on a line 10 cm long to indicate the level of pain. A score of 0 indicates no pain while a score of 10 indicates the most amount of pain that one could experience. Within an individual, changes in levels of pain and the effect of treatment can be assessed. These scales are useful in veterinary medicine. Numerical rating scales, in which an evaluator assigns a score of 0 to 10 on behalf of the patient, have been used. Again, these scores are subject to the observer's bias and are not consistent. The most reliable indicator of pain in animals is related to changes in behaviour.
Behavioural Assessment of Pain
A statistically evaluated behavioural pain scale for dogs with acute pain has been published. Ten categories or expressions of pain were defined. These categories were then associated with behaviours or signs of pain. This was then reduced to seven categories based on responses from private veterinarians and used of the scale to test the statistical validity of the scale. The final pain score questionnaire is known as the Glasgow Pain Scale. The behaviours and signs incorporated into this questionnaire are either above average indictors of pain (rigid, screaming, aggressive, crying, snapping), good indicators (hunched, tense, restless, crying, whimpering, chewing, licking, rubbing, depressed, disinterested, fearful, stiff, lame, slow to rise, flinch, growl), poor indicators (quiet, indifferent) or indicate an absence of pain (none or neither, comfortable, ignoring, happy, content, bouncy). In the assessment of these behaviours, the prior disposition of the patient needs to be taken into account. A dramatic change in this disposition may be an important indicator of pain. The aggressive dog that becomes quiet and withdrawn, the friendly dog that now snaps readily, the lap dog that hides in the corner of its cage and this list can carry on. The concept is a change from what was previously exhibited. A more detailed review of changes in pain behaviour has been published.
A number of scales are available for the assessment of chronic pain in dogs. These scales are the Helsinki Chronic Pain Index, The Cincinnati Orthopaedic Disability Index and the Canine Brief Pain Inventory. These scales assess the quality of life of the dog. By regularly applying these scales, an assessment in the progress of pain management can be made.
Pain Assessment in Cats
Pain may be particularly difficult to assess in cats due to their sedentary nature. This makes the diagnosis of osteoarthritis difficult, as lameness is not readily evident. Osteoarthritis is most probably as common in cats as it is dogs, but the diagnostic rate is significantly lower. The first basic step is to assume that if it is painful to us, it is painful to our cat. Behavioural observation is probably the best method of pain assessment, but this is seldom useful in the acute setting. Acute pain related to trauma may result in a depressed, immobile, and silent cat that is tense, distanced from its environment and that tries to hide and does not respond to stroking or attention. Alternatively, cats can be manic and aggressive, growling, hissing, and rolling around their cage. Cats with abdominal pain adopt a hunched sternal posture, with their head lung lower than their body, elbows drawn back, stifles forward and abdominal muscles tensed. Cats may lick, chew, and self-mutilate an injured area.
A chronic pain scale (FMPI questionnaire) has been developed by the North Carolina State University. This scale again reflects the effect of pain, activity and quality of life of the cat.
Reference are available upon request.