18 Feb 2020
Karen Walsh
Job Title
Image: Syda Productions / Adobe Stock
As the canine and feline populations live longer, it is likely they will have an increased number of comorbidities, which may result in chronic pain. Untreated pain – whether acute or chronic – may be a reason why owners choose euthanasia for their older pet.
To ensure the welfare and well-being of our patients, it is important to identify and treat chronic pain; however, this is not always easy to achieve. Full owner involvement is required as these patients can be difficult to assess in the clinic setting and they may be required to give feedback on the effects of the treatment prescribed.
Chronic or maladaptive pain can be the result of an ongoing noxious stimulus, which is then sustained by neuroplastic (sensitisation and alterations in receptors) changes well after healing. No obvious inciting cause may exist for the chronic pain. Chronic pain is likely to decrease the quality of life of those experiencing it. See Panel 1 for some of the definitions associated with pain.
The clinician will need to convey to the pet owner the difficulties inherent in treatment as it can often be very time consuming and frustrating to treat. The mechanisms (Voscopoulos and Lema, 2010) associated with chronic pain are described in detail in other texts.
Chronic: Pain persistent for greater than three to six months. The initial cause may not be apparent.
Neuropathic: Pain associated with a lesion or disease on sensory or motor nerves. This can be central or peripheral in origin.
Allodynia: A usually non-painful stimulus that is painful.
Hyperalgesia: A greater than normal degree of pain associated with a painful stimulus.
Nociceptive: Pain associated with non-neural tissues, nociceptors themselves are activated.
Peripheral sensitisation: Alteration of the nociceptors or their environment resulting in increased responsiveness to stimulation.
Central sensitisation: Increased responsiveness of nociceptive neurons in the CNS to afferent inputs, mainly at the level of the spinal cord.
Detecting/measuring chronic pain in dogs and cats is perhaps the biggest challenge in management. Our patients are not able to self-report, so it is up to pet owners and veterinary staff to make some interpretation of the way a pet feels.
Health-related quality of life questionnaires appear to allow an interpretation of the patient’s well-being as a whole, and may make it easier to assess efficacy of treatment and possibly initiate therapies earlier in a disease process. In a recent review article by Lascelles et al (2019), it was highlighted that chronic pain is a multidimensional issue, with many contributory factors, and will be different with each patient as comorbidities will vary.
A variety of scoring systems are available, but many have been validated for assessing patients with OA. A comprehensive review of these has been conducted by Lascelles et al (2019; Panels 2a and 2b).
American College of Veterinary Surgeons canine orthopaedic index – OA
Canine brief pain inventory – OA and bone cancer
Helsinki chronic pain index – OA
Liverpool OA in dogs – OA
Montreal instrument for cat arthritis testing by care giver or veterinarian
A relatively new system is the Newmetrica online tool that uses questions to assess four domains: energy, happiness, comfort and calmness. This system generates an output that looks at the overall well-being of the pet, and an interesting aspect is it seems to help measure health improvement and decline. Importantly, the aspect of assessment that both these scales look at is quality of life and an interpretation of how the pet “feels”, not a quantifying of the degree of pain.
Many owners are most concerned with quality of life rather than duration, but this should be discussed when deciding on a treatment programme. Using one of the assessment tools may help establish some objective goals for treatment, such as the brief inventory of pain (Brown et al, 2008) or the Newmetrica system. Supplying a copy to the owner at this time might also improve compliance and follow-up.
It is almost inevitable that these patients will require multimodality treatment – this includes drug therapies, physical therapies and adjunctive therapy. A holistic approach is important in many of these elderly patients as they will often have multiple medical conditions that will influence quality of life.
Table 1 shows an overview of drug treatment options that are commonly used.
| Table 1. Commonly used analgesic drugs for chronic pain | |||
|---|---|---|---|
| Drug group | Example | Licensed product available | When to use |
| NSAIDs | Meloxicam, firocoxib | Y, multiple on the market. | Often first line treatment. |
| PLT tablets | Y, stop if not effective after three days. | When NSAIDs not effective, appropriate warning of side effects should be given. Not for use in cats. | |
| Paracetamol | PardaleV | Y with codeine. | If NSAIDs contraindicated. Not for use in cats. |
| Antiepileptics | Gabapentin, pregabalin | N | In combination for neuropathic and chronic pain. |
| Opioids | Tramadol, buprenorphine (oral transmucosal) | T- Tralieve 20mg and 80mg. Buprenorphine licensed, but not for oral transmucosal and chronic use. | In addition to NSAIDs and for breakthrough pain. |
| N-methyl-D-aspartate receptor antagonists | Amantadine, memantine | N | Chronic pain, neuropathic pain. |
| Tricyclic antidepressants | Amitriptyline | N | Neuropathic pain. |
Food or dietary supplements are defined as a product taken by mouth containing a dietary ingredient intended to augment the diet. A nutraceutical, as defined by the North American Nutraceutical Council, is a substance in a purified or extracted form that, when administered orally to patients, aims to provide them with the necessary elements for their structure and normal function.
Significant number of these are available in the UK and EU, and do not require evidence of efficacy for market authorisation. Vandeweerd et al (2012) conducted a systematic review of efficacy with relation to OA. They found that omega-3 fatty acid supplementation had the highest strength evidence for efficacy.
A large body of evidence exists to suggest that, once it has been established, the disease process will not be worsened by it, and that exercise and physical therapies will help improve quality of life. The exercise programme can be combined with weight control to reduce the load on joints. Tailoring exercise to the general physical abilities is, of course, important and will tend to reduce the incidence of detrimental side effects. This will have an additional effect of helping improve the pet-owner bond – which may, in turn, improve the owner’s ability to detect and assess discomfort in their pets. This type of exercise would include physiotherapy and hydrotherapy.
As with more conventional treatments for pain and disease, it is important that a full clinical examination and working diagnosis is established before embarking on adjunctive therapies.
Acupuncture: acupuncture can be defined as the insertion of a solid needle into the body with the purpose of therapy, disease prevention or maintenance of health (Acupuncture Regulatory Working Group, 2003). In the UK, acupuncture is an act of veterinary surgery. The mechanism of action of acupuncture is not fully understood, but an intact nervous system is a requirement for it to work. It appears some effects are mediated by endorphins and other opiates (opiate antagonists such as naloxone can block the effect of acupuncture). Evidence also exists that acupuncture upregulates messenger RNA for pre-encephalin – this is likely to be the mechanism of the increasingly prolonged effects of acupuncture over time. The placement of needles stimulates Aδ fibres, which are associated with acute pain. This means the onward transmission from C fibres is suppressed and so chronic pain is decreased. This is most effective when the needle is placed near the source of the pain.
Extracorporeal shockwave therapy: this type of treatment has been used to treat tendon and ligament injuries in humans. It involves the use of high pressure sound waves emitted at a high velocity. It can be applied in two ways:
The evidence for effectiveness is weak and conflicting in the dog.
Mesenchymal stem cell therapy: mesenchymal skin cell (MSC) therapy is a popular area of research in both human and veterinary treatment of arthritis.
The basic premise is to harvest mesenchymal stem cells (often harvested from fat cells) and then treated before injection into arthritic joints. The theory behind their use is that the growth factors and cytokines they produce may stimulate recovery and reduce inflammation.
MSC and carrier media: some studies have investigated the addition of bioactive carriers – for example, platelet-rich plasma. Platelets are thought to supply a broad spectrum of compounds – for example, growth factors – which may help improve the effect of MSCs. Bench top research success has not been translated into clinical success as yet. However, it is an area in which a greater amount of research is warranted.
Chronic pain is a multifaceted condition that is often difficult to treat. Where possible, the initiating cause should be treated, but equally, this may never be identified. A close working relationship with the pet owner is vitally important for success.
Many of the treatments – both pharmacological and non-pharmacological – still require further studies to show if they have a truly beneficial effect. However, it must be kept in mind that the variation in disease process and pain experienced is very variable between patients necessitating individual treatment plans.
