Analgesics for cats. Which is the correct choice?
Introduction
The treatment of pain in cats was largely ignored by veterinary professionals until only recently. This was because of the difficulty in recognising pain in cats and the limited availability of analgesics with correctly validated usages and dosages for cats. However, in recent years, there have been significant advances in the way pain is recognised and treated. In fact, it is now seen as the fourth vital sign after temperature, pulse and respiration. Recognition of the associated behavioural changes and the use of appropriate pain assessment scales are crucial in the identification of pain in cats.1
Analgesic treatment is considered a fundamental element in the management of many processes. However, we must remember that analgesia is affected by the anatomical and physiological idiosyncrasies specific to cats. This means that analgesic protocols for cats cannot be extrapolated from those for other species.1
Characteristics of pain in cats
Pain is a complex phenomenon that courses with pathophysiological changes that affect behaviour and quality of life.
Acute pain is usually associated with tissue damage and its purpose is to alter animal behaviour to minimise damage and optimise conditions for healing. The pain disappears when healing is complete. Pain associated with surgery, trauma or sudden illnesses are examples of acute pain.
Chronic pain, on the other hand, persists after the tissues have healed, has a mixed nociceptive component, can occur even in the absence of active disease and has no biological purpose and is therefore considered a maladaptive pain that affects the patient’s wellbeing.2,3 Examples include oncological pain, joint pain secondary to osteoarthritis, periodontal pain, persistent postsurgical pain and neuropathic pain.4
Choice of analgesics for cats
DIFFERENCES IN METABOLISATION
There are significant differences in the way many drugs are metabolised by cats compared to humans or dogs, which obviously affects the dose and frequency of administration.
- Substances that are eliminated after metabolic conjugation (e.g., paracetamol, carprofen, ketoprofen, morphine) are excreted more slowly in cats, which may lead to significant side effects (including death) if the dose is extrapolated from another species.
- Substances metabolised by oxidation (e.g., meloxicam, buprenorphine), however, are eliminated more quickly.1
- It should also be noted that chronic kidney disease can affect the pharmacokinetics of many anaesthetic and analgesic agents. CKD is highly prevalent (30–40%) in geriatric cats; consequently, this age group receives these drugs most frequently.
DRUGS
The goal of treatment for acute pain is to resolve the underlying cause and block the nociceptive signals in the nervous system. This is usually achieved through the use of opioids and nonsteroidal anti-inflammatory drugs (NSAIDs), although other drugs, such as local anaesthetics, α-2 agonists, ketamine or gabapentin, can sometimes be used.
The control of chronic pain requires multidisciplinary and holistic management of the patient’s quality of life.2
The different distributions, densities and number of opioid receptors in the central nervous system between species explains the differences in efficacies and side effects of opioid analgesics in cats compared to other species.1 At the correct dose, opioids provide analgesia in painful procedures. They may cause purring, kneading and rubbing behaviours, and also euphoria. However, morphine, oxymorphone and hydromorphone can cause vomiting and hypersalivation, which may affect the cat’s wellbeing.
μ-Receptor agonists (morphine, methadone, fentanyl, remifentanil, pethidine) provide dose-dependent analgesia and are recommended for the treatment of moderate-to-severe pain.
- Fentanyl transdermal patches (authorised for use in humans) have been tried on cats, but high individual variability in the absorption rate can lead to treatment failure. They have an onset of action of up to 7 hours and the effect lasts for the same length of time after being removed.
- Opioid infusions provide adequate analgesia with the added benefit of potential dose escalation.
- Epidural administration of opioids provides good analgesia, especially when combined with local anaesthetics. It is important to void the bladder before their administration to avoid the urinary retention caused by epidural morphine.1
Buprenorphine is a partial μ-agonist with a dose-independent effect and is recommended for managing mild-to-moderate pain or as part of multimodal analgesia. Like methadone, it can be administered through the oral mucosa, although intravenous or intramuscular routes are preferred.1
Nalbuphine and butorphanol are κ-agonists with limited analgesic effect and are generally used in sedation protocols together with a-2 agonists or acepromazine.1
Tramadol is a dual-action analgesic; it acts as a weak synthetic opioid agonist and as a noradrenaline and serotonin reuptake inhibitor. Liquid formulations have a bitter taste that can cause profuse salivation, limiting their use in cats. Tramadol is believed to have a better analgesic effect in cats than in dogs, but it is still not considered a potent analgesic. It can be used for the treatment of acute or chronic pain when other options are unavailable or contraindicated. It can also be combined with acepromazine or dexmedetomidine for sedation.1
NSAIDs are widely used as analgesics for cats and are generally safe drugs. In any case, when considering their use in the treatment of chronic pain, it is important to select drugs authorised for analgesic use in cats and monitor kidney function and for the appearance of side effects.1-5
Conclusions
For a long time, veterinary surgeons beleived that cats hardly ever feel pain, so in many cases its treatment was overlooked. Fortunately, we have learned to recognise the changes in a cat’s behaviour, including changes in their facial expressions, that help us identify when they are in pain. The author strongly advocates the use of pain assessment scales to grade the severity of pain as objectively as possible. Hence, you can select the most appropriate treatment for each particular case and monitor its effectiveness over time.
