The effects of analgesic drugs depend on the pain that they are used to treat.
There are lots of different ways of classifying pain; probably the most commonly
used in people is to divide pain into nociceptive (ie, in response to a noxious
stimulus) and neurogenic (where nerve damage produces abnormal signals which
mimic pain signals and there is not any obvious damage to tissues). Different
types of drugs are used to treat the two types of pain in man but most types
of pain in animals are assumed to be nociceptive. This may change as we learn
more about pain.
Pain can also be classified as somatic, visceral or central. Central pain is
assumed to be neurogenic; there is some evidence that visceral pain (eg, colic)
is produced by a different mix of neurotransmitters than in the periphery.
Assessing the intensity of pain, and thus the effectiveness of analgesics, can
be tricky. A variety of pain scales have been advocated, which assign numbers
(subjectively) to the severity of the pain. Beware statements such as “drug
X is twice as good as drug Y since it halves the pain score”.
Successful management of pain requires more than just analgesic drugs. Other things to be considered include:
emotional aspects - nursing, food, warmth
treat the condition!
physiotherapy??
TENS? / acupuncture?
If pain cannot be successfully treated, euthanasia must be considered. If an animal's owner refuses analgesia on the grounds of expense. this is the only option.
Many drugs have analgesic effects but few are clinically useful. No drug works in every case. The commonly used groups of drugs are:
There are many other drugs used in people which are less useful in animals (but may be used as adjuvants to one or more of the above drugs):
psychotropics (mainly used for neurogenic pain in man) - tricyclic antidepressants (TCAs), anticonvulsants
odds and sods - capsaicin etc
| site | drug |
| peripheral nerve endings | local anaesthetics, NSAIDs (opioids?) |
| peripheral nerve | local (opioids? GABA agonists?) |
| spinal cord dorsal horn | local, opioids, α2 agonists (NSAIDs??) |
| brain stem | opioids, α2 agonists, TCAs, carbamazepine |
| ventral tegmental area | opioids |
| cortex | opioids, α2 agonists, TCAs, carbamazepine |
It is often a good idea to use combinations of drugs which work at different sites (balanced analgesia), but more of that later.
Drugs are more effective if given before central sensitisation occurs; ie, before the pain starts (sometimes incorrectly called pre-emptive analgesia). After central sensitisation has occurred higher doses are required. Ketamine may be able to reverse central sensitisation, presumably by blocking NMDA receptors. This means that for surgical pain, animals should be given an analgesic in their premed.
mild pain - NSAIDs
inflammatory pain - NSAIDs
severe pain - opioids
surgical pain - opioids + local + NSAIDs, depending on op
Analgesia in food animals can cause problems; giving drugs nearly always involves
withholding times for meat or milk (as well as cost), but as a vet you will
have a responsibility to try to relieve pain. Persuading farmers that animals
in pain are not productive may help.
Pain Management in Animals. eds. Flecknell and Waterman, W.B.Saunders, 2000
