Page 50 - BSAVA Guide to Pain Management in Small Animal Practice

5 | Pharmacological treatment of pain

VetBooks.ir Central nervous system: Opioids can cause pant, which is a common e ect of opioid
administration in this species (Monteiro et al.,
sedation, dysphoria and euphoria. Sedation can
be useful when opioids are used as part of
resetting of the hypothalamic thermoregulatory
pre-anaesthetic medication or in a sedative 8 . his is probably due to opioid induced
combination. he sedative e ect is synergistic threshold, resulting in activation of cooling
with that of other sedatives (e.g. acepromazine mechanisms (panting), to reach the lower set
or alpha agonists . Sedation may be an temperature. Attempts to warm the animal are
unwanted e ect in hospitali ed animals that likely to result in resumption of panting (Adler et
require high levels of analgesia for several days, al., 1988 . Panting may be a sign of pain in dogs,
as their appetite and mobility may be impaired, so this can be confusing, emphasizing the
and it is more di cult to assess their mental importance of proper pain assessment.
status. In this case, use of multimodal analgesia
and careful titration of the dose of opioid is Ocular effects: Opioids tend to cause miosis in
required. Sometimes opioids can cause dogs and mydriasis in cats. Mydriasis may result
dysphoria, characterized by abnormal in increased light sensitivity and decreased
behaviour, agitation, vocalization and appearing visual acuity, and possibly contributes to clinical
to ‘see things’, in both dogs and cats. This is less signs of dysphoria. It may last longer than the
likely to happen at appropriate doses, in analgesic e ect of the opioid up to 1 hours
combination with sedative agents or if the following methadone administration (Slingsby
animal is in pain at the time of drug et al., 16 . Apart from this, the changes in
administration. Cats in particular may pupil si e are of little clinical signi cance.
experience euphoria, demonstrating purring, Several opioids (pethidine, fentanyl,
kneading with their paws and a ectionate butorphanol and morphine) have been
behaviour. In the author’s [CJ] experience this is documented to cause decreased tear
often seen following buprenorphine production, and regular corneal lubrication is
administration in cats, making buprenorphine a indicated for patients at risk of corneal
useful drug to improve feline well being and ulceration when receiving opioids.
compliance while hospitalized.
Urinary system: Opioids cause urine retention
Thermoregulation: pioids a ect by increasing urethral sphincter tone and
thermoregulation, probably by resetting the detrusor muscle relaxation (Baldini et al., 9 .
hypothalamic ‘thermostat’, and can cause Epidurally administered opioids are often
hyperthermia in cats, and hypothermia in dogs. thought to generate a higher incidence of this
he e ect of opioids on temperature in an problem compared with other routes, but a
individual may also depend on ambient recent report demonstrated that epidural
temperature and the degree of mobility. morphine administration was not associated
Hyperthermia in cats has been demonstrated with urine retention de ned as no urination
following administration of hydromorphone, within 4 hours , and that urine retention can
morphine, buprenorphine and butorphanol also occur following parenteral opioid
(Posner et al., 1 , and also non opioid administration (Peterson et al., 14 . Urine
anaesthesia-related drugs (Posner et al., 7 . retention usually resolves without treatment. It
It is likely to occur with other opioids. The is unlikely to result in complications such as
aetiology is not well understood, but the bladder rupture unless the bladder wall is weak,
increase in temperature does not seem to be of for example, following trauma, cystotomy or in
clinical signi cance and is self limiting. pioid the presence of neoplasia. In these situations,
administration should be considered as a an indwelling catheter can be used to prevent
di erential diagnosis for hyperthermia in cats. urine retention.
Dogs may become hypothermic after A separate e ect of opioids is decreased
administration of opioids, especially when they urine output, possibly due to an arginine

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