Page 78 - BSAVA Guide to Pain Management in Small Animal Practice

5 | Pharmacological treatment of pain

VetBooks.ir glutathione, depleting its stores; risk of (Eisenberg et al., 7 . he hypothesi ed
mechanism of action is blockade of sodium
toxicity directly correlates with the extent of
and calcium channels. Blockade of
hepatic glutathione depletion and therefore
indirectly with the glucuronidation capabilities sodium or calcium channels stabilizes cell
of the species. membranes of damaged nerves, decreasing
the chances of spontaneous depolarization
Clinical use and contraindications at the site of injury, which is one of the
The only licensed preparation of paracetamol mechanisms sustaining neuropathic pain
contains paracetamol and codeine and is (Taylor et al., 1998 .
licensed for use in the dog only N A , 16 . Of the antiepileptics used in humans to treat
he licence allows treatment for up to days, neuropathic pain, the most commonly used one
and concurrent use of other NSAIDs is in dogs and cats is gabapentin. Despite its
contraindicated. The dose indicated in the common use, especially in dogs, the evidence
datasheet is up to approximately mg kg of supporting its analgesic e ect is limited to a
paracetamol and .7 mg kg of codeine, small number of studies and case reports.
administered every 8 hours. Administration in a
clinical setting is usually started at 1 mg kg of Mechanism of action and
paracetamol and, although the licence does not disposition
mention this and there is no study suggesting The analgesic mechanism of action of
that at this lower dose the treatment can be gabapentin and pregabalin is mediated by
administered for longer periods, it is plausible binding to the N-type voltage-gated calcium
that due to the mechanisms of toxicity channel, inhibiting the release of excitatory
(glutathione depletion), administration can be neurotransmitters mediated by calcium in ux.
extended for longer than days, especially The pharmacokinetics of gabapentin have
when the drug is used in terminally ill animals. been characteri ed in reyhounds. After oral
Use of the licensed preparation is administration of 1 or mg kg, bioavailability
contraindicated in animals with hepatic and was 8 , the time to peak plasma
renal disease, dehydrated, hypovolaemic and concentration was 1. and 1. hours, and the
hypotensive animals, or when gastrointestinal terminal half life was . and .41 hours,
ulceration or bleeding is possible, although the respectively. abapentin is metaboli ed to
authors often use generic paracetamol N-methyl-gabapentin in dogs. In contrast to the
(intravenous or oral) in patients with case in humans, it seems that gabapentin
gastrointestinal ulceration or renal issues. gastrointestinal absorption is not a ected by the
Interestingly, there is no contraindication for use dose administered. Based on the e ective
in pregnancy, or concurrent use of steroids, plasma concentration in humans, administration
which are two major areas where NSAIDs are of an oral dose of 1 mg kg every 8 hours
contraindicated, and animals could bene t should be appropriate in dogs (KuKanich and
from paracetamol/codeine administration Cohen, 11 . ral bioavailability of gabapentin
N A , 16 . Clinical evidence supporting the in cats is high 88 , with a time to peak plasma
e cacy of analgesia is limited, and one concentration of approximately 1 minutes.
investigation suggested that both paracetamol/ Based on the pharmacokinetic parameters, oral
codeine and tramadol are inadequate if used dosing with 8 mg kg every 6 hours should
as the sole analgesic after a tibial plateau result in a plasma concentration similar to that
levelling osteotomy (Mburu et al., 1988; Benite e ective in humans Siao et al., 1 .
et al., 1 .
The pharmacokinetics of pregabalin after
single dosing has been reported in the dog,
Gabapentin although there are no clinical reports about the
any antiepileptic drugs are e ective in e cacy of this drug in treating pain Sala ar et
treating neuropathic pain in humans al., 9 .

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