In a clinical scenario where a pure μ-opioid agonist is not used (ie a partial μ-opioid like buprenorphine is administered instead), an intratesticular block may be indicated. However, where a pure may μ-opiod is used, an adjuctive intratesticular block may not provide significant benefit.
A clear pattern
Analysis of seven studies presented across two Knowledge Summaries in Veterinary Evidence – one for cats and one for dogs – initially demonstrated extreme inconsistency in the effect of intratesticular blocks on post-operative pain scores. However, when the study samples were considered separately by the type of premedication used, a clearer pattern emerged:
For dogs, in studies that used pure μ-opioid agonists, there was less significance between control and intratesticular block groups in post-operative pain scores. In all studies where a pure μ-opioid agonist was not used, there was clinical significance in the use of intratesticular blocks (to favourable effect).
For cats, in the study that used a pure μ-opioid agonist, there was marked improvement of the pure μ group over intratesticular groups (and epidural groups) in pain scores.
As such, the most significant factor in the reduction of post-operative pain, in either species, appears to be the analgesic used perioperatively. It must be noted, however, that much of the available literature was based on small-scale study populations, included non-standard clinical environments or had the potential for bias. That being said, the pattern was reproduced across the studies.
Other considerations for the vet
The clinical questions of the two Knowledge Summaries specifically targeted intratesticular blocks as adjunctive analgesic. If the questions were modified to include other forms of analgesia, such as isoflurane, propofol and rescue analgesic sparing effect, the conclusions may be different. Similarly, the majority of studies only used lidocaine as the intratesticular block, with the alternative bupivacaine rarely utilised.
Only one study utilised pure μ-opioid premedication and bupivacaine but it had positive outcomes, so this is an area for further study. However, there are concerns about bupivaciane toxicity, while lidocaine led to no significantly aversive events in any of the studies.
Lidocaine is relatively inexpensive and is easy to administer. Although it may not be necessary to use it as an adjunctive analgesic to premedication with a pure μ-opioid agonist, in light of the low cost and minimal risk, an argument could be made that it is still worthwhile to administer in most clinical scenarios.
Authors: Erik Fausak, Elizabeth Rodriguez, Anna Elizabeth Simle, Netwarat Merman and Dakota Cook