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Developments in small animal pain control

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01 September 2011, at 12:00am

DEBBIE GRANT reviews some of the latest developments in controlling pain, from electroacupuncture to progress with both anaesthetics and analgesics

THERE has been a small flurry of papers recently published investigating the therapeutic effects of electroacupuncture, including its analgesic efficacy in dogs. Electroacupuncture (EA) involves placing needles at specific points along the body, as in traditional acupuncture. The needles are then attached via clips to a small device generating continuous electrical impulses. Both the frequency and intensity of the impulses can be adjusted, depending on the condition being treated. A randomised, positively-controlled pilot study has just been published comparing preoperatively administered EA to butorphanol for the control of post-operative pain in 12 dogs undergoing ovariohysterectomy.1 Dogs that received EA required significantly lower end-tidal isoflurane concentrations than the butorphanol dogs during traction of the second ovary. In addition, EA dogs appeared to have prolonged post-operative analgesia of at least 24 hours. Another recent publication investigated the efficacy of silver spike point needle-free EA for post-op analgesia in women undergoing hysterectomy.2 The needle free technique allowed for a sham placebo group in this double-blinded randomised study involving 47 patients. Patients receiving low-frequency EA reported significantly lower mean pain scores from three to 24 hours post-op. The total doses of morphine required post-op by the EA group were significantly lower than those required by the control patients. Thus, this type of EA was found to provide useful adjunctive analgesia after hysterectomy and was significantly opioid sparing. New work has also been published demonstrating the potential mechanism of action of EA analgesia.3 EA appears to induce release of endogenous endomorphins that activate μ opioid receptors in the brain, which in turn leads to the activation of serotonergic neurons that inhibit pain.

Epidural anaesthesia

Epidurally administered opioids with or without local anaesthetic produce a much greater duration of pain relief (up to 24 hours) compared to other routes of administration, and also appear to be associated with minimal side effects. Publications in the past
two years have clearly demonstrated these benefits. In one study involving 20 dogs undergoing ovariohysterectomy, epidural
morphine was shown to produce better post-op analgesia and cause fewer adverse effects than fentanyl patches.4 Another study compared the postop analgesia produced by a lidocaine plus morphine epidural with a lidocaine plus tramadol epidural in 36 dogs undergoing castration (orchiectomy). Neither treatment group required rescue analgesia, and the tramadol combination produced analgesia comparable to that of the morphine combination, without significant side effects.5 One of the major concerns of using local anaesthetics in epidural anaesthesia is the potential for the loss of motor function post-operatively. However, a study involving 16 clientowned dogs given an epidural for splenectomy found no significant problem.6 Dogs were administered either a morphine epidural or a combination of morphine plus low dose bupivacaine epidural after surgery. There was no significant difference in motor function between the two treatment groups. In addition, none of the dogs had profound motor dysfunction or respiratory depression at any time during the study. A novel use for epidural local
anaesthetic in male cats with urethral obstruction was recently described in the literature.7 Coccygeal epidurals with local anaesthetic have been safely used to provide analgesia of the penis and urethra during the unblocking procedure. This technique avoids the use of general anaesthesia, which can
be high risk, due to the metabolic abnormalities that usually develop in these patients. 

Wound soaker catheters

Reviews and meta-analysis of the human literature suggest that local anaesthetic wound infusion or infiltration following laparotomy and caesarean section can significantly decrease post-op pain scores and reduce the total amount of patient opioid requirements.8,9 Results of the latest canine studies looking at wound soaker catheters have been mixed.10,11 An American study reviewed the potential complications in 52 dogs that had undergone limb amputation.12 The dogs had received continuous lidocaine infusions via wound soaker catheters for an average of 1.6 days post-op. Complications included disconnection of the catheters, one seroma and one suspected lidocaine toxicity. However, incision infection rate was the same for controls at 5.3%. 

Novel routes of administration of analgesics

In humans, intra-articular IA ketorolac, a NSAID, has been shown to provide additional analgesia after joint surgery.13 The efficacy of IA meloxicam was recently investigated in dogs undergoing cranial cruciate ligament rupture repair14 in a double-blinded placebo controlled randomised study. Three groups of nine dogs received either pre-op IA meloxicam, or pre-op subcutaneously administered meloxicam or placebo. Disappointingly, the results did not find any significant effect of treatment between groups for any of the pain scoring measures. The availability of buprenorphine transdermal patches for humans has led to their initial evaluation in veterinary patients. A comparative study in 24 dogs undergoing ovariohysterectomy found that a 70μg/hr transdermal patch provided post-operative analgesia comparable to 20μg/kg of buprenorphine administered subcutaneously at the time of surgery.15 

Methadone 

Eurovet Animal Health has recently been granted a UK licence for Comfortan 10mg/ml, a methadone HCL solution for injection for dogs. It is licensed for use as an analgesic, premedication for general anaesthesia or in combination with a neuroleptic for neuroleptanalgesia. Methadone is a synthetic opioid and a full μ agonist that produces profound analgesia of approximately four hours duration, with some sedation. It is similar to morphine but
with a lower incidence of vomiting. Onset of action is one hour following s/c administration, 15 minutes after i/m and within 10 minutes after iv injection. As with all opioids there can be some individual variation in duration (1.5 to 6.5 hours) and it may contribute to respiratory depression. Methadone is highly protein bound (60-90%). The majority of the drug is metabolised and excreted by the liver; only a small amount, 3-4%, is excreted unchanged in urine.16 

  • Please note that some of the drugs mentioned in this article are not licensed in veterinary species or for all the routes of administration discussed.
  1. Groppetti, D., Pecile, A. M., Sacerdote, P. et al (2011) Effectiveness of electroacupuncture analgesia compared with opioid administration in a dog model: a pilot study Br J Anaesth 12th July (epub).
  2. Lee, D., Xu, H., Lin, J. et al (2011) Needle-Free Electroacupuncture for Postoperative Pain Management. Evidence-Based Complementary and Alternative Medicine 10: 1,155-1,162.
  3. Zhang, Y., Li, A., Xin, J. et al (2011) Rostral ventromedial medulla μ but not ϰ opioid receptors are involved in  lectroacupuncture anti-hyperalgesia in an inflammatory pain rat model. Brain Res 1,395: 38-45.
  4. Pekcan, Z. and Koc, B. (2010) The post-operative effects of epidurally administered morphine and transdermal fentanyl patch after ovariohysterectomy in dogs. Vet Anaesth Analg 37 (6): 557-65.
  5. Almeida, R. M., Escobar, A. and Maguilnik, S. (2010) Comparison of analgesia provided by lidocaine, lidocaine-morphine or lidocaine-tramadol delivered epidurally in dogs following orchiectomy. Vet Anaesth Analg 37 (6): 542-549.
  6. Abelson, A.L., Armitage-Chan, E., Lindsey, J. C. et al (2011) A comparison of epidural morphine with low dose bupivacaine versus epidural morphine alone on motor and respiratory function in dogs following splenectomy. Vet Anaesth Analg 38 (3): 213-223.
  7. O’Hearn, A. K., Wright, B. D. (2011) Coccygeal epidural with local anaesthetic for catheterization and pain management in the treatment of feline urethral obstruction. J Vet Emerg Crit Care (San Antonio) 21 (1): 50-52.
  8. Karthikesalingam, A., Walsh, S. R., Markar, S. R. et al (2008) Continuous wound infusion of local anaesthetic agents following colorectal surgery: systematic review and meta-analysis. World J Gastroenterol 14 (34): 5,301-5,305.
  9. Bamigboye, A. A. and Hofmeyr, G. J. (2009) Local anaesthetic wound infiltration and abdominal nerves block during caesarian section for postoperative pain relief. Cochrane Database Syst Rev 8 (3). 
  10. Savvas, I., Papazoglou, L. G., Kazakos, G. et al (2008) Incisional block with bupivacaine for analgesia after celiotomy in dogs. J Am Anim Hosp Assoc 44 (2): 60-66.
  11. Fitzpatrick, C. L., Weir, H. L. and Monnet, E. (2010) Effects of infiltration of the incision site with bupivacaine on post-operative pain and incisional healing in dogs undergoing ovariohysterectomy J Am Vet Med Assoc 237 (4): 395-401.
  12. Abelsen, A. L., McCobb, E. C., Shaw, S. et al (2009) Use of wound soaker catheters for the adminstartion of local anaesthetic for postoperative analgesia: 56 cases. Vet Anaesth Analg 36 (6): 597-602.
  13. Fajardo, M., Collins, J., Landa, J. et al (2011) Effect of a perioperative intra-articular injection on pain control and early range of motion following bilateral TKA. Orthopaedics 34 (5): 354.
  14. Moak, P., Hosgood, G., Rowe, E. et al (2011) Evaluation of intra-articular and subcutaneous administration of meloxicam for postoperative analgesia following stifle surgery in dogs. Vet Comp Orthop Traumatol 24 (1): 32-38.
  15. Moll, X., Fresno, L., Garcia, F. et al (2011) Comparison of subcutaneous and transdermal administration of buprenorphine for pre-emptive analgesia in dogs undergoing elective ovariohysterectomy. Vet J 187 (1): 124-128.
  16. SPC Comfortan 10mg/ml, solution for injection for dogs: EMEA website.