34th Annual Scientific Meeting proceedings

The surgical colic patient is typically (very) painful, agitated (or dull), dehydrated if not severely hypovolaemic, with compromised GI motility and/or GI mucosal barrier function. Analgesic treatment for post-operative colic patients should aim to optimise patient comfort and expedite return to normal function, while minimizing risk of adverse effects. Barriers to effective pain relief post-colic surgery include fear of adverse effects, concerns over costs, hesitance to adopt new or evolving evidence into daily practice, and insufficient randomized trials being performed reporting patient- and client relevant outcome parameters (e.g. comparative cost, survival to hospital discharge, duration of hospitalisation).

Multimodal analgesia aims to combine agents acting on different pathways to achieve optimized analgesia with reduced incidence of side effects. Importantly, ‘multimodal’ should not be taken as advocacy of ‘polypharmacy’ – the goal is to provide effective pain relief by making sensible choices of complementary drugs, bearing in mind that under-analgesia, e.g. due to limitations in detecting pain in others, poses significant risk for morbidity. Postoperative pain relief in surgical colic patients should be guided by frequent (q4hr) clinical assessment incorporating multiparameter pain scales. While the PASPAS scale was officially validated for pain assessment in postoperative colic patients, it was recently proven less reliable in inexperienced hands (Lemonnier et al. 2022). CPS, EQUUS-COMPASS or EQUUS-FAP have been found to have good to excellent inter-observer agreement and can be used effectively for monitoring (postoperative) pain in colic patients (Van Loon et al. 2014, van Loon and van Dierendonck 2015, Rosenzweig et al. 2021). Future work should focus on reducing redundancy and overlap between pain scale items and signs of endotoxaemia, e.g. by excluding physiological parameters (T, P, R; Barreto da Rocha et al. 2021).

Cost aspects aside, analgesic treatment of postoperative colic patients is typically affected by concerns over potential side effects, mainly renal (NSAID), GI mucosal barrier (NSAID) and GI motility (alpha2 agonists, opioid), as well as possible behavioural / neurological effects (ketamine, lidocaine). When considering risks of adverse effects, it is crucial to bear in mind that pain is a strong physiological stressor, activating the sympathetic nervous system and HPA axis. This causes a variety of downstream effects that are detrimental to postoperative healing, including increased protein catabolism, reduced intestinal perfusion and motility, and reduced skin blood flow. In short, the nervous, endocrine and metabolic system of a patient in clinical pain is vastly different from that of a healthy pain-free patient. As a result, research on analgesic drug side effects performed in healthy pain-free patients may be much less applicable to the effects of the same analgesic drug on horses in clinical pain states (Bowen et al. 2020). All this comes on top of obvious ethical considerations for patient welfare in case of un(der)treated pain.

Optimising postoperative pain relief starts pre- and intraoperatively, with the anaesthetist providing analgesics, intravascular volume and blood pressure support to optimise tissue perfusion, and the surgeon minimizing surgical trauma through delicate tissue handling. Postoperative analgesia following colic surgery should address: Incisional pain; visceral pain (e.g. traction, distension); any preexisting/underlying painful conditions; and pain related to complications incurred during anaesthesia/recovery (e.g. myopathy, soft tissue trauma). Notably, a 2024 systematic review found a mean reported incidence of postoperative complications after colic surgery of 55.6% (95% CI 45.8 - 65.0; OR vs. elective surgery 6.6; p < 0.0001; Loomes, de Grauw et al. under review).

A useful systematic approach to analgesia post-colic surgery works by analogy to the WHO analgesic ladder, extended with ancillary drugs and locoregional options for surgical pain. Like any ladder, it is designed to go up and down: Peri-operatively, one starts high on the ladder, at a tier fitting with the anticipated level of surgical pain, aiming to optimally control acute postoperative pain and allow early de-escalation postoperatively. For colic surgery, the ladder approach would suggest NSAID +/- adjunct +/- opioid (weak/strong) +/- locoregional technique +/- parenteral CRIs.

The analgesic, anti-inflammatory, anti-endotoxaemic and antipyretic effects of NSAIDs are much needed in the postoperative colic horse. While this population carries multiple risk factors for NSAID adverse effects (e.g. fluid deficit, GI disease, fasting), the evidence base for NSAID efficacy is strong, and they should be used unless contraindicated; indeed, a recent survey showed 95% of respondents include NSAIDs in their standard of care after colic surgery (Gibbs et al. 2022). In the critically ill patient, selecting an NSAID with the most favourable safety profile would seem prudent. A recent systematic review of NSAID choices for horses with abdominal pain concluded that sadly no clear recommendation can be made for or against any individual NSAID in this population (Citarella et al. 2023). Importantly, the authors noted that only 2 randomized clinical trials were conducted on postoperative colic patients (Naylor et al. 2014, Ziegler et al. 2019; n = 116 horses in total). In the absence of evidence-based recommendations, flunixin remains the most commonly prescribed NSAID for postoperative colic patients (Gibbs et al. 2022), dictated mainly by historical use. Concerns over potential for limited analgesic efficacy of newer, more COX-2 preferential NSAIDs over flunixin have been raised (Naylor et al. 2014, Citarella et al. 2023); Ziegler et al. (2019) used an extra-label 0.3 mg/kg loading dose for firocoxib. Again, inferiority nor superiority can be substantiated at this time (Citarella et all 2023), and the potential patient benefit and/or survival impact of COX-2 sparing NSAIDs in horses remains unclear. In systemically unwell postoperative colic horses, phenylbutazone likely is best avoided as it has a smaller safety margin (Flood and Stewart 2022), and more selective drugs may have a place in subsets of patients (Ziegler & Blikslager 2020). As an aside, the preventive use of omeprazole with NSAIDs has come under some scrutiny after studies showed worse GI ulcerative damage with the combination treatment (Ricord et al. 2021); further research using clinical NSAID dosages +/- omeprazole may be warranted (Flood and Stewart 2022).

With a dearth of evidence to help guide NSAID selection, it should be no surprise that even less is known about efficacy of oral adjunct analgesics in postoperative colic patients. While paracetamol did not increase gastric ulcer scores in healthy horses (Mercer et al. 2020) and in an experimental endotoxaemia study was equally anti-pyretic to flunixin, flunixin produced a more sustained heart rate reduction, and endotoxaemia reduced paracetamol bioavailability (Mercer et al. 2023); the safety or efficacy of combining with NSAID in horses likewise remains unknown. While paracetamol may have a place for those horses that cannot receive NSAIDs due to proven toxicity, it should not replace NSAIDs for fear of their adverse effects. We likewise lack a convincing body of evidence of analgesic efficacy of gabapentin or tramadol for acute postoperative pain in horses.  As these drugs also come with risk of human diversion, they cannot currently be recommended for routine postoperative pain management in equine ICU.

Postoperative opioid use carries the risk of reduced GI motility but should be considered on a risk/benefit basis for severely painful horses (Bowen et al. 2020). A recent study on postoperative opioid use after elective or emergency non-colic surgery or general anaesthesia without surgery, identified prolonged (>24 hr) morphine use, but not methadone or butorphanol CRI, as a risk factor for postoperative colic (Haralambus et al. 2024). While this study was retrospective and importantly excluded post-laparotomy patients, its results interestingly highlight the potential for surgical pain and fasting (in addition to opioid use alone) to affect post-operative motility, as well as possible differences between opioids. The use of opioids should always be restricted to as short a time period as possible, based on pain score evolution, and be accompanied by methods for reducing the risk of impaction (e.g. locoregional use vs systemic, laxative diet, pre-emptive NG tubing).

The common use of IV lidocaine CRI in the postoperative colic horse has been contended (Salem et al. 2016, Freeman 2019) – the controversy seems to focus most on its putative small intestinal prokinetic effects and (lack of) possible survival benefit. Its analgesic potential has been better established, and the author favours its use after surgery for strangulating intestinal lesions (colonic torsions, small intestinal strangulation) to supplement analgesia without negatively affecting GI motility. The risk/benefit analysis for the individual patient should also include cost, and the risk of accidental recumbency after inadvertent IV bolus delivery. The results of the currently ongoing multicentre trial, which is in its final stage of enrolment, are highly awaited.

Ketamine CRI can be particularly effective for patients with incisional complications and/or postoperative trauma / myopathy. Like lidocaine, it needs a dedicated infusion system and comes with a risk of inadvertent bolus administration and traumatic recumbency. While low dose alpha2 agonist CRI may be advantageous and considered in refractory cases to maintain patient comfort and ‘buy time’, sedative and motility depressing effects currently limit their practical implementation in ICU.

Locoregional techniques for abdominal surgery are underutilised in horses compared to other species, and we will likely see a surge in interest and approaches in coming years, parallelling trends in human and small animal surgery to reduce postoperative pain and opioid reliance. Cadaver studies, as well as clinical reports on abdominal wall blocks (transverse abdominus plane block (TAP), rectus sheath block) in foals and ponies have emerged over the past 5 years. Practical uptake in equine colic surgery is likely hindered by concerns over abdominal incisional healing and wound dehiscence, as well as time needed to perform the blocks. As blocks can technically be performed within 10 minutes post-operatively (on the less bloated horse) just before transfer to the recovery box, acceptance may come with time.

References

1. Barreto da Rocha et al. PLoS One 2021
2. Bowen et al. Equine Vet J 2020
3. Citarella et al. Animals 2023
4. Flood and Stewart Animals 2022
5. Freeman Vet Clin N Am Eq Pract 2019
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