Discussion

Pain management is necessary for ethical, legal, medical and in horses especially - practical reasons. Acute pain behaviours in horses are ostentatious and may be violent; effective analgesia may be required for diagnosis. In contrast, chronic pain behaviours may be misleadingly undramatic. Equine pain can be managed in 5 broad ways: 1) pharmacologically; 2) surgical/ manipulative (wound inspection/dressing); 3) neuro-physiological, 4) euthanasia; 5) complementary (evidence-free). The relative importance of each component depends on circumstances. These may vary from animals injured 'in the field' to those presenting with severe colic requiring transport to a referral centre for laparotomy. The effective management of acute laminitis pain remains elusive.

1) Pharmacological analgesia: The most important drug classes are: 1) opioids; 2) local anaesthetics; 3) NSAIDS; 4) alpha2 agonists; 5) ketamine; 6) spasmolytics; 7) general anaesthetics; 8) SAIDs; 9) benzodiazepines; 10) anticonvulsants; 11) anti-depressants.
Pure mu-agonists are effective analgesics in horses and synergise the sedative effects of other drugs. Concerns over their side effects have been critically over-stated in the past (Clutton 2010). The choice of opioid(s) used depends on these factors: analgesic efficacy; likelihood of side effects; sedative properties; autonomic nervous effects; onset time; duration of action; duration of action and concurrent alpha2 agonist administration; CDR requirement; VMD requirement; personal preference; and cost.
Local anaesthetics are true analgesics insofar that certain routes of administration eliminate all pain sensation. There are 7 ways to administer local anaesthesia: topically; by infiltration; conduction blockade; intra-articular; intravenous regional analgesia and by neuraxial anaesthesia (extra [epi-] dural and spinal, sub- arachnoid anaesthesia). The drugs used depend on efficacy, onset time, duration and toxicity. Those most commonly used in horses are: lidocaine; mepivacaine; bupivacaine; proxymetacine.
NSAIDs are important for providing musculoskeletal analgesia, but some exert useful anti-endotoxic effects. Used in conjunction with other analgesics as part of PMPT, it is often difficult to identify the superiority of one NSAID over another.
Alpha-2 agonists are useful because they sedate, whilst providing visceral analgesia. The principle choices (xylazine, detomidine and romifidine) are chosen on the basis of their analgesic efficacy, onset time, duration, cardiovascular effects and the sedation muscle relaxation and ataxia produced.
Ketamine produces profound analgesia and chemical restraint at 'normal' doses, but the use of 'stun' doses at 1/20th may also be beneficial. Ketamine infused at very low doses may prevent central sensitisation.
Smooth muscle relaxants, e.g. anti-muscarinic drugs, reduce pain arising from gastro-intestinal spasm.
General anaesthetics only control pain for the duration of anaesthesia, and some may promote pain appreciation post operatively.
The versatility of many of these drug classes in providing pain relief has expanded over recent years as new methods and routes of administration have been devised, e.g. both opioids and alpha2 agonists can be used within synovial structures or the extradural space. Lidocaine can be infused intravenously as well as applied peri-neurially. Ketamine dose manipulations have similarly improved its versatility. The use of transdermal 'patches' provides one opportunity for the long-term administration of drugs, e.g. fentanyl. Another option achieving the same goal is the use of constant rate infusions. The use of battery driven syringe drivers facilitates this in the ambulatory animal.
Developments in therapeutic strategies have also provided greater options for more effective pain management: 1) pre- emptive analgesia; 2) polymodal pain therapy; 3) partial intravenous anaesthesia; 4) prolonged post operative pain therapy. 2) Surgical/manipulative measures
Wound dressing, the effective casting of fractures, dental and foreign body extractions and abscess lancing are obvious ways surgery relieves (rather than creates) pain. Hot and cold compresses may have some useful effect. Good nursing practice, i.e. grooming, wound management, feeding, watering and exercise, may improve the animal's comfort 3) Neurophysiological methods: TENS (transcutaneous nerve stimulation) which stimulates nerves at 100 Hz does not appear to have been tested in horses whereas TSE (transcutaneous spinal electroanaesthesia) has. Using stimulation frequencies of 1800 - 2500 Hz acupuncture, this was alleged to be effective in treating lumbar injuries in Australian racehorses. 4) Euthanasia
The most effective means of treating pain in horses. 5) Complementary methods; Unproven and faddish methods of treating pain are important on
2 counts: at worst, they may aggravate the pain, at best, they will delay the time when the animal receives effective treatment or euthanasia. Practical analgesia (surgical) equine colic: To facilitate diagnosis (paracentesis, nasogastric intubation and rectal examination) xylazine (1.1 mg/kg bwt i.v.) and pethidine (2 mg/kg bwt i.m.) would be suitable as both are short-acting sedative and analgesics. For transfer to a referral centre, detomidine (10 microgramg/kg bwt i.v.) flunixin (1.1 mg/kg bwt i.v.) and Buscopan would provide a long duration of action. NB the receiving centre would require pretransfer information arising from physical examination, e.g. heart and respiratory rate and the doses of transfer drugs given and time. For cases going immediately to surgery, preanaesthetic medication with an alpha2 agonist and opioid, followed by ketamine (2.5 mg/kg bwt i.v.) would provide
conditions for endotracheal intubation, allowing anaesthesia, but not necessarily analgesia to be maintained with an inhalational anaesthetic. The depressant effects of inhalation agents could be reduced by infusing lidocaine and, or morphine. The infusion alpha2 agonists, i.e. medetomidine and ketamine is not uncommon practice these days. Nonsteroidal anti-inflammatory drugs should be given before surgery begins. Many would give an additional alpha2 agonist dose to smooth the initial parts of recovery.