13 Jul 2015
Tom Hughes
Job Title
Figure 1. Epidural analgesia in a horse is very useful where a caudal lesion – in this case a penile lesion – is present.
Drugs available for analgesia in horses can, on the surface, appear somewhat limited – particularly given the raft of analgesics available to small animal colleagues.
It is still important to consider the groups of drugs that can be used to provide analgesia, either on their own or in combination, and to combine them with treatment of the underlying cause of discomfort so that pain is effectively managed.
NSAIDs such as phenylbutazone form the backbone of any analgesic protocol and are very effective in many cases. Several alternatives are available and some individual variation in effect appears to exist. The author still regularly uses phenylbutazone (2.2mg/kg to 4.4mg/kg) intravenously for many cases of orthopaedic pain and either oral phenylbutazone or suxibuzone (6.25mg/kg) for orthopaedic cases that are to be treated at home.
In small ponies such as Shetlands and foals, where it is easier to accurately measure the correct dose of a solution using an oral dosing syringe, meloxicam can be very useful. In horses suffering soft tissue/visceral pain the author has a preference for flunixin meglumine (1.1mg/kg), usually given intravenously, even in cases of colic. The more cyclooxygenase-2 selective NSAID firocoxib is available as an oral paste (0.1mg/kg) and an intravenous solution (0.09mg/kg) and may be indicated where renal compromise is a particular concern.
Opioid use in horses is often discussed and, in the short-term, provides excellent analgesia. However, for the vast majority of opioids the adverse effects on gut transit and relatively short duration of action make them difficult to use day after day. For short-term relief, morphine (0.1mg/kg) is excellent, but will only provide analgesia for four to six hours and will rarely be carried in the car by ambulatory vets. Buprenorphine is licensed in the horse for postoperative analgesia at a dose of 10mcg/kg providing analgesia for up to eight hours.
Butorphanol may be given as a single intravenous bolus (0.1mg/kg) or as a constant rate infusion (13mcg/kg/hr), but in clinical cases it appears to provide only mild analgesia. It is interesting to note the data sheet for butorphanol states studies in the horse have shown it alleviates abdominal pain associated with torsion, impaction, intussusception, parturition, and spasmodic and tympanic colic, without mention of orthopaedic pain.
Gabapentin has been used extensively for the treatment of pain of nerve origin in horses, and, more recently, for the treatment of other painful conditions including laminitis. It has been used at a dose rate of 5mg/kg to 20mg/kg, although the author has only been convinced of clinical effect at the top end of that range.
Paracetamol (20mg/kg) was reportedly used to good effect as an adjunctive analgesic in a laminitic pony (West et al, 2011) and the author has found it to provide effective analgesia in combination with other NSAIDs without adverse effects. A soluble epoxide hydrolase inhibitor called t-TUCB was found to provide sufficient analgesia in a horse with laminitis where NSAIDs and gabapentin had not (Guedes et al, 2013) and it may be this class of drug will find broader use.
The alpha-2 adrenergic agonist drugs xylazine (0.2mg/kg to 1.1mg/kg IV or IM), detomidine (0.005mg/kg to 0.04mg/kg IV or IM) and romifidine (0.04mg/kg to 0.12mg/kg IV or IM) are very useful for short-term pain relief. The sedation they provide can also be useful in horses that are agitated and distressed as a result of severe pain or acute injury. The analgesic effects of xylazine last 20 minutes to 60 minutes, detomidine 90 minutes and romifidine 120 minutes (Rohrbach et al, 2009). These drugs may have a synergistic effect when given in combination with opioids such as butorphanol.
Nerve blocks can be a very useful way to provide relief from orthopaedic pain, although they can be challenging in an ambulatory setting. Blocking the palmar digital nerves at the level of the proximal sesamoid bones (abaxial sesamoid nerve block) is sufficient in most horses suffering from foot pain.
Where foot pain is associated with severe laminitis, a low four-point nerve block may be required in the forelimbs and a low six-point nerve block in the hindlimbs.
Intra-articular analgesia may provide some relief in horses suffering severe joint pain and may be considered when a horse must be transported for treatment. Mepivacaine hydrochloride has a rapid onset of action and will last three hours. Bupivacaine hydrochloride has a slower onset of action, but will last up to eight hours. A combination of the two agents (for example, 2ml of each injected subcutaneously over the palmar digital nerves) will mean a rapid onset of action that will last many hours. Even when bupivacaine is used, nerve blocks will have a relatively temporary effect and some horses will show an increase in discomfort when the block wears off.
Local anaesthetics, alpha-2 agonists and opioids may all be administered via the epidural route to provide analgesia to the tail, anus, perineum, rectum, vulva, vagina and bladder. The injection is made by passing a needle through the first intercoccygeal space, which is the first mobile space after the sacrum. The first intercoccygeal space is found by pumping the tail while palpating the dorsal aspect of the tail head. The skin over the space is clipped and aseptically prepared. A skin bleb of local anaesthetic solution may be placed subcutaneously to aid needle placement and can be very useful if the operator is inexperienced.
A 3.75mm to 7.50mm 18G to 20G needle is introduced perpendicular to the skin with the bevel positioned cranially and advanced through the skin and subcutaneous tissues until the ligamentum flavum is penetrated (often with a popping sensation). Correct placement of the needle in the epidural space is verified by the hanging drop technique and negligible resistance to air or sterile water injection.
Once the needle has been successfully placed, local anaesthetic solution, an alpha-2 agonist or an opioid is usually injected (Figure 1). The volume of injection is particularly important when local anaesthetic solution is injected to ensure motor function of the hindlimbs is not affected. The author’s preference in a 500kg horse is for 2ml 10% xylazine hydrochloride combined with 3ml 2% mepivacaine hydrochloride injected together in the same syringe.
When considering analgesia in the horse it is tempting to concentrate on local and systemic drug administration, but effective coaptation must not be forgotten as a means to provide significant and important analgesia in cases of orthopaedic discomfort.
The type of coaptation employed will vary with the injury involved. For example, if a horse has severely injured the superficial digital flexor tendon, hyperextension of the metacarpophalangeal joint must be prevented. A Robert Jones bandage can be placed from the distal aspect of the carpus to the ground, but will only be effective if it is a true Robert Jones bandage, which is very large and very expensive (Figures 2a and 2b).
A more effective way to provide coaptation, particularly for transportation, is to reinforce the dressing with a splint or cast tape. A piece of wood taped in place so it contacts the floor will be effective, but may slip. The commercial aluminium Kimzey Leg Saver Splint will provide excellent coaptation and therefore analgesia, and is useful for transporting a horse with such an injury. If longer-term support is required then a cast may be a more practical alternative – again providing rigid support to the limb and, as a result, improving comfort levels when weight is borne.
In the case of laminitis, placing foot pads that increase the load taken through the frog and caudal part of the foot can provide significant relief. Most people will have their favourite type of pad for this application. In the author’s hands dental impression material placed around the frog and caudal part of the foot, combined with a relatively hard foam pad, give the most consistent results. In the case of a foot abscess or region trauma, thought should be given as to whether any dressings placed can be arranged so that weight is not borne on the affected area of sole.
Remedial farriery may be employed in the longer term, to relieve pressure from a lesion, using a hospital plate shoe. In the case of fractures of the distal phalanx, coaptation with a suitable shoe, either with multiple clips or combined with a hard plate that unloads the frog, is essential. It can be surprising how much extra pain relief is provided by a single roll of cast tape wrapped around the foot and shoe extending up the hoof wall to 5mm below the coronary band.
