When an owner makes the difficult decision to euthanize a horse with laminitis, it's often because the horse is simply in too much pain to justify prolonging treatment. For this reason, researchers are continually trying to come up with improved analgesic (pain relief) methods. Andrew van Eps, BVSc, PhD, MACVSc, Dipl. ACVIM, senior lecturer in Equine Medicine at The University of Queensland School of Veterinary Science, reviewed current pain management options for laminitic horses at the 2013 International Equine Conference on Laminitis and Diseases of the Foot, held Nov. 1-3 in West Palm Beach, Fla.
The pain laminitic horses experience might stem from several sources, van Eps said, including inflammation, submural (beneath the hoof wall) pressure, tearing of submural tissues, ischemia (lack of blood flow), the distal phalanx (P3, or coffin bone) applying pressure to the sole, and neuropathic mechanisms (those caused by nerve damage and described, in humans, as throbbing, stabbing, sharp pain).
We, as humans, want to control horses' pain not only for welfare reasons but also because it interferes with the animal's function. However, we can't simply eliminate pain completely, as it does serve an important purpose in self-preservation, van Eps said.
"Without pain, weight-bearing and ambulation are not restricted in horses with laminitis, so consequently there can be increased mechanical distractive forces on the lamellar tissue and progression of the lesion itself," he explained.
The first step in determining how to manage a laminitic horse's discomfort is to assess his pain level objectively and routinely, said van Eps. This might include:
- Performing a lameness exam.
- Observing the horse's posture and stance.
- Hoof testing, although this can vary among cases, van Eps cautioned.
- Obtaining systemic measurements such as heart rate, respiratory rate, and blood pressure. "These are reasonably sensitive, especially in hospital settings where horses are monitored serially," he said.
- Using tools such as force plates and pedometers to evaluate weight distribution. "Incessant shifting of weight is one of the first and most subtle signs of laminitis pain," van Eps said. "Recently, the use of human pedometers (to document weight-shifting frequency) … proved to be much more sensitive than visual assessment alone."
- Performing behavioral analysis. "Serial recording of events such as teeth grinding, interactions with people, and position in the stall (back corner vs. close to the door) can be useful markers of pain that are more subtle," he said.
Van Eps said owners and veterinarians also need to consider disease stage and what they are trying to achieve—long-term or short-term pain relief. Then they can choose from the two categories of analgesic therapies what makes the most sense for that individual horse: a systemic or regional approach.
Systemic Therapy
Systemic therapies are those that travel through the bloodstream. This primarily involves non-steroidal anti-inflammatories (NSAIDs) such as phenylbutazone, flunixin meglumine, and ketoprofen. While this drug class is potent and effective in most acute and chronic laminitis cases, van Eps said it's also associated with side effects such as gastric/colonic ulcers and kidney damage because it targets not only the pain and inflammation-causing COX 2 enzyme, but also the protective COX 1 enzyme.
Thus, "more COX 2-selective NSAIDs, such as firocoxib and meloxicam, are potentially associated with fewer side effects," he said. "However, their analgesic efficacy and long-term safety compared with non-COX-specific NSAIDs requires further investigation." He added that some studies have recently confirmed these drugs are both safe and effective, although expensive.
When NSAIDs alone are insufficient, van Eps suggested using a combination of other systemic drugs to provide additional analgesia.
"The concept of multimodal analgesia involves the use of different types of analgesic drugs that act by different mechanisms at different sites in the nervous system to provide superior analgesia with fewer side effects," he explained. "The use of multimodal analgesia is often reserved for laminitis cases that do not respond to conventional therapy."
Drugs van Eps said veterinarians might use in a multimodal approach including:
- Intravenous (IV) lidocaine, which he said can be effective for acute laminitis, but comes with a risk of toxicity/neurologic sign development during infusion if an overdose occurs. "This can happen if the horse receives even a transient overdose (inadvertent increase in the infusion rate)," van Eps explained. "Therefore this therapy is best done in hospital situation with infusion pumps and a dedicated IV catheter."
- Opioids, which van Eps said are not particularly effective alone, but might be useful in combination with drugs such as lidocaine, ketamine, morphine, detomidine, and acepromazine;
- Ketamine, an anesthetic that acts through decreasing central sensitization;
- Alpha-2 agonists, which he described as short-lasting sedatives useful for managing unrelenting pain;
- Amitriptyline, which van Eps said has been used effectively as an antidepressant and analgesic in humans, but not in horses; and
- A promising anti-inflammatory (soluble epoxide hydrolase inhibitor) currently under investigation at the University of California, Davis.
The last systemic therapy van Eps described was gabapentin, a drug originally used to treat neuropathic pain and seizures in humans. Much is still unknown about this drug's effects and appropriate dosage in horses, although he said IV or oral gabapentin might be useful when treating acute laminitis cases.
"We routinely administer gabapentin to clinical cases with acute and chronic laminitis without ill-effect," he said. "Unfortunately, it is very difficult to accurately assess whether gabapentin improves analgesia in these cases.
"The main side effect of gabapentin administration in horses is mild sedation and tranquilization," he added. "However, this may actually be of benefit in acute laminitis cases, as recumbency and reduced ambulation are encouraged, as they protect the tissues from further damage caused by weight-bearing."
Regional Therapy
Regional analgesic techniques, as the name implies, aim to treat a specific area. Examples of regional methods used in laminitis cases include:
- Continuous peripheral nerve block of the palmar nerves. Van Eps said veterinarians might use this method when performing a painful procedure such as hoof resection.
- Epidural analgesia, in which the veterinarian administers long-term pain management (e.g., opioids, alpha-2 agonists, local anesthetics) via an epidural catheter. "It can be useful to control pain associated with hind-limb laminitis," van Eps said. "Morphine is probably the most useful, as its analgesic effects are long-lasting (12-14 hours) and it does not cause ataxia (incoordination)."
- Therapeutic hypothermia, or cryotherapy, has both anti-inflammatory and nerve conduction effects.
Take-Home Message
Because there's no one-size-fits-all formula to managing laminitic horses' pain, van Eps recommended owners work with their veterinarians to tailor a multimodal approach for each individual case. "Equine analgesia is evolving," he said. "There's more evidence for COX 2 selective drugs than 10 years ago, and now that we recognize the contribution of neuropathic pain in laminitis, we're beginning to investigate drugs that can help to control this."
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.
