Q: My 21-year-old Arabian gelding, a retired endurance horse, was diagnosed with fibrotic myopathy a little over a year ago. One hindquarter is more affected than the other, and he exhibits a mild “slapping” gait on that leg until he warms up. My veterinarian told me that, although my horse was not in pain, it would be best for him to work only on flat trails.
Can you tell me more about the cause and progression of the disease? There was no trauma in the limb that I’m aware of, and I’m always very careful about proper warming up before work. Does the myopathy eventually stabilize, or does the muscle continue to shorten/scar over time? If it stabilizes, can the horse be slowly exercised on more than flat ground? When I turn him out in the arena he gallops around and seems to enjoy it. Does this further damage the muscle? Other than the surgery, are there any treatments?
A: Fibrotic myopathy is a condition that occurs when scar tissue develops in the big muscles on the back of the thigh (think hamstrings in people). This group of muscles pulls the leg back during the stride and thus is responsible for much of the impulsion a horse generates with his rear legs. Scarring prevents the muscles from ex-tending to their full length; the shortened muscles act like a giant rubber band, pulling the foot down and back, resulting in the “slapping” gait you described. Your veterinarian is absolutely right that this is not a painful lameness but simply a limitation of function.
The scarring in the muscles usually results from some form of injury, possibly a laceration or a kick. In rare cases, a horse who habitually leans on the butt chain in a trailer will develop fibrotic myopathy. Another possibility is tearing of the muscle fibers caused by extreme exertion. Because your horse is affected in both legs, it is possible that he gave everything he had in some of his endurance rides and damaged the muscles. It is uncommon for fibrotic myopathy to progress in severity unless the horse repeats the injury or continues to hurt himself. Galloping in the pasture should not be a problem.
Severe cases of fibrotic myopathy may be treated with surgery to cut the semitendinosus tendon or muscle in an attempt to release the limiting effect of the scar. The procedure is not always successful because scarring may reoccur at the surgical site. Shockwave treatments and possibly deeper penetrating laser therapy can help in some cases. From your description, it sounds like your horse is only mildly affected and likely would not benefit substantially from any treatment.
You are fortunate because most horses with fibrotic myopathy do not warm up out of the lameness. Continuing to pay special attention to warming up properly at the start of each ride makes sense for you. Also, because there are limitations to how far your horse can step with his hind legs, you’ll need to be careful in deep or uneven terrain. But as long as you remember all of this, I suspect you can still participate in quite a few athletic endeavors with your horse.
Bruce A. Connally, DVM, MS
This article first appeared in EQUUS issue #454, July 2015.
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