FAQ – Fibrotic Myopathy

By Dr. Kimberly May

Fibrotic myopathy is caused by scar tissue formation in the horse’s muscles after trauma or a muscle injury. The most commonly affected muscles are the hamstring muscles, which run down the back of the rump and thigh above the hock (see the yellow-boxed area in the image to the right for the muscles most commonly affected by fibrotic myopathy). When an injury tears the muscle tissue, the muscle tissue in the injured area is replaced with fibrous scar tissue during the healing process.

We often think of muscles as contracting to produce movement, but it’s also important that muscles have some elasticity (stretch) to allow smooth movement. Think about doing a biceps curl at the gym: when you extend your arm, your biceps muscle is slightly stretched, and when you curl your arm with the weight, your biceps contract to pull your hand (and the weight) toward your shoulder. Too much stretch causes weakness, but the right amount of stretch allows more fluid movement.

With fibrotic myopathy, the scar tissue prevents the normal stretch of the
hamstring muscles. To demonstrate this, stretch a rubber band between your fingers and see how far it stretches (without breaking it). Then do it again,
but have someone pinch the rubber band somewhere between your fingers:
you’ll notice the rubber band doesn’t stretch as far now. This is what’s
happening to a horse’s muscle with fibrotic myopathy: the scar tissue
prevents the muscle from moving normally to advance the leg in a fluid
motion, and you get the “jerky” step of fibrotic myopathy. As the horse is
walking and moves the leg forward, it moves forward until it hits the scarred
muscle’s limit and then “slaps” backward to hit the ground. This video from
Oklahoma Equine Hospital shows the classic fibrotic myopathy gait. This horse
has it in both hindlegs, and it’s very easy to see.

The good news is that although the original injury that led to the fibrotic myopathy was likely painful, fibrotic myopathy itself is not – it’s a mechanical lameness, not a painful lameness. As a results, they
don’t need pain medications for this condition. Horses with fibrotic myopathy can be ridden, although
their performance in some disciplines may be hindered. They don’t “warm out of it,” and it’s often more noticeable at the walk than at other gaits. If the horse reinjures the area and tears some of the scar tissue (or muscle tissue connected to it), they may be painful until that injury has healed and formed more scar tissue. There is a surgical procedure that can be performed on more severely affected horses, but the results of the surgery vary and it’s not necessary for most horses with fibrotic myopathy.

2020 Copyright Dr Kimberly May and Horse Vet Corner. Do not remove or alter without the author’s permission.

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