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Equine Health Resource

Fibrotic Myopathy

Fibrotic myopathy is the classic “mechanical lameness” of the hind limb. The gait change is caused by a mechanical limitation on movement of the hind limb and is not necessarily caused by pain.

Injury and scarring of the hamstring muscles in the upper hindquarter of the horse causes a classic change in gait with shortening of the front phase of the stride, loss of the stretchability of the normal muscle, and slapping of the foot to the ground. The injury can result from a single traumatic injury to these muscles (hyperextension injury) but can also can result from repetitive strain, seen most commonly in Western performance horses, especially reiners, cutters and rope horses. In some cases, scarring can occur from repeated IM injections into the hamstring muscles.

“Goose-stepping” is the name given to horses that move their hind limb in this characteristic way – the hoof is raised, then brought rearward and slapped to the ground. Fibrotic myopathy is very different from “stringhalt” as are the causes. Importantly, other lameness conditions can cause similar but rarely this classic gait.

Other signs usually associated with this condition include change in contour and feel of the rear contour of the hindquarter, where the hamstring muscle has been torn and replaced by bands of scar tissue.

This chronic scarring causes a “mechanical lameness”, meaning that the gait deficit is caused by a mechanical limitation and is not necessarily painful to the horse. Depending upon the severity of the gait alteration, and the horse’s use, the condition may or may not be performance-limiting for the horse.

The initial tearing of the hamstring causes lameness and reluctance to move the limb, and pain and swelling in the rear of the limb. By 30-60 days post injury there is enough scarring to cause the typical gait. By 90 days, the scar is fully mature.

This is one of the rare conditions that can be diagnosed by the classic gait, along with an obvious rod-like band of scar tissue in the hamstring area. Ultrasound can be helpful to determine extent and location of injury.

There are effective surgical treatments for fibrotic myopathy. Cutting of the scar itself, or the tendon below it may be helpful to restore normal length and elasticity of the hamstring and a more normal gait. The surgery is often performed in the standing horse. Rest is advised during the early period after injury to the muscle, as the initial injury is painful. In mild cases, physical therapy and massage may be helpful, especially soon after injury.

Prognosis & Relevant Factors

The prognosis for fibrotic myopathy depends on the severity of the injury, gait deficit and scarring, as well as whether or not there is ongoing pain. Mild, chronic cases can go on performing normally and may not require treatment.

Horses not used for riding or performance typically do fine with the gait deficit. For riding and performance horses: severe cases of fibrotic myopathy can interfere with performance and require treatment. Surgery is generally successful at improving the gait.

QUESTIONS TO ASK MY VET

  • Would surgery help this condition?
  • Is my horse really lame or is this purely a mechanical problem?
  • Is this a recent problem or more chronic?
  • Is this condition affecting my horse's ability to do his job?
  • PREVENTION

    Be on the lookout for injury to this area, and commence physical therapy right away to minimize scarring and gait deficit.

    Remember that in some cases, repeated IM injection into this area can create this problem.

    Prevent horses from "leaning on the butt bar" of a trailer or stocks, as this can injure these muscles. Hard stops can cause overload of these muscles. Good footing and proper conditioning may reduce the likelihood of this and other injuries.

    Author: Doug Thal DVM Dipl. ABVP

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