Fibrotic Myopathy

What is fibrotic myopathy?

Fibrotic myopathy is a form of mechanical lameness that occurs when abnormal scar (fibrotic) tissue forms in the muscle. Scar tissue does not behave the same way as normal muscle fibers. Notably, it is inelastic and can restrict the muscle’s normal elasticity, altering its function. It can be caused by trauma (getting kicked, slipping and falling, catching a foot in a fence, sustaining a laceration), hyperextension, repetitive strain (often seen in reining, cutting, and roping horses), infection, or repeated IM injections. There is also a congenital (present at birth) form of the disease. Fibrotic myopathy most commonly affects the hamstring muscles in the upper part of the hind limb, particularly the semitendinosis and semimembranosus muscles. One hind limb is usually affected, but in some cases, both hind limbs are involved. It is not typically associated with pain, but can limit or prohibit performance.

What are the clinical signs of fibrotic myopathy?

Clinical signs of fibrotic myopathy in horses can include sudden onset of warm, painful muscles. Chronic cases exhibit hardened areas of muscle.  Affected horses display a characteristic gait abnormality (primarily at the walk) in which the leg stops abruptly mid-stride and does not continue its forward motion, with the foot slapping the ground (“goose stepping”). This is a different gait abnormality than exhibited by horses with stringhalt, which has different causes.

How is fibrotic myopathy diagnosed?

Fibrotic myopathy is diagnosed based on the clinical signs, including the characteristic gait abnormality and patient history. Ultrasound imaging can be used to determine the location and extent of the muscle damage.

How is fibrotic myopathy treated?

Fibrotic myopathy in horses may be treated with rest, cold therapy, therapeutic ultrasound, massage, stretching, shockwave, laser therapy, underwater treadmill therapy, cavaletti exercises, and passive range-of-motion exercises. Surgery to break up the band of scar tissue may be indicated for some cases. However, scarring can also occur at the surgery site, so this approach does not always result in a return to full range of motion of the affected limb(s).

What is the prognosis for fibrotic myopathy?

The prognosis for a horse affected with fibrotic myopathy is generally good, with many horses returning to full work within a month. The condition usually does not progress in severity unless the causative injury is repeated.

How can fibrotic myopathy be prevented?

Prevention of fibrotic myopathy includes ensuring that horses are warmed up prior to exercise, including daily exercise and/or turnout, and limiting IM injections in the hamstring muscles.

For more information

Noll, C.V., Kilcoyne, I., Vaughan, B., Galuppo, L. 2019. Standing myotomy to treat fibrotic myopathy: 22 cases (2004-2016). Veterinary Surgery 48(6): 997-1004.

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