Fibrotic myopathy refers to the abnormal presence of scar (or "fibrotic") tissue within muscle tissue. Since the collagen fibers that make up scar tissue do not act like normal muscle fibers, the presence of fibrotic tissue results in altered muscle function (known as "myopathy"), particularly with regard to pliability or "stretchability".
Scar tissue develops pursuant to muscle injury, such as occurs during limb hyperextension, intramuscular infection, or external trauma. It generally takes 6-8 weeks post-injury for the scar tissue to mature and organize.
The presence of organized scar tissue typically imposes biomechanical restrictions on the affected muscle(s). Associated lameness, therefore, is a result of physical limitations within the muscle rather than inflammation or pain. In other words, fibrotic myopathies usually don't hurt.
The most common manifestation of fibrotic myopathy in the horse is that affecting the hamstring muscles of the hind limb, which include the semitendinosis, semimembranosis, and biceps femoris groups. Of these groups, the semitendinosis muscle is most commonly affected.
Pelvic limb myopathies are most frequently observed in Quarter Horses due to the type of work they perform. Pelvic (hind) limb hyperextension frequently occurs during reining, cutting, and roping.
Normally, the hamstring muscles move independently of each other and are able to stretch very easily when the limb is extended cranially (out in front). However, trauma in the form of muscle strain/tearing (resulting from hyperextension of the pelvic limb) or reaction to intramuscular injection results in the development of scar (fibrotic) tissue within the muscle(s). The scar tissue organizes, matures, and contracts, creating a "rope-like" band where there was once normal pliable muscle tissue. This band is often visible along the back-side of the hind limb (fig 1).
Scar tissue is most easily palpable at the distal extremity of the band, just as it courses around to the inside aspect of the limb (fig 2).
The scar tissue's lack of elasticity causes the pelvic limb to be pulled caudally (backward) before the full length of the hind limb stride is reached. The foot "slaps" the ground as a consequence. The slapping of the back foot is often referred to as "goose stepping" and is a classic (pathogpneumonic) symptom for fibrotic myopathy of the hamstring muscle(s).
Fortunately, this problem is very easy to treat, and carries a good prognosis. Treatment involves a minor surgical procedure that is performed with the horse standing/sedated and the area locally anesthetized. Scar tissue is transected (cut) through a 1-to-2-inch incision along the back of the leg. Once the rope-like band is severed, the limb has the potential to once again reach full extension. Most horses can return to normal work after 3-4 weeks.