Fibrotic myopathy is a rare condition in horses characterized by the development of fibrous (scar) tissue within the muscles. This condition typically affects the inner thigh muscles of one or both hindlimbs. [1][2][3]

Affected horses develop hindlimb stiffness, a distinctive hoof-slapping gait, lameness and reduced flexibility. This can lead to impaired mobility and athletic performance.

Fibrotic myopathy primarily develops in horses that train and compete in disciplines involving sudden stops and sharp turns, such as cutting and barrel racing.

This makes Quarter horses and other stock breeds more susceptible to the condition. These disciplines require rapid changes in direction, placing significant stress on the hindlimb muscles. [4]

Horses with fibrotic myopathy may experience varying degrees of movement issues, requiring careful management by veterinarians and owners. Understanding its causes, symptoms, diagnostics, and treatment options enables effective management and minimizes risks for horses with this condition.

Fibrotic Myopathy in Horses

Fibrotic myopathy in horses describes the formation of fibrous tissue within muscles, often affecting the semitendinosis and semimembranosus muscles in the hind limbs. [1][2][3]

It is typically caused by trauma, injury or repetitive strain, resulting in abnormal healing and the development of scar tissue in the muscle.

The initial injury triggers an inflammatory response, leading to the deposition of inelastic connective tissue within the muscle fibers. Over time, this fibrous tissue accumulates and causes progressive stiffness and contraction of the affected muscle.

This fibrous tissue formation can lead to gait abnormalities, impacting the horse’s movement and ability to participate or compete in athletic disciplines.

Symptoms

Clinical signs of fibrotic myopathy in horses vary depending on the severity and progression of the condition. In acute cases, the affected muscles are painful and warm upon deep palpation due to inflammation and tissue damage. [5]

As the condition progresses, the damaged muscles begin to bond together and harden, a process known as fibrosis. This leads to the formation of dense, fibrous scar tissue that impairs muscle function and flexibility. [3]

Horses with fibrotic myopathy may exhibit the following clinical signs: [4][6][7][8]

  • Hindlimb stiffness: affected horses may exhibit hindlimb stiffness, which can be more pronounced during movement or after periods of rest.
  • Hoof-slapping gait: this distinctive movement abnormality is characterized by an abrupt halt in the forward swing phase of the affected hind limb, causing the leg to jerk and the hoof to forcefully slap down onto the ground. This gait irregularity is particularly noticeable at a walk.
  • Lameness: affected horses develop mechanical lameness, as opposed to pain-related lameness. This means the abnormal movement patterns are primarily due to structural changes within the muscle and are not associated with acute discomfort or pain.
  • Reduced flexibility: fibrosis within the muscles can lead to stiffness and decreased flexibility, limiting the affected horse’s range of motion, particularly in the hind limbs.
  • Hardened muscle tissue: palpation of the affected muscles may reveal firm or hardened areas, indicating the presence of fibrous scar tissue and potentially ossification (formation of bone within the muscle).

Muscle Anatomy

Fibrotic myopathy in horses primarily affects the muscles of the hindlimb, which are essential for locomotion and overall performance. By understanding which muscles may be involved in cases of fibrotic myopathy, owners can equip themselves to support recovering horses more effectively.

The specific muscle groups associated with fibrotic myopathy include: [2][8][9]

  • Semitendinosus Muscle: plays a key role in extending the hip, stifle, and hock joints during movement
  • Semimembranosus Muscle: involved in extending the hip and stifle joints and is essential for smooth, coordinated limb movement
  • Biceps Femoris Muscle: assists in extending the hip, stifle, and hock joints, as well as in abducting the hindlimb
  • Gracilis Muscle: involved in adducting the hindlimb and assisting in hip flexion

Causes

Fibrotic myopathy in horses can arise from various factors, each contributing to the development of fibrous scar tissue within the muscles.

Sudden Stop and Turn Exercise

Activities like polo or cattle cutting, which involve sharp turns and sliding stops, place significant strain on the hindlimb muscles of horses. These maneuvers require rapid deceleration and changes in direction, which can cause the muscles to contract forcefully against the extended joints. [1][4]

Injuries

When a hindlimb is injured, such as from getting kicked, slipping, or stuck in the fence, the muscles may sustain strain and tears. Injuries initiate an inflammatory response as the body attempts to repair the damaged tissue. [2][4]

Although inflammation is a normal part of healing, severe or repetitive trauma can cause chronic inflammation, leading to persistent fibroblast activation. Fibroblasts are cells that build connective tissue.

Prolonged, abnormally high levels of fibroblast activity in the muscle results in fibrosis, characterized by excessive collagen deposition and the formation of dense, fibrous scar tissue.

Intramuscular (IM) Injections

Intramuscular (IM) injections can lead to fibrotic myopathy when they introduce pathogens (disease-causing agents) into the muscle tissue, resulting in infections and abscess formation. The body’s response to infections involves inflammation, which, if chronic, causes ongoing tissue damage and repair. [2][3][4]

This repair process often results in the production of fibrous scar tissue by fibroblasts. Over time, fibrous tissue accumulates, leading to fibrosis, where the muscle becomes thickened and less flexible. Chronic inflammation can also lead to ossification, where calcium deposits form within the scar tissue and create bone-like structures within the muscle.

Proper injection techniques, using sterile, single-use equipment, and following good hygiene practices are essential to minimize the risk of such complications from injections.

Congenital

In some cases, fibrotic myopathy may have congenital origins, where foals develop the condition due to injury at or soon after birth, which may indicate a congenital cause. [1]

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Diagnosis

Diagnosing fibrotic myopathy in horses involves a combination of physical examination, palpation, and advanced imaging techniques.

The diagnostic process begins with a thorough physical examination by a veterinarian, who observes the horse’s gait and overall movement in addition to taking a health history and checking other vital signs. [10]

Palpation

Palpation describes feeling and squeezing different parts of the body by hand. In cases of suspected fibrotic myopathy, veterinarians use palpation to assess the hindlimb muscles, particularly the semitendinosus, semimembranosus, and biceps femoris.

In acute cases, these muscles may be warm and painful on deep palpation. In chronic cases, palpation may reveal hardened areas within the muscles, indicating fibrosis and potential ossification. Abnormal thickening behind the stifle, around the insertion points of the affected muscles, may also be detected. [4]

Diagnostic Imaging

Accurate diagnosis of fibrotic myopathy in horses relies on advanced diagnostic imaging. These diagnostic tools provide detailed insight for assessing the extent of muscle damage and planning appropriate treatment strategies.

Types of imaging may include: [4]

  • Ultrasound: helps identify areas of fibrosis, scar tissue, and any ossification within muscle fibers.
  • Thermography: detects temperature variations on the horse’s body surface, indicating underlying inflammation or abnormal muscle function. Areas affected by fibrosis often appear cooler due to reduced blood flow.
  • Scintigraphy (bone scan): uses radioactive tracers to detect increased metabolic activity, helping identify muscle injuries and active fibrotic regions.

Differential Diagnosis

When diagnosing fibrotic myopathy in horses, other conditions with similar symptoms have to be ruled out. Conditions to rule out include: [2][4]

Treatment

In the past, fibrotic myopathy was managed with partial or complete semitendinosus myotomy or myotenectomy, techniques involving the partial or complete cutting of the semitendinosus muscle or the removal of a portion of it. [7][10]

However, these methods are associated with the creation of dead space during surgery, a void or cavity that remains unfilled. Dead spaces can accumulate blood or other fluids, potentially leading to complications like hematomas, seromas, and increased risk of bacterial infection. [10] As such, myotomy and myotenectomy are no longer considered the gold standard of care for fibrotic myopathy.

Modern treatment for horses with fibrotic myopathy involves surgical intervention to remove the thickened and scarred tissue or cutting the ligament attached at the stifle (tenotomy) to regain range of motion in the limb. [4]

Conservative Treatment

In mild cases of fibrotic myopathy, a conservative approach to treatment may be preferred to avoid the invasiveness of surgery. The outcome of conservative methods depends on the severity of the condition, compliance with rehabilitation protocols, and the individual horse.

Conservative treatment usually requires a multi-modal approach to regain flexibility and range of motion in the affected limb(s). Modalities include: [1][11][12]

  • Physical therapy: specific rehabilitation exercises aim to enhance muscle function, alleviate stiffness, and support overall recovery.
  • Therapeutic ultrasound: involves the application of high-frequency sound waves to targeted muscle areas. This technique stimulates deep tissue healing, boosts blood flow, enhances tissue elasticity, and remodels scar tissue.
  • Stretching exercises: gently elongate affected muscles to prevent stiffness and mitigate scar tissue formation. Controlled and gradual stretching routines improve range of motion while preventing contraction that could further impair movement.
  • Massage therapy: promotes muscle relaxation, enhances circulation, and reduces tension. Massage also aids in breaking down adhesions and scar tissue, potentially improving muscle function and mobility over the course of treatment.
Always work with a qualified practitioner when seeking body work modalities for your horse

 

Surgery

Small fibrotic masses (<1.5 cm across) may be removed using minimally invasive procedures, such as laser surgery, through a thin transecting incision. This is the preferred method of treatment for fibrotic myopathy. [4]

Another treatment option is a semitendinosus tenotomy, where the insertion of the semitendinosus tendon of the hindlimb is cut near its attachment site on the femur bone. [2][4][10][13]

This intervention is typically conducted under general anesthesia, with the horse positioned in lateral recumbency and the affected limb positioned downward. This procedure aims to relieve the mechanical restriction caused by fibrotic tissue within the muscle, thereby increasing range of motion and alleviating associated lameness.

Prognosis

While some horses may experience significant improvement in mobility and function with appropriate treatment, full recovery is uncommon, with only about half of cases achieving complete resolution. [2][3][4]

Complications may occur following surgery, which can delay healing, such as reopening of surgical wounds, fluid buildup and hemorrhage. Additionally, necrosis (tissue death) may occur under tension-relieving bolster skin sutures, further complicating the healing process. [4]

These factors contribute to a cautious prognosis for horses diagnosed with fibrotic myopathy, as mechanical lameness and impaired mobility may persist despite treatment efforts. Close monitoring and attentive veterinary care are important aspects of ongoing recovery and reduce the risk of complications.

Riding Horses with Fibrotic Myopathy

Horses with milder cases of fibrotic myopathy, or those that respond well to treatments and physical therapy, might be able to return to light and less strenuous riding. However, these horses may require ongoing management to prevent fibrotic myopathy from worsening. [4]

For horses with severe fibrotic myopathy or those that do not respond adequately to treatment, riding may remain limited or not be possible. These horses might continue to suffer from stiffness and impaired flexibility, making it difficult to perform under saddle.

Overall, the prognosis for riding depends on individual circumstances and should be assessed on a case-by-case basis, in close consultation with a veterinarian.

Prevention

While certain measures can mitigate the risk of fibrotic myopathy in horses, this condition may still occur depending on the horse’s discipline and predisposition. Factors such as unexpected accidents can contribute to its development, despite having preventive measures in place.

Preventive measures to reduce the risk of fibrotic myopathy include: [1][3][14][15]

  • Warm up and cool down: incorporating warm up and cool down into training routines is essential as this optimizes blood flow to the muscles, improving muscle elasticity and reducing the risk of injury during exercise.
  • Appropriate training: gradual conditioning and using effective training techniques helps build muscle strength and flexibility safely, which may reduce the likelihood of muscle strain and subsequent fibrosis.
  • Recovery time: allowing sufficient rest between workouts gives muscles time to recover and repair, building strength over time.
  • Safe injection practices: ensuring sterile techniques and proper management of injection sites can minimize the risk of intramuscular infections that might contribute to fibrotic lesions. Hamstring muscle injections should be limited or avoided completely.

In addition, providing a balanced diet tailored to your horse’s level of work ensures they receive all the nutrients required to build strength and recover effectively from training.

If you are unsure whether your horse’s diet is meeting their needs, consider working with an equine nutritionist to review your horse’s feeding program today.

Summary

Fibrotic myopathy is a condition in horses characterized by the development of fibrous scar tissue within the hindlimb muscles, typically affecting mobility and performance.

  • It can result from activities requiring sudden stops and turns, traumatic injuries, intramuscular injections leading to infections, or even congenital factors
  • Signs include hindlimb stiffness, a distinctive hoof-slapping gait, mechanical lameness, reduced flexibility, and hardened areas within affected muscles
  • Diagnostic methods include clinical evaluation, palpation to detect muscle abnormalities, and advanced imaging techniques like ultrasonography and scintigraphy to assess muscle damage
  • Treatment involves surgical interventions such as tenotomy or laser surgery to remove scar tissue, with varying degrees of recovery

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References

  1. Young, A., Fibrotic Myopathy. UC Davis School of Veterinary Medicine. 2022.
  2. Pickersgill, C.H. et al., Surgical Treatment of Semitendinosus Fibrotic Myopathy in an Endurance Horse – Management, Complications and Outcome. Equine Veterinary Education. 2000.
  3. Valberg, S.J., Muscle Disorders in Horses. Merck. 2019.
  4. Adair, S. et al., Muscle: myopathy – fibrotic in Horses (Equis). Vetlexicon.
  5. Valentine, B.A. et al., Denervation Atrophy in Three Horses with Fibrotic Myopathy. J Am Vet Med Assoc. 1994.
  6. Adams, S.B., Components of the Musculoskeletal System of Horses. Merck. 2019.
  7. Magee, A.A. and Vatistas, N.J., Standing Semitendinosus Myotomy for the Treatment of Fibrotic Myopathy in 39 Horses (1989-1997). AAEP. 1998.
  8. Adams, S.B., Overview of Lameness in Horses. Merck. 2023.
  9. Valberg, S.J. et al., Chapter 42 – Diseases of Muscle. In: Large Animal Internal Medicine (Sixth Edition). Mosby. 2020.
  10. Suarez-Fuentes, D.G. et al., Tenotomy of the Semitendinosus Muscle under Standing Sedation versus General Anesthesia: Outcomes in 20 Horses with Fibrotic Myopathy. Veterinary Surgery. 2018.
  11. Moll, D. and Munroe, G., Musculoskeletal: therapeutic ultrasound in Horses (Equis). Vetlexicon.
  12. Frick, A., Stretching Exercises for Horses: Are They Effective?. Journal of Equine Veterinary Science. 2010.
  13. Bramlage, L.R. et al., Semitendinosus Tenotomy for Treatment of Fibrotic Myopathy in the Horse. J Am Vet Med Assoc. 1985.
  14. Frippiat, T. and Votion, D.-M., Warm-Up Strategies and Effects on Performance in Racing Horses and Sport Horses Competing in Olympic Disciplines. Animals (Basel). 2024. View Summary
  15. Janczarek, I. et al., Effect of Different Water Cooling Treatments on Changes in Rectal and Surface Body Temperature in Leisure Horses after Medium-Intensity Effort. Animals (Basel). 2022. View Summary