Myopathies in Horses

Important Concepts

• Myopathies: Muscle disorders that can be classified based on their cause (traumatic, toxic, or anesthetic).

Types and Clinical Features

1. Fibrotic Myopathy:

   • Cause: Trauma, IM injections, congenital.

   • Signs: Gait abnormalities, muscle fibrosis, no pain chronically.

   • Diagnosis: Palpation, ultrasound, thermography, scintigraphy.

   • Treatment: Rest, therapeutic ultrasound, surgery for chronic cases.

2. Ionophore Toxicity:

   • Source: Contaminated feed.

   • Signs: Colic, myoglobinuria, hypokalemia, cardiomyopathy.

   • Treatment: Supportive care, prevent exposure.

3. Malignant Hyperthermia:

   • Cause: Genetic mutation (RYR1) in Quarter Horses.

   • Signs: Hyperthermia, acidosis, muscle necrosis under anesthesia.

   • Treatment: Dantrolene pre-treatment, supportive care during episodes.

4. Postanesthetic Myopathy:

   • Cause: Muscle hypoperfusion during anesthesia.

   • Signs: Muscle pain, weakness, hard muscles, sweating, tachycardia.

   • Treatment: Pain relief, fluid therapy, proper positioning during anesthesia.

Pathological Changes

• Fibrosis and Ossification: In fibrotic myopathy.

• Cardiac Damage: In ionophore toxicity.

• Muscle Necrosis: In malignant hyperthermia.

For more details, visit the Merck Veterinary Manual on Myopathies in Horses.