Horses with muscle disease can have several clinical signs (symptoms) including:
- Tying-up (rhabdomyolysis). Horses that tie-up are reluctant to move due to muscle stiffness, muscle cramping, and muscle pain (myalgia). The medical term for tying-up is rhabdomyolysis that refers to physical damage and destruction of skeletal muscle cells. Tying-up most commonly occurs when a horse is exercising (where it is termed exertional rhabdomyolysis), but can also occur without exercise (non-exertional rhabdomyolysis).
- Muscle wasting (atrophy). Muscle atrophy refers to a decrease in muscle mass. Atrophy can affect part of a muscle, the entire muscle, or several muscles at the same time. When muscles atrophy it causes weakness because the overall strength of a muscle is directly related to its overall mass.
- Muscle weakness. Muscle weakness is a lack of muscle strength. Horses may be reluctant to move or may have difficulty rising depending on the severity and medical basis of the weakness.
- Muscle twitching (fasciculations). Muscle fasciculations are tiny muscle contractions in one or more muscles.
- Dark urine (myoglobinuria). Dark, coffee-colored urine results from the presence of the muscle protein myoglobin, released from the muscle cells due to cell damage, being excreted in the urine. Myoglobinuria is a result of rhabdomyolysis.
- Exercise intolerance. Horses may demonstrate an inability or decreased ability to perform physical exercise, or perform at a greatly diminished level, due to the underlying muscle disease.
- Abnormal gait or lameness that cannot be attributed to joints, tendons, or ligaments.