Exertional Rhabdomyolysis (ER), commonly known as tying-up or azoturia, is an exercise-induced condition in horses characterized by the excessive breakdown of muscle tissue. [1]

This results in muscle spasms, pain and impaired performance following bouts of exercise. [1] In severe cases, affected horses may exhibit signs of colic or even collapse.

Episodes of tying-up may be influenced by factors such as poor conditioning and abrupt changes in exercise intensity. Some horses inherit a genetic predisposition to ER.

Treatment of this condition includes a combination of stall rest, fluids, dietary adjustment and medication. To prevent tying-up, implement gradual exercise conditioning to build muscle strength, maintain hydration, and ensure a balanced diet with sufficient electrolyte levels.

If you suspect your horse has ER, contact your veterinarian for assessment. Early intervention and treatment improve your horse’s prognosis and reduce the risk of future episodes.

Exertional Rhabdomyolysis in Horses

Exertional Rhabdomyolysis is a muscular disorder affecting horses that is frequently triggered by exertion, stress, hormonal factors or nutritional imbalances. In horses, the condition is characterized by muscle pain, stiffness, and reluctance to move.

This condition is associated with the breakdown of muscle fibers, leading to the release of muscle enzymes and myoglobin (a protein found in blood cells) into the bloodstream, some of which can be harmful to the kidneys.

Laboratory tests of affected horses typically show elevated muscle enzyme levels in the blood, such as creatine kinase (CK) and aspartate aminotransferase (AST). In some cases, muscle biopsies may be performed to assess the extent of muscle damage or to identify specific muscular disorders.

Clinical Signs

Clinical signs of ER often appear during exercise or periods of excitation. Episodes of tying-up can vary in intensity, from mild clinical findings on bloodwork with no visible symptoms to severe cases where the horse may lie down or even collapse. [2] The severity of symptoms may even fluctuate in the same horse over time.

Common signs of ER in horses include: [3][4][5]

  • Reluctance to move
  • Sweating
  • Increased temperature
  • Muscle stiffness and pain over the loin and croup
  • Tachypnea (rapid breathing)
  • Tachycardia (fast heart rate)
  • Muscle fasciculation (visible muscle tremors)
  • Myoglobinuria (dark urine due to the presence of myoglobin)
  • Reduced exercise tolerance

Types of Exertional Rhabdomyolysis

Exertional Rhabdomyolysis in horses can manifest in two primary forms: sporadic and chronic. Understanding the distinctions between these types is crucial for effective management and prevention of the condition.

Sporadic Exertional Rhabdomyolysis

Sporadic ER is an occasional occurrence that typically affects horses subjected to unusual or excessive physical exertion beyond their normal level of training or fitness.

This form is often seen in horses that are otherwise healthy and do not have an underlying predisposition to muscle disorders. In horses affected by sporadic ER, there is no inherent or intrinsic disorder causing abnormal breakdown of the muscle tissue.

Instead, episodes of tying up are caused by external factors, such as intense exercise or environmental stressors, that contribute to muscle cell damage. These episodes may not follow a consistent pattern and may be separated by periods of normal muscle function.

Examples of external factors that can trigger sporadic ER include: [4][6]

  • Overtraining and muscle strain: ER commonly occurs when horses are exercised beyond their conditioning level, especially after a long period of rest followed by a sudden increase in training intensity. Signs may include muscle stiffness and gait alterations, as well as increased muscle enzyme activity. Overexertion and repetitive muscle use can lead to exercise intolerance.
  • Nutritional deficiencies: Deficiencies in electrolytes or antioxidant nutrients such as selenium and vitamin E can contribute to ER in some horses. Horses may develop electrolyte imbalances during periods of prolonged exercise in hot weather. Dietary imbalances, particularly deficiencies of sodium, potassium and calcium, have also been linked to ER episodes.
  • Exercising while sick: Horses exerting themselves while fighting herpes, the influenza virus or other infections could be at a higher risk for an ER episode. Horses with fever, coughing, nasal discharge or other respiratory symptoms should not be exercised unless under the guidance of a veterinarian.

Chronic Exertional Rhabdomyolysis

Horses with chronic ER experience repeated episodes of muscle breakdown during exercise. Chronic tying-up is often linked to an underlying genetic abnormality affecting muscle cell function, such as such as Polysaccharide Storage Myopathy. [7]

Type 1 Polysaccharide Storage Myopathy

Type 1 Polysaccharide Storage Myopathy (PSSM1) is a genetic disorder in horses that affects muscle metabolism. It is commonly found in Quarter horses, draft horses and some warmblood breeds. [8] PSSM1 horses are typically found in halter and pleasure horse disciplines, as they do not perform well at high speeds.

Horses with PSSM1 experience abnormal glycogen storage in muscle tissue. Glycogen is a complex carbohydrate that serves as a primary storage form of energy for muscle cells in horses. It can be broken down into glucose during exercise when the body needs energy to fuel muscle contraction.

PSSM1 is caused by a mutation in the glycogen synthase gene, which leads to continuous glycogen production in the horse’s muscles. This results in difficulty for the horse when switching to burning gl