MIM - PSSM1 - PSSM2 - RER

Horses have developed an extraordinary muscular system through the course of their evolution and selective breeding. Hardly any other animal species is capable of generating such large amounts of energy, performing extremely powerful muscle contractions, and recovering so rapidly after exertion.

Numerous genes are involved in these abilities. Their variability results in an enormous range of individual performance capacity — from the reliable leisure horse to the highly competitive sport or racehorse.

If genetic defects are present and/or the horse’s environment is adversely affected, this finely balanced interaction can become disrupted. The consequences range from mild exercise intolerance and muscle stiffness to severe forms of so-called “tying-up” with extensive muscle damage or chronic muscle wasting. This spectrum of clinical signs is collectively referred to as exertional myopathy. Specific forms for which a genetic basis is known or suspected (PSSM, MIM, RER) are distinguished on the basis of their mechanism of action.  

Exertional myopathy is therefore a multifactorial disorder caused by disturbances in the complex interplay between genetic predisposition, training, nutrition, management, stress exposure, and individual exercise tolerance. 

For some genetic factors, specific diagnostic tests are available to support further evaluation.

Exertional myopathy of type MIM, PSSM or RER should be considered when characteristic symptoms occur during normal or intense exercise, often also following periods of particular stress, and cannot be explained by injuries, infections, or similar conditions.

Typical signs may include:

• hard or tense musculature
• stiffness in movement
• uncoordinated gait
• signs of pain
• nonspecific or shifting lameness
• aggression or irritability
• excessive sweating
• sudden bolting or resistance
• refusal during saddling or riding

If generalized muscle weakness, muscle wasting, or a failure to develop musculature are also present, the condition is likely already chronic.

It is important to understand that the symptoms of exertional myopathy can be caused by a wide variety of different factors. These factors interact with one another; in addition to environmental influences, genetic factors with varying degrees of impact may also contribute.

More about symptoms | More about contributing factors

In clinical practice and scientific literature, numerous terms are used that may describe different causes, specific subtypes, or particular clinical manifestations. These include, among others:

• PSSM – PSSM1 / PSSM2
• Tying-up syndrome
• Rhabdomyolysis
• RER
• MIM
• Monday Morning Disease

All these terms and more are often used interchangeably or synonymously, although they may represent different underlying mechanisms.

Based on the mode of effect these types of Exertional Myopathies can be defined:

The complex interplay of causes requires the integration of multiple diagnostic approaches:

• Veterinary history and clinical evaluation to determine whether external or clinically recognizable causes are present
  • Assessment of the horse’s medical and performance history
  • If necessary, extended differential diagnostics, including muscle biopsy
• Evaluation of management, nutrition, and training
• Assessment of known genetic factors and proper interpretation of DNA test results:
  • DNA Test PSSM1
  • MIM-6-Var. Test

Decisions and management strategies should never be based solely on genetic findings, but rather on an appropriate interpretation of their significance within the overall clinical context.

Please contact us, if you have any questions. Our team of veterinarians, geneticists and feeding experts will find a way to assist you.