RER - Recurrent Exertional Rhabdomyolysis
Recurrent Exertional Rhabdomyolysis (RER) is a form of exercise-induced muscle disorder in horses. It is characterised by recurrent episodes of muscle stiffness, muscle cramps and a decline in performance associated with physical exertion. In more severe cases, it can lead to so-called ‘cross-strain’ with significant muscle cell damage.
RER is one of the main causes of exercise-induced muscle problems, particularly in horses used for sport. Thoroughbreds and other performance-oriented horse breeds are particularly frequently affected. The condition typically occurs in young to middle-aged horses in training.
In contrast to PSSM1, current understanding suggests that RER is less characterised by a primary disorder of carbohydrate metabolism and more by a dysregulation of muscle cell excitability and calcium metabolism within the muscle cells.
Ursachen und genetischer Hintergrund
The exact causes of RER have not yet been fully elucidated. It is currently believed to be a multifactorial condition in which genetic predisposition, exercise, husbandry, and feeding interact.
Current research focuses on disturbances in the intracellular calcium regulation of muscle cells. Excessive release of calcium from the sarcoplasmic reticulum can lead to uncontrolled muscle contractions and subsequent muscle cell damage.
To date, no single causative mutation has been identified for RER, as has been the case with PSSM1. However, familial clusters, particularly in Thoroughbreds, suggest a genetic predisposition.
Contributing factors may include:
- intensive training
- stress and nervousness
- interruptions in training
- high-energy diets with high starch content
- electrolyte imbalances
- inadequate adaptation of training and recovery
Clinical appearance
The severity of symptoms can vary greatly.
Typical symptoms, which also occur in other forms of exercise-induced myopathy, include:
- Muscle stiffness after exercise
- Restricted or stiff movements
- Painful muscles
- Sweating despite minimal exertion
- Reluctance to move
- Decreased performance
- Stiff gait in the hindquarters
- Muscle tremors
- Elevated heart rate and respiratory rate
In more severe cases, signs of acute rhabdomyolysis appear:
- pronounced cross-lameness
- inability to move
- dark urine due to myoglobinuria
- massive elevation of the muscle enzymes CK and AST
In cases of ‘true RER,’ horses recover completely between episodes and are clinically unremarkable.
Affected horse breeds
RER is particularly commonly reported in:
- English Thoroughbreds
- Arabian Thoroughbreds
- Standardbreds
- warmbloods used in sports
Nervous, spirited, and performance-oriented horses in particular appear to be at increased risk.
Studies show that Thoroughbreds prone to RER perform better in racing.
Diagnostics
RER is diagnosed based on a combination of:
- clinical symptoms
- exercise history
- recurrent episodes
- laboratory tests
- ruling out other muscle disorders
Key diagnostic procedures:
Blood test for
- CK (creatine kinase)
- AST (aspartate aminotransferase)
Elevated levels indicate muscle cell damage.
Genetic Testing
Genetic testing for PSSM1 and MIM is recommended to rule out these supporting conditions as part of the differential diagnosis.
Muscle biopsy
In certain cases, a histological examination of the muscle tissue may be performed.
Stress and Training Assessment
Training management, husbandry, and feeding should be included in the overall assessment.
Therapy and management
In acute cases, immediate veterinary care is required. Treatment includes pain medication and intravenous fluids to “flush out” the myoglobin so that it does not lead to secondary kidney damage.
For less severe forms, the approach is similar to that for other forms of exercise-induced myopathy:
- Assessment of other contributing factors (environment, genetics)
- Feeding, management, stress reduction
Training management
- Regular daily training
- Avoiding long breaks in training
- A controlled build-up of training
- A sufficient warm-up
- A prolonged cool-down
Feeding
- Reducing high starch and sugar content
- Energy intake tailored to requirements
- Adequate supply of electrolytes
- Partially increased fat content in the diet
Stress reduction
As stress is considered a major trigger, many horses benefit from:
- consistent housing conditions
- calm management
- a tailored training routine
Distinction from other exertional myopathies
RER falls under the umbrella term of exertional myopathy.
Important differential diagnoses include:
- PSSM1 and PSSM2
- MIM
- inflammatory myopathies
- toxic muscle damage
- electrolyte imbalances
Whilst PSSM is primarily characterised by a disorder of glycogen metabolism, RER is more typically characterised by dysregulation of muscle contraction and MIM as based in structural defects.
Significance for breeding and sport
As genetic factors are thought to play a role, scientific research into the hereditary predisposition to RER is becoming increasingly important.
The following are particularly important for breeding and sport:
- early identification of at-risk horses
- tailored training management
- differentiated diagnosis of various exercise-induced myopathies
- combination of genetic, clinical and management-related information
Modern research increasingly assumes that the clinical term ‘RER’ may conceal several biological causes.