Equine Exertional Rhabdomyolysis - 'tying up syndrome'

Description

Exertional myopathies (‘azoturia’ or ‘tying-up’) in horses are a condition of muscle fatigue, pain, or cramping associated with exercise. Most commonly, exertional myopathies produce damage to skeletal muscle and are termed exertional rhabdomyolysis (ER). Previously, ER was considered to be a single disease, however, it is now known to comprise several different myopathies, which, despite similar clinical presentation, differ significantly in cause.

Episodes range from sub-clinical to severe muscle necrosis with horses unable to get up and potentially fatal kidney damage. The severity can vary extensively between individuals and episodes.

ER can be sporadic, with single or infrequent episodes of exercise-induced muscle damage, or chronic, with repeated episodes and increased serum creatine kinase (CK) and/or aspartate transaminase (AST) brought on even by mild exertion.

Clinical Signs

Clinical signs usually appear during or shortly after exercise or at times of excitement and stress. They include:

Poor performance
Excessive sweating
Tachypnoea (elevated respiratory rate)
Tachycardia (elevated heart rate)
Muscle fasciculation’s (twitching)
Reluctance or refusal to move
Hard, swollen and/or painful lumbar and gluteal muscles
Dehydration
Discoloured urine (severe)
Recumbency (inability to stand) (severe)

Causes / Risk Factors

Excessive dietary soluble carbohydrates
Frequency or exertion changes in the exercise program
Excessive exercise for fitness level
Stressful/exciting occurrence’s
Research has shown the following factors contribute to the process:
Dehydration & electrolyte imbalance; calcium regulation and phosphor ratio,

vitamin E or Selenium deficiency

Hormone imbalance
Heritability
Genetic mutation or abnormality
Infection
Underlying muscle-damaging causes can be present including type 1 and 2

polysaccharide storage myopathies (PSSM), malignant hypothermia, abnormal regulation of intracellular calcium in skeletal muscles, etc.

Diagnosis

Patient history and clinical signs will usually provide the information to make a provisional diagnosis.

A Physical Exam is fundamental in examining the muscular system. Palpating the muscle groups for tension, heat, swelling, pain, observation for fasciculation’s, percussion of the muscle groups, lameness evaluation and neurological examination may all be used during the examination.

Blood Chemistry Analysis establishes whether there are abnormal increases in serum muscle enzyme’s CK and AST. Blood analysis can be useful in identifying severity, recovery, sub-clinical cases and in disease monitoring. Tests are usually conducted during, after 4-7days and then again at 1 month post-episode. Basal blood tests should be done on the horse in its healthy state as a reference.

Urinalysis can be used to determine whether myoglobin is present, indicating muscle fibre breakdown.

Further diagnosis

Exercise Tolerance Testing is a series of serum chemistry analysis’ to identify chronic ER and can assist in determining the exercise load appropriate for a recovering horse. Serum AST is slower to be released and can also come from the liver, elevated AST can in some cases require further investigating. Serum CK activity is muscle specific and usually reflects the magnitude of acute muscle damage. Tests are done prior to and 4-6 hours post exercise on regular occasions throughout the recovery and management process to ensure the horse is not being overexerted and that no further muscle or organ damage is occurring.

Muscle Biopsy or Genetic Testing may be prescribed for ongoing and recurrent episodes of ER or with susceptible breeds to diagnose the possibility of underlying myopathies.

Treatment

If you suspect your horse has had an ER episode cessation of exercise and immediate veterinary care are advised. In cool weather rug the horse and provide a clean stable with comfortable bedding.

Veterinary treatment may include pain relief, NSAID’s (non- steroidal anti inflammatories), administration of a vasodilator, oral fluids and electrolytes and/or intravenous fluid therapy to rehydrate and mitigate serious kidney damage. Further management and treatment is dependent upon the individual’s initial cause, response to treatment and severity of the episode. Pharmacologic administration of calcium regulators may be recommended.

Management and Prevention

Management of ER is always individualised for the situation and all changes should be made gradually and under the guidance of your veterinarian with knowledge of the full clinical history of your horse. Susceptible horses will be prone to recurrent episodes and vigilant management is required to prevent future relapse.

Prevention is aimed at decreasing the triggering factors and remedying imbalances. These can include:

Limiting stable confinement through exercise or turn-out
A gradual, regular and consistent exercise program with thorough warm up and

warm down

Feed according to workload
Reducing/removing soluble, non-structural carbohydrates
Feed a minimum of 1.5-2% b.w. in hay and pasture
Constant access to clean drinking water
Provide compatible company
Control stress and excitement inducing activities
Supplement with a balance of omega-3 and omega-6 fats
Balanced mineral, vitamin and electrolyte supplementation
Specialised diet and amino acid supplements can be beneficial in some cases
Medications may be prescribed to reduce excitability, alter calcium transport, or to