Recommended Diagnostic Work-up for Myopathy

Recommended Diagnostic Work-up for Myopathies 

Clinical Signs 

Recommended Work-up 

Work-Up Includes:

• Physical Exam
• Serum Chemistry
• Exercise Challenge Test
• Fractional Excretion of Electrolytes
• Vitamin E and Selenium
• Muscle Biopsy
• Genetic Testing vs. Muscle Biopsy Decision Tree

Clinical Signs

Signs of a muscle disorder: 

• Muscle stiffness, cramping, pain
• Muscle fasciculation
• Muscle atrophy
  • unilateral
  • generalized
• Exercise intolerance

Signs of acute rhabdomyolysis (damage to muscle tissue) 

• Pain, sweating
• High heart and respiratory rate
• Firm muscles, stiffness, lameness
• Discolored brown urine (myoglobinuria)
• Recumbency, thrashing
• Weakness
• In endurance horses, may only have high heart rate and poor recovery

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Recommended Diagnostic Workup

Physical Exam

A physical exam is always required.

• Inspection for muscle atrophy
• Lameness evaluation
• Palpation of all muscle groups
• Evidence of fasciculations (palpate, auscultate)
• Percussion of muscle groups

Serum Chemistry 

• This test measures enzymes normally found in muscle that leak into the blood stream with muscle damage. The enzymes include AST, LDH, CK.

  • AST and LDH can come from liver so ensure that liver disease is not present if AST and LDH are elevated
  • Serum CK activity is muscle specific and usually reflects the magnitude of acute muscle damage
  • CK activity does not necessarily indicate the amount of pain involved because;
    • Individual differences in pain tolerance exist
    • Contracture (cramping) may be more painful than just muscle damage

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Exercise Challenge Test 

This test is useful for identifying chronic exertional rhabdomyolysis and can be helpful in determining how much work to give a recovering horse.

Objective: To produce an elevation in serum CK activity without clinical signs.  Horses with chronic ER in treadmill studies were more likely to have subclinical elevations of CK at submaximal exercise levels than maximal exercise levels. If the CK activity is elevated (greater than 800 U/L) 4 hours after the light work without clinical signs, muscle damage (rhabdomyolysis) is occurring even with a small amount of exercise.

• Procedure: Two minutes of walk, followed by a maximum of 13 minutes of trot on the lunge line. With unfit horses use 2 min intervals of walk and trot. Horses should be relaxed, not collected and not worked in deep sand. Measure CK levels before and 4- 6 hours after exercise.  Stop if the horse seems stiff, allow horse to stand 1 minute and then walk forward. If stiffness persists, discontinue the exercise challenge test.

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Fractional Excretion of Electrolytes

Electrolytes, small charged ions like sodium, potassium and calcium, play a key role in muscle contractility. Depletion of electrolytes due to strenuous exercise or dietary deficiency can cause clinical signs of exertional rhabdomyolysis (ER). When electrolyte levels are chronically low due to dietary imbalances, serum levels may not reflect total body electrolyte levels, and analyzing serum and concurrent urinary levels of creatinine and electrolytes is necessary to determine whether the horse's clinical signs are the result of electrolyte imbalances.

The fraction of a given electrolyte that is excreted is calculated by:

Particularly high or low fractional excretion of a certain electrolyte can indicate electolyte imbalances. Normal values may vary somewhat with diet.

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Vitamin E and Selenium deficiencies

Vitamin E and Selenium are both important to maintaining healthy muscle, and deficiencies in either can cause signs of rhabdomyolysis. Because Vitamin E and Selenium deficiencies can contribute to tying up, testing a horse for deficiencies in Selenium or Vitamin E can potentially help clarify the cause of the horse's rhabdomyolysis, particularly if in an area of the country where Selenium soil content is low, such as the Northeast, Northwest, and Great Lakes regions.  Both act to control the damaging effects of free radicals: Vitamin E limits the effect free radicals can have on cell membranes and Selenium enhances an enzyme that directly limits the number of free radicals present in the cytoplasm. Free radicals occur when the muscle cell obtains energy through oxidative respiration, an energy pathway that is particularly active during exercise.

Muscle Biopsy

The analysis of muscle biopsies can help characterize the cause of a horse's rhabdomyolysis or muscle atrophy. PSSM, RER, GBED and other myopathies have distinctly different patterns of damage at the cellular level and can be diagnosed based on the results of the muscle biopsy. Click here for a shortcut to forms for biopsy submission.

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