​Clinical Signs

Possible signs of a muscle disorder:

  • Muscle stiffness, cramping, pain
  • Muscle fasciculations (twitching)
  • Muscle atrophy
    • Focal
    • generalized
  • Exercise intolerance
  • Stiff stride, reluctance to collect, difficulty holding a canter

Signs of acute rhabdomyolysis (tying up)

  • 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

​Physical Exam is fundamental to examining the muscular system

Physical Exam is fundamental to examining the muscular system

  • Palpation of all muscle groups for heat, pain, muscle tone
  • Palpate and observe for fasciculations twitching
  • Percussion of muscle groups
  • Lameness evaluation
  • Neurologic examination

Serum Chemistry

  • AST can come from the liver so ensure that liver disease is not present if AST is elevated
  • Serum CK activity is muscle specific and usually reflects the magnitude of acute muscle damage. CK peaks in the blood 4 to 6 h after the muscle damage occurs
  • 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

​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.

​Genetic Testing

Selection of a genetic test should be based on the clinical signs, age, breed and knowledge of the prevalence of the disease within a specific breed. A decision tree is provided to help determine if genetic testing is a reasonable choice.

Genetic testing is available at various laboratories or through the AQHA panel test for the following muscle diseases:

  • Type 1 polysaccharide storage myopathy
  • Malignant hyperthermia
  • Glycogen branching enzyme deficiency
  • Hyperkalemic periodic paralysis

​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 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.

Muscle biopsies taken at Michigan State University are done with a muscle biopsy needle that is the size of a pen. The procedure is done often without tranquilization using a local anesthetic and a ¼ inch incision. Horses can be ridden immediately after the procedure. In the field, muscle biopsies can be obtained through an open surgical technique. Horses usually need to rest for 10 to 14 days after an open surgicial biopsy to allow the wound to heal.

The needle muscle biopsy procedure performed at the Mcphail Equine Performance center at Michigan State University is shown on the right. It requires a local anesthetic and horses can go right back to work. The open surgical biopsy procedure is shown on the left. Horses need 7-10 days of rest after the open biopsy procedure.