A physical examination 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

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; and
  • Contracture (cramping) may be more painful than just muscle damage.

An exercise challenge 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.

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

The analysis of muscle biopsies can help characterize the cause of a horse’s rhabdomyolysis or muscle atrophy. PSSM, MFM, 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.

Muscle biopsies taken in the field are usually open biopsies of the semimembranosus muscle. Horses need 7-10 days of rest after the open biopsy procedure for sutures to heal. Click here for more information on how to take a biopsy.

For some owners the size of a conventional open surgical biopsy and the 7-10 day time off period required after the procedure mean that a needle biopsy is a more attractive option. Instructions on the technique are detailed in this PDF. The procedure is done with tranquilization using a local anesthetic and a 1/4 inch incision. Horses can be ridden the day after the procedure. Needles can be purchased for about $300 at Covetrus. Tissue cassettes are also necessary for the procedure, such as those purchased from Fisherbrand (category #15182701A).