Exercise associated myopathies (Proceedings)


Exercise associated myopathies (Proceedings)

Nov 01, 2010

Exertional rhabdomyolysis is muscle necrosis precipitated by exercise or exertion, resulting in myoglobinuria, elevated serum CK (usually > 5,000 in acute cases) and/or elevated AST in animals with normal liver function.

Local Muscle Strain

Muscle strain may be induced by inadequate warm-up, pre-existing lameness, exercise to the point of fatigue and insufficient training. The lumbar and gluteal muscles are frequently injured in jumpers, dressage and harness horses and the semimembranosus and semitendinosus muscles are more frequently damaged in working Quarter Horses performing abrupt turns and sliding stops (and may also occur secondary to intramuscular injections in this region). The gracilis muscle (adductor) can be torn through misadventure and creates swelling and pain in the medial thigh, and elevated serum CK.

Diagnosis: History, altered stride and reluctance to engage the hind quarters, and pain or heat on deep palpation of the muscles. CK and AST are usually only mildly elevated and thermography may assist with identification of affected muscle groups. Severe muscle trauma may be identifiable on ultrasound. In horses with chronic damage to the semimembranosus and semitendinosus, fibrotic myopathy may occur resulting in a characteristic gait.

Treatment: Rest and non-steroidal anti-inflammatory medication. Hand walking once the initial stiffness has dissipated. Rehabilitation: icing for acute injuries, therapeutic ultrasound, massage and stretching. Resume exercise gradually with appropriate warm-up. Check saddle for fit. If treating fibrotic myopathy surgical procedures can be tried (tenotomy of the tibial insertion of the semimembranosus tendon and/or transection (± excision) of the fibrotic area of muscle).

Exertional Rhabdomyolysis "Tying Up"

Exertional rhabdomyolysis represents a pathological description of a number of muscle diseases which have common clinical signs.

A useful way to classify tying-up is the following:

1. Sporadic Exertional Rhabdomyolysis
     - Occurs in healthy horses that have no previous history of muscle disease
     - Responds well to treatment
     - Environmental causes e.g. over-exertion (muscle strain), dietary imbalances of electrolytes, vitamin E and selenium, excessive grain in diet, viral infections.

2. Chronic Exertional Rhabdomyolysis
     - Occurs repeatedly in horses often following light exercise due to an intrinsic muscle defect
     - Recurs in spite of treatment
     - Two disorders are well described;
          a. Polysaccharide Storage Myopathy
          b. Recurrent Exertional Rhabdomyolysis
     - Idiopathic chronic exertional rhabdomyolysis also exists

Clinical signs:
     • Ranges from mild stiffness following exercise to recumbency, myoglobinuria and severe muscle damage. May have painful muscle cramps especially in the gluteal area.
     • Horses develop a short stiff stride, sweat profusely, and have an elevated respiratory rate. Reluctant to move, may posture to urinate +/- myoglobinuria.
     • Muscle pain usually persists for several hours.
     • Endurance horses may show other signs of exhaustion including a rapid heart rate, dehydration, hyperthermia, synchronous diaphragmatic flutter and collapse.

     • Decide if Sporadic or Chronic
     • History of muscle cramping and stiffness during or following exercise
     • Mild to marked elevations in serum CK, LDH and AST at rest or, if necessary perform an exercise challenge test (pre and 4 h post exercise serum CK, using 15-30 min of lunge work at a trot).
     • Serum vitamin E and whole blood selenium concentrations
     • Urinalysis and determination of renal excretion of electrolytes (Na, K, Phosphorus)
     • Genetic testing (or muscle biopsy if needed) to identify underlying cause