Mitochondrial Myopathy

Mitochondria are the celular structures in which energy (in the form of heat and work) is produced from the oxidation of fuels such as glucose and fat. A number of biochemical defects have been discovered in mitochondria. Inherited disorders of carnitine metabolism are one of the causes of mitochondrial myopathy. Many mitochondrial diseases are known to be due to abnormalities of mitochondrial DNA.

A myopathy is a neuromuscular disease in which the muscle fibers dysfunction for any one of many reasons, resulting in muscular weakness. Muscle cramps, stiffness, and spasm can also be associated with myopathy. Because myopathy is such a general term, there are several classes of myopathy which include congenital myopathies, muscular dystrophies and mitochondrial myopathies among others. Because different types of myopathies are caused by many different pathways, there is no single treatment for myopathy. Treatments range from treatment of the symptomes to very specific cause-targeting treatments. Drug therapy, physical therapy, bracing for support, surgery, and even acupuncture are current treatments for a variety of myopathies.

In mitochondrial disorders, the most severely affected organs are those most dependent on oxidative metabolism (chemical reactions involving oxygen), including brain, skeletal, and cardiac muscles; sensory organs; and kidney. The mildest form of an existing mitochondrial myopathy may cause only slight muscle weakness most notable in the arms associated with exercise intolerance. The severe form seen in humans called infantile mitochondrial myopathy is invariably fatal and characterized by muscle weakness and lactic acidosis found shortly after birth and prove fatal in the first year of life.

Few cases of mitochondrial diseases have been described in veterinary medicine. In dogs, mitochondrial myopathy has been reported in young (less than 6 months of age) Clumber spaniels and Sussex spaniels, German shepherd dogs as well as young adult Old English sheepdog. Dogs affected with mitochondrial myopathies have severe exercise intolerance that can be consistently demonstrated with even mild exercise. They have stiff, stilted gait, bunny-hopping in the pelvic limbs, reluctance to move, and spontaneous pain. Laboratory tests reveal exercise-induced metabolic acidosis.

Diagnosis of mitochondrial myopathies requires demonstration of elevated resting or post-exercise plasma lactate and pyruvate concentrations, and light and electron microscopic evaluation of mitochondria within muscle biopsy sections. Precise characterization is dependent on specialized biochemical tests and molecular studies.

Adapted from:
1. Clinical Neurocardiology. Louis R. Caplan, J. Willis Hurst, Marc I. Chimowitz
2. A Practical Guide to Canine and Feline Neurology. Curtis W. Dewey, Anton G. Hoffman, Carol Rudowsky.
3. Mitochondrial Myopathy in a German Shepherd Dog. O. PACIELLO, P. MAIOLINO, G. FATONE, AND S. PAPPARELLA. In: Vet Pathol 40:507–511 (2003)
4. Neuromuscular Disorders affecting Young Dogs and Cats (1999). G. Diane Shelton. In: Vet Neurol Neurosurg J. January 1999;1(1):1.

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