Disorders of Muscle Mass and Tone.

Muscle disease has various clinical manifestations that range from exertional and non-exertional rhabdomyolysis, fasciculations, weakness, rigidity, stiffness, gait abnormalities, poor performance, and alterations in muscle mass and tone. Neurogenic disorders and non-neurogenic disorders such as primary muscle disease can cause muscle atrophy and changes in muscle tone. Myotonic disorders can have a genetic (eg, inherited channelopathies) or acquired (eg, electrolyte derangements) origin. Normal muscle enzyme activities do not rule out a myopathic disorder as the underlying cause of muscle atrophy and changes in muscle tone. Genetic testing to facilitate responsible breeding practices is recommended.