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George E. Carvell, PhD, PT   University of Pittsburgh
 
PERIPHERAL NERVE DYSFUNCTION II: MYOPATHY
Ben.  fig 13-31  p 421
Myopathies are diseases of skeletal muscle that attack muscle fibers. Many primary muscle diseases are inherited, and are progressive (Musclular Dystrophies or MD). The muscle weakness begins early in childhood for most forms of MD. Cardiac myopathy may accompany skeletal muscle degeneration. Degeneration of the contractile proteins results in a reduced ability to generate force. Motor units have a reduced innervation ratio and to generate tension the individual must increase recruitment by firing more motor units at a higher rate of discharge. Clinical Electromyo-
graphic Needle Exams reveal small amplitude, polyphasic potentials (myopathic potentials). Electromyographers report an early interference pattern (motor unit potentials overlap
long before a maximal volitional effort is reached). Normally an interference is seen only when maximal effort is requested. Nerve Conduction is normal. There are no sensory deficits, no evidence of central motor or other brain dysfunction. Most types of primary muscle disease involve proximal musculature. Reduced power in shoulder and hip girdle muscles and axial muscles make reaching, transfers, standing and walking a challenge that eventually becomes disabling. Weakness typically spreads to more distal muscles. A waddling gait, lordotic standing posture, and difficulty in rising from a recumbent position are characteristic of the Duchenne and other forms of muscular dystrophy. Clinical examination typically includes a test for loss of muscle power in rising from a lying position on the floor. Individuals with proximal weakness tend to climb up upon themselves to rise to a standing position (often called a Gower's sign). Despite the severe weakness in the affected muscles, those muscles may actually appear to be well developed (pseudohypertrophy). The involved muscles have an increase in connective tissue, evidence of immature multinucleated muscle fibers, and fibrotic replacement of degenerated muscle fibers. Late in the disease, muscle biopsies may reveal a high percentage of fat that has replaced the lost muscle tissue.
GMOMM  2001
Recruit more motor units/
Unit of force