Congenital monomelic muscular hypertrophy of the upper extremity.

Pathological muscular hypertrophy results from either muscular or neurogenic damage. Rarely, it is caused by a congenital malformation consisting of a unilateral muscular hyperplasia of the upper extremity. We report on a young woman with an enlargement of the right upper extremity. Electromyography showed polyphasic, large motor unit potentials in the affected muscles. MRI and ultrasound assessment demonstrated diffuse enlargement of muscle mass without signs of edema. Muscle biopsy revealed sections with marked variations in fiber size with no signs of inflammation or marked loss of muscle fibers. Factors assumed to be important in the pathophysiology of this phenomenon are discussed.