Late-onset Mcardle's disease with unusual electromyographic findings.

Symptoms of McArdle's disease (muscle phosphorylase deficiency) commonly begin in childhood or adolescence. Late onset of the disease is rare. We describe a 76-year-old man whose symptoms began at age 74 years with sudden onset of proximal muscle weakness and fatigability. Electromyography disclosed substantial spontaneous activity and myopathic features as seen in inflammatory muscle disease. The diagnosis of McArdle's disease was made by histochemical studies of muscle, an abnormal ischemic lactate test, and absence of myophosphorylase activity.