Myophosphorylase deficiency (McArdle syndrome) is an uncommon condition characterized by exercise intolerance, muscle cramping, and myoglobinuria. Although the original report by McArdle dealt with a sporadic case, subsequent cases reported in the literature show high familial incidence and consanguinity, implying that the defect is transmitted as a rare recessive gene or a possible sex-limited mode of inheritance. The present report describes the clinical, histoenzymatic, and biochemical findings in a 40-year-old woman with myophosphorylase deficiency. The family history reveals that four other members are also affected: an older sister, a younger brother, a 10-year-old son, and her 75-year-old mother, and possibly her maternal grandmother. Because of this particular pattern of direct transmission in this family, a dominant inheritance is postulated.

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