Atypical McArdle’s disease with asymmetric weakness and Atrophy

McArdle’s disease is an autosomal recessive disorder of glycogen metabolism, which occurs due to deficiency of myophosphorylase, an enzyme required for conversion of glycogen to glucose-1-phosphate in the muscle [1]. It is also known as Glycogen Storage Disease Type V or Glycogenosis Type V. This syndrome typically presents in the first two decades of life with exercise intolerance, muscle pain, cramps, rhabdomyolysis and myoglobinuria. Patients notice stiffness after exercise and a second-wind phenomenon during exercise [2]. It can present in infancy with progressive weakness, hypotonia, respiratory distress and early death. In the adult onset form, the findings are atypical with symmetric progressive limb-girdle weakness and delayed onset wasting [3]. Heart and liver have a different isomer of the phosphorylase, and are thus not affected. An ischemic exercise test can be used to screen for this condition. In a diseased person, there is absence of increase in serum lactate with an increase in ammonia production.