Résumé :
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Communication n° 48. Muscle-specific phosphorylase b kinase (PbK) deficiency is an unusual form of glycogen storage disorder. PbK converts the inactive form of glycogen phosphorylase into its active form. Most patients are male, complain of exercise intolerance and are aged 15 to 35 at the time of diagnosis. We report a 60 year-old man with PbK deficiency. At 55, the patient experienced cramps, increasing difficulty running and climbing stairs, then walking, which led him to give up his job as an oyster farmer. He had no exercise-induced myoglobinuria. Muscle weakness of lower limbs was predominantly proximal and slightly asymmetric; upper limb muscles were normal. The family history was negative. CPK was slightly raised. EMG revealed a mixed pattern of neurogenic and myogenic changes. Inclusion body myopathy was suspected and a biopsy specimen was taken from the left vastus lateralis muscle. The muscle fibers displayed abnormal variation of size, some being atrophic and others hypertrophic; a few atrophic fibers were angulated; some were regenerating; there were a few invaded necrotic fibers without endomysial inflammation, but no invasion of non-necrotic fibers. Many showed small subsarcolemmal vacuoles and some had slightly enlarged intermyofibrillar spaces that contained PAS-positive material. Vacuoles were found in both type I and type II fibers. The stain for myophosphorylase was normal. In vitro anaerobic glycogenolysis screening showed decreased glycogen utilization by the muscle, glycolysis being normal. Glycogen content of muscle was moderately elevated; total phosphorylase activity was normal; active phosphorylase activity, in the absence of AMP, was reduced; amylo 1-6 glucosidase pathway of glycogenolysis was enhanced and PbK activity was decreased (10% of controls). Pbk deficiency must be included in the differential diagnosis of proximal lower limb weakness in elder subjects even with no history of exercice intolerance.Only a muscle biopsy can confirm the diagnosis
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