P.17.18 Contribution of muscle MRI and 13C nuclear magnetic resonance spectroscopy in McArdle disease

Abstract

McArdle disease is a rare metabolic myopathy due to mutations in the PYGM gene, encoding for the myophosphorylase. The alteration of muscle glycogenolysis manifests as muscle pain and cramps during exertion and episodes of rhabdomyolysis. The natural course of the disease is usually benign, but a small number of patients develops permanent muscle weakness, usually after 50 years of age. The aims of the study were the evaluation of muscular morphology and structure with muscle MRI (whole body or lower limbs) and the evaluation of muscle glycogen storage as measured by 13C nuclear magnetic resonance spectroscopy in a single centre cohort of McArdle patients. Fifteen patients with a confirmed diagnosis of McArdle’s disease, aged 28 to 83 years (median: 49 years), were included in the study and evaluated by MRI either whole-body (11 cases) or lower limbs (4 cases). Carbon-13 NMR spectroscopy was available 8 cases. The results of these tests were compared with the clinical examination, CPK and grip test results. Five patients (age 44–83 years) presented morphological alterations on whole-body muscle MRI, with fatty substitution of axial muscles (4 cases) or lower limb muscles (1 case). In all but one patients the alterations were mild. All these patients had clinically evident muscle weakness. Carbon-13 NMR showed glycogen accumulation in all patients, with a range of ratio glycogen/creatine between 1.6 and 5.28 (laboratory controls 0.83 ± 0.18). This study is the largest study of muscle imaging in McArdle disease. We confirm the absence of alterations of muscle morphology in patients with McArdle’s disease and normal strength. Furthermore, muscle imaging can accurately evaluate the involvement of axial muscles. 13C NMR spectroscopy showed in all cases an abnormal accumulation of glycogen.