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The ratio between muscle strength and muscle cross-sectional area is called the specific force. Fatty replacement of muscles is seen in many myopathies, affecting the specific force, without necessarily affecting the ability of the remaining muscle fibers to contract. This ability is called the contractility and is the ratio between muscle strength and the lean muscle cross-sectional area, i.e. the contractile cross-sectional area. We hypothesized that contractility is disrupted in patients with congenital myopathy, because of defects in contractile proteins of the sarcomere. Peak torque across ankle and knee joints was measured by isokinetic dynamometry in 16 patients with congenital myopathy and 13 healthy controls. Five patients only participated partially in the dynamometer measurements due to severe muscle weakness. Dixon MRI technique was used to quantify muscle fat fractions and calculate cross-sectional area. Patients with congenital myopathy had lower cross-sectional area in all muscle groups (P<0.01), higher fat fraction (P<0.01) and less strength (P<0.005) in all studied muscle groups. Their fat content was more than doubled and peak torque lower than half that in healthy controls. Muscle contractility was reduced (P<0.01) in three of four patient muscle groups. In conclusion, muscle contractility was reduced in patients with congenital myopathy, across different diagnoses, and was independent of the level of muscle fat fraction, suggesting that intrinsic defects of the myocyte are responsible for reduced contractility., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Bag3opathy is a rare myofibrillar myopathy (MFM) caused by a mutation in the Bcl-2 associated-athanogene-3 gene. Less than twenty patients have been described, almost all with severe cardiac involvement. We present a 26-year-old man with a c.626C>T (p.Pro209Leu) mutation in the Bcl-2 associated-athanogene-3 gene (BAG3). Our patient presented with problems running before he turned 10 and rapidly progressing, proximal muscle weakness and rigidity of the neck and back. Muscle biopsy showed Z-disc streaming, vacuoles, which is typical findings of Bag3opathy, as well as accumulation of filamentous materials. He rapidly developed respiratory insufficiency necessitating assisted ventilation, and became wheelchair bound by age 13. The progression of his muscle disease is characteristic of Bag3opathy, but unlike other reported cases, he had no evidence of cardiac involvement at age 25 years, despite serial Holter monitoring, ECG and echocardiographs. This case illustrates that counseling of patients with BAG3 myopathy should not predict an inevitable occurrence of cardiomyopathy., (Copyright © 2018 Elsevier B.V. All rights reserved.)