Congenital myopathies constitute a very heterogeneous group of neuromuscular diseases both clinically and genetically. They are related to the presence of intrinsic lesions or the accumulation of inclusions inside the muscle fiber. Classically described as responsible for global hypotonia and little or no progressive, when the critical period of the first days or months of life is over, congenital myopathies sometimes may show clinical pictures out of the ordinary.
In an article published in September 2020, French clinicians and researchers report two original observations of patients with asymmetric muscle deficit since birth, with delayed motor acquisitions, and having progressed to scoliosis as well as respiratory failure with decrease of vital capacity. In one patient, the asymmetry of the deficit, although real, was less obvious than in the other. In both cases, rods were found on the muscle biopsy. Genetic studies have confirmed the existence of de novo mutations (distinct in the two observations) in the ACTA1 gene, a gene frequently implicated in congenital myopathies, notably in nemaline. The originality of the results lies in the fact that these mutations were the result of a so-called mosaic distribution of the molecular defect. In other words, these mutations were only present and detectable in the muscle biopsy but not in the circulating blood. Considering this rare mechanism could help resolve cases of congenital myopathy still awaiting aetiological diagnosis.
