255P Relevance of muscle biopsies in the neonatal period: a 52-year retrospective study in the gene-sequencing era.

Neuromuscular disorders (NMD) with neonatal or perinatal onset are usually severe. The establishment of a definite diagnosis relies on the combination of clinical examination, enzymatic tests, muscle morphology analyses, and genetic investigations, and is essential to provide a vital prognosis, propose adequate genetic counseling, consider treatment options, and include the patients into clinical trials for innovative therapies. However, the recent and rapid advancements of panel, exome, and genome sequencing questions the utility of invasive muscle biopsies and the contribution of histological and ultrastructural studies of muscle sections to the diagnosis process. Here, we explored the relevance of muscle biopsies collected at the Institute of Myology in Paris over a period of more than 50 years (1970-2021) in light of the evolution of Sanger and next-generation sequencing (NGS) technologies. Altogether, 82% of the biopsies showed typical structural myofiber anomalies highly suggestive of specific NMD classes (congenital myopathies, metabolic myopathies, lower motor neuron (LMN) and neuromuscular junction (NMJ) disorders, muscular dystrophies, inflammatory myopathies), while the remaining 18% showed no or only non-specific histological abnormalities. The diagnosis success rate differed among the NMD classes and changed over time following the identification of major disease genes like SMN1 or DMD , shifting the focus to direct gene sequencing and reducing the necessity of muscle biopsies. Ultrastructural investigations were of particular importance for the diagnosis of congenital myopathies and less relevant for the other NMD categories. Typical myofiber lesions found in congenital myopathies as intranuclear rods in ACTA1-related nemaline myopathy are barely discernible under the light microscope and solely detectable by electron microscopy (EM). This shows that the higher resolution of EM is not only a feature complementing light microscopy, but also a powerful diagnostic method. From our experience, morphological muscle biopsy examinations were frequently accurate and either pointed to a group of myopathies or to single genes. In conclusion, this retrospective study describes a unique collection of neonatal muscle biopsies and illustrates that the thorough analysis of muscle sections was of major diagnostic relevance in the past and still plays an important role nowadays - either by directing purposeful gene sequencing or by complementing clinical findings and biochemical analysis methods. [ABSTRACT FROM AUTHOR]