Diagnostic Value of Muscle Biopsy for the Evaluation of Adult Myopathy in Daily Clinical Practice

<b>Background/Objectives</b><b>:</b> Muscle biopsy is traditionally considered a cornerstone in the diagnosis of myopathies. Advances in the clinical and laboratory evaluation of myopathies warrant re-evaluation of the diagnostic yield. <b>Methods:</b> Results of muscle biopsies performed between 1 January 2009 and 31 January 2023 in patients with symptoms indicative of myopathy were evaluated and set in relation to clinical diagnosis, based on phenotype, electromyography, laboratory results, and available antibody testing. Biopsies were classified as diagnostic (changed or specified clinical diagnosis), confirmative (same as clinical diagnosis), or non-informative (normal/unspecific findings). Genetic testing followed muscle biopsy at later follow-up, upon availability of genetic testing. <b>Results:</b> One-hundred sixty-two patients were included and divided into five groups based on clinical phenotype: inflammatory myopathy, <i>n</i> = 54; mitochondrial myopathy, <i>n</i> = 33; muscular dystrophy, <i>n</i> = 23; metabolic myopathy, <i>n</i> = 3; and non-specific phenotype (isolated hyperCKemia/myalgia), <i>n</i> = 49. Muscle biopsy was diagnostic in 21.0%, confirmative in 38.3% and non-informative in 40.7% of patients. The percentage of diagnostic biopsies was 66.7% in metabolic myopathy, 54.5% in mitochondrial myopathy, 17.4% in muscular dystrophy, 14.8% in inflammatory myopathy, and 4.1% in the non-specific phenotype. <b>Conclusions:</b> Overall, in our cohort, muscle biopsy yielded a new diagnosis or additional information in 21.0% of patients. In the majority, a diagnosis was established based on clinical and laboratory evaluation, and muscle biopsy was either confirmative or non-informative. We propose muscle biopsy in cases where serological and genetic tests are inconclusive, in the presence of specific signs indicative of myopathy, or when in-tissue genetic testing is necessary to obtain a comprehensive diagnosis.