A rare case of mitochondriopathy with autosomal dominant progressive external ophthalmoplegia diagnosed through skeletal muscle biopsy.

: Mitochondriopathies are a heterogeneous group of genetic diseases of all ages, with a very diverse clinical presentation related to genetic heteroplasmy. The clinical symptoms display a large variability and generally, the more severe phenotypes have an early onset, even from the neonatal period, while milder ones are manifested later in the adulthood. Most publications have already demonstrated deletions or point mutations in mitochondrial deoxyribonucleic acid (DNA), but in recent years, the field of investigation has expanded to syndromes caused by mutations in the nuclear DNA (nDNA), with a Mendelian inheritance. We present the case of a male patient with a mitochondriopathy with phenotype of chronic progressive external ophthalmoplegia (PEO), due to an autosomal dominant mutation in nDNA, in the DNA polymerase subunit gamma (POLG) gene, the pathogenic variant c.2864A>G (p.Tyr955Cys), morphologically investigated and diagnosed using a skeletal muscle biopsy. The aim of this presentation is to emphasize the diagnostic value of the muscle biopsy both in cases of clinical suspicion and in more challenging cases of mitochondrial diseases with atypical or unusual features. Although genetic testing may be the initial test of choice in cases with suggestive clinical presentation, muscle biopsy is an alternative diagnostic aid with high value even in our molecular era. We present pathological and ultrastructural data to confirm the diagnosis.