Background A 27-year-old male with perinatal hypoxia presented with global developmental delay, progressive hearing loss, ataxia, dysarthria, and intellectual disability. Whole-exome sequencing revealed compound heterozygous ACAD9 variants: c.456del (p.Ile153Serfs*46) and c.869G > A (p.Gly290Glu). Brain MRI showed bilateral cerebellar atrophy and a prominent cisterna magna. OCT confirmed optic atrophy. The diagnosis of mitochondrial encephalomyopathy (complex I deficiency type 20) was establishe