Congenital core myopathy linked to <i>SOX5</i> : Expanding the phenotypical spectrum of Lamb-Shaffer syndrome

Haploinsufficiency of <i>SOX5</i> causes Lamb-Shaffer syndrome, a rare condition with developmental delay, impaired language and intellectual disability and optic nerve abnormalities. Muscle involvement is poorly characterized. We report a 20-year-old woman with neonatal hypotonia, delayed motor milestones, proximal weakness and learning difficulties. Serum creatine kinase was normal. Electroneuromyography revealed a mild myogenic pattern. Muscle biopsy revealed myopathic changes, with cores and protein aggregates. Whole genome sequencing disclosed a heterozygous pathogenic variant in the <i>SOX5</i> gene. This case expands the phenotypic spectrum of Lamb-Shaffer syndrome, confirming primary muscle involvement with core myopathy.