Spatiotemporal Patterns of Fat Replacement in <scp>SELENON</scp> ‐Related Myopathy: A Whole‐Body Imaging Study

INTRODUCTION/AIMS: Variants in the SELENON gene cause a myopathy with axial weakness, scoliosis, and respiratory failure. This study aimed to characterize the temporal and spatial distribution of fat replacement across the whole body in patients with SELENON-related myopathy. METHODS: We conducted a retrospective study of genetically confirmed SELENON-related myopathy patients at a single center who had available whole-body skeletal muscle computed tomography or magnetic resonance imaging data. The degree of fat replacement in 47 skeletal muscles was scored using the modified Mercuri score (mMS). Spatial and temporal progression patterns of fat replacement were identified using k-means clustering and hierarchical clustering. RESULTS: The time-series imaging data from a single patient revealed that muscles could be classified into four patterns: those showing fatty replacement from the initial stage, those that undergo fatty replacement in the early phases, those affected in the advanced stages, and those that show minimal fatty replacement throughout the disease course. In the early stage, fat replacement was most prominent in axial and hip girdle muscles. Subsequently, thigh muscles were affected. In contrast, the muscles of the arms and lower legs were preserved even in advanced-stage patients. The whole-body mMS increased linearly with age. No significant correlations were observed between the mMS and walking ability or respiratory function. DISCUSSION: Fat replacement progresses with age, and serial imaging over time may allow more accurate prediction of muscle-specific involvement. These data may be useful for monitoring disease progression and serve as potential biomarkers for future therapeutic interventions.