Background and PurposezzHyperkalemic periodic paralysis (hyperKPP) is a muscle sodi-um-ion channelopathy characterized by recurrent paralytic attacks. A proportion of affected individuals develop fixed or chronic progressive weakness that results in significant disability. However, little is known about the pathology of hyperKPP-induced fixed weakness, includ-ing the pattern of muscle involvement. The aim of this study was to characterize the patterns of muscle involvement in hyperKPP by whole-body magnetic resonance imaging (MRI). MethodszzWe performed whole-body muscle MRI in seven hyperKPP patients carrying the T704M mutation in the SCN4A skeletal sodium-channel gene. Muscle fat infiltration, sugges-tive of chronic progressive myopathy, was analyzed qualitatively using a grading system and was quantified by the two-point Dixon technique. ResultszzWhole-body muscle MRI analysis revealed muscle atrophy and fatty infiltration in hyperKPP patients, especially in older individuals. Muscle involvement followed a selective pattern, primarily affecting the posterior compartment of the lower leg and anterior thigh muscles. The muscle fat fraction increased with patient age in the anterior thigh (r=0.669, p=0.009), in the deep posterior compartment of the lower leg (r=0.617, p=0.019), and in the superficial posterior compartment of the lower leg (r=0.777, p=0.001). ConclusionszzOur whole-body muscle MRI findings provide evidence for chronic progres-sive myopathy in hyperKPP patients. The reported data suggest that a selective pattern of muscle involvement—affecting the posterior compartment of the lower leg and the anterior thigh—is characteristic of chronic progressive myopathy in hyperKPP. Key Wordszz hyperkalemic periodic paralysis, chronic progressive myopathy, muscle MRI, fat quantification, two-point Dixon technique

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