The clinical and pathological characteristics of a hypokalemic periodic paralysis family with muscle atrophy due to SCN4A R672G mutation and review of literatures

Objective To report a family of hypokalaemic periodic paralysis (HypoPP) with muscle atrophy due to SCN4A gene R672G mutation. The clinical, pathological and MRI characteristics of HypoPP were summarized combining literatures review. Methods and Results The proband was a 27⁃year⁃old male patient who firstly presented periodic muscle weakness from 7 years old. The episode frequency increased with age, and gradually accompanied with muscle atrophy and permanent weakness. His mother has the similar episode but lower frequency. Serum potassium level decreased at the episode. The long⁃term EMG showed the compound muscle action potential (CMAP) on ulnar nerve gradually decreased from 0-45 min. The CMAP in abductor digiti minimi was decreased by 75.8% after long⁃time exercise test. Muscle MRI showed edema in vastus lateralis, vastus medialis, gastrocnemius lateralis, gastrocnemius medialis, soleus, et al. Muscle pathology showed eosinophilic light⁃stained sediment under the sarcolemma. Transmission electron microscopy (TEM) showed the sediment under the sarcolemma were primitive myofilaments with disordered arrangement. Gene test showed heterozygous mutation on exon 12 c.2014C＞G (Arg672Gly) in SCN4A gene in proband and his mother. Finally, the proband was diagnosed HypoPP, and the family was confirmed HypoPP due to SCN4A gene R672G mutation. Conclusions HypoPP due to SCN4A gene R672G mutation can have pathological feature of muscle atrophy and sediment of primitive myofilament. The nature of the sediment needs further study.