Abstract
BACKGROUND: Myotonia congenita (MC) is mainly caused by variants in the CLCN1 Gene, which is characterized by having difficulty in relaxing the muscle after active contraction, known as myotonia. This study aims to investigate the clinical characteristics and gene mutations of myotonia congenita caused by CLCN1 mutation. CASE PRESENTATION: Five children with myotonia congenita due to CLCN1 mutations admitted to Nanjing Children's Hospital were included. All five children had a juvenile onset of the disease (1 to 11 years of age). Two had onset before 2 years of age, and three had onset after 10 years of age. All patients experienced muscle stiffness (5/5, 100.0%), two reported delayed relaxation of the hand after forceful grasping (2/5, 40.0%), and three reported that the muscle stiffness worsened with changes in motor status (3/5, 60.0%). These symptoms improved with exercise (warm-up phenomenon) (5/5, 100.0%).Two children had elevated CK (2/5, 40.0%), and EMG showed muscle tonic potentials in all five children (5/5, 100.0%). Eight CLCN1 gene mutation sites were identified in five patients, including four unreported variants: c.688G > A (p.G230R), c.2653_c.2654insC (p.A885Afs*27), c.1938G > T (p.M646I) and c.1825 A > G (p.M609V). In this paper, we also summarized the Chinese CLCN1 mutation sites reported in the last 10 years, revealing that exons 8 and 15 may be the hotspot regions of mutation in Chinese children. CONCLUSION: This study expands the clinical and genetic spectrum of Chinese children with myotonia congenita. The clinical manifestations observed in these children were similar with those previously reported in the literature. Additionally, exons 8 and 15 may be the hotspot regions for gene mutations in Chinese children with myotonia congentia.