CACNA1S-associated triadopathy presenting with myalgia, muscle weakness, and asymptomatic hyperCKemia.

CACNA1S variants can alter the structure and function of the calcium channel, resulting in abnormal calcium influx and homeostasis. It is well established that pathogenic variants in CACNA1S can lead to hypokalemic periodic paralysis, malignant hyperthermia, and congenital myopathy. Nevertheless, the clinical presentations and disease progression of exertional myalgia and weakness associated with CACNA1S variants remain elusive. In this study, four affected individuals from an autosomal-dominant family were described, exhibiting symptoms of severe exertional myalgia, followed by flaccid weakness or rhabdomyolysis, along with asymptomatic hyperCKemia during the interictal period. Long exercise test showed a late decrease in compound muscle action potential amplitude. Muscle MRI revealed edema-like changes in the early stage, and fatty degeneration and substitution in prolonged disease courses, while closely aligned with the features of chronic myopathy. Ultrastructural examination revealed dilation of the sarcoplasmic reticulum and myofibrillar structural disarrangement. Genetic screening identified a c.3724A>G (p.Arg1242Gly) mutation in the CACNA1S gene. A literature review revealed that 15 patients exhibited the exertional myalgia and weakness phenotype associated with CACNA1S mutations, presenting similar clinical, electrophysiological, radiological, and pathological features. As the disease progressed, these patients developed severe muscle weakness, ultimately leading to wheelchair dependency. This exertional myalgia-weakness phenotype represented a unique CACNA1S-related phenotype that broadened the spectrum of CACNA1S-associated myopathy, bridging between periodic paralysis and congenital myopathies. The similarities between CACNA1S-associated myalgia-weakness and RyR1-associated myalgia-weakness underscored a shared pathogenesis of excitatory-contractile coupling at the triad of skeletal muscle.