A disorder of the musculoskeletal system caused by pathogenic variants in the RYR1 gene, which encodes the ryanodine receptor type 1 protein. These variants are associated with a variety of overlapping features characterized by symmetric proximal muscle weakness, often with pronounced facial weakness with or without dysmorphism and ophthalmoparesis/ophthalmoplegia with ptosis, bulbar weakness, significant respiratory involvement, severe neonatal hypotonia, scoliosis, orthopedic deformities including arthrogryposis, hip dislocation, club feet, and King Denborough syndrome (pectus carinatum or excavatum, short stature, joint contractures, facial and skeletal deformities), malignant hyperthermia susceptibility, anesthesia-induced rhabdomyolysis, fatigue, exercise-induced hyperthermia/exertional heat stroke, and exertional myalgia. Histologic findings on skeletal muscle biopsy reveal a wide range of structural abnormalities and can include central core disease, multiminicore disease, cone-rod myopathy, centronuclear myopathy, and congenital fiber-type disproportion. [ https://clinicalgenome.org/affiliation/40031/ http://www.ncbi.nlm.nih.gov/pubmed/30406384 ]