Antioxidant Therapy in RYR1-Related Congenital Myopathy

Background: \- Ryanodine receptor type 1-related myopathies (RYR1-RM) are the most common non-dystrophic muscle diseases that people are born with in the U.S. They affect development, muscles, and walking. Researchers want to test a new drug to help people with these diseases. Objectives: \- To see if the drug N-acetylcysteine decreases muscle damage in people with RYR1-RM. To see if it improves their exercise tolerance. Eligibility: \- People age 7 and older with a confirmed genetic diagnosis of RYR1 or a clinical diagnosis of RYR1 and a family member with a confirmed genetic diagnosis. Design: * Participants will be screened with a checklist of criteria. Adult participants may have a muscle biopsy. A needle will remove a tiny piece of muscle in the lower leg. * Study visits will take several days. * Visit 1: * Medical history * Physical exam * Blood, urine, and saliva tests * Questions about symptoms and quality of life * Heart, lung, and walking tests * Muscle Oxygenation Capacity Test. A blood pressure cuff around the thigh will be tightened for up to 10 minutes. * Biodex testing, stretching the leg against resistance * Muscle ultrasounds. A probe will be moved over the skin. * Participants may be photographed or videotaped during procedures. * They may have a muscle biopsy. * Six months later, visit 2 will repeat visit 1. Participants will start taking the study drug dissolved in water or placebo three times a day for 6 months. * Participants will stay at NIH for 2 days after starting the study drug. * Participants will be contacted by phone during the study to monitor side effects * Six months after starting the study drug, study visit 3 will repeat some or all of visit 1.