Genetic and Physical Study of Childhood Nerve and Muscle Disorders

Background: \- Some nerve and muscle disorders that start early in life (before age 25), like some forms of muscular dystrophy, can run in families. However, the genetic causes of these disorders are not known. Also, doctors do not fully understand how symptoms of these disorders change over time. Researchers want to learn more about genetic nerve and muscle disorders that start in childhood by studying affected people and their family members, as well as healthy volunteers. Objectives: \- To better understand nerve and muscle disorders that start early in life and run in families. Eligibility: * Individuals at least 4 weeks old with childhood-onset muscular and nerve disorders, including those who have a later onset of a disorder that typically has childhood onset. * Affected and unaffected family members of the individuals with muscular and nerve disorders. * Healthy volunteers at least 4 weeks old with no nerve or muscle disorders. Design: * Participants will be screened with a physical exam and medical history. Genetic information will be collected from blood, saliva, cheek swab, or skin samples. Urine samples may also be collected. * Healthy volunteers and unaffected family members will have imaging studies of the muscles. These studies will include magnetic resonance imaging (MRI) and ultrasound scans. Results will be compared with those from the affected participants. * All participants with nerve and muscle disorders will have multiple tests, including the following: * Imaging studies of the muscles, including ultrasound and MRI scans. * Imaging studies of the bones, such as x-rays and DEXA scans. * Heart and lung function tests. * Eye exams. * Nerve and muscle electrical activity tests and biopsies. * Video and photo image collection of affected muscles. * Speech, language, and swallowing evaluation. * Lumbar puncture to collect spinal fluid for study. * Tests of movement, attention, thinking, and coordination. * Participants with nerve and muscle di