Muscular Dystrophy Muscular dystrophy causes the muscles in the body to become very weak. The muscles break down and are replaced with fatty deposits over time. Myasthenia Gravis in Children Myasthenia gravis is a complex, autoimmune disorder that causes problems with the nerves that communicate with muscles, resulting in weakness of the skeletal muscles. Spinal Muscular Atrophy Spinal muscular atrophy is a genetic disease that affects the spinal cord and nerves, resulting in muscle wasting and weakness.