There are several types of nemaline myopathy, caused by mutations in different genes. It primarily affects skeletal muscles, which are muscles that the body uses for movement. Nemaline myopathy causes muscle weakness throughout the body, but it is typically most severe in the muscles of the face, neck and limbs. This weakness can worsen over time. The muscle problems associated with nemaline myopathy are caused by abnormal buildup of thread-like structures in certain muscle tissue.