Onset in early childhood of slowly progressive muscle weakness starting in legs (children walk on tip-toe) followed by weakness in shoulder muscles; atrial conduction abnormalities, chest pain, and recurrent syncope occur in young adulthood. Condition may be associated with mental retardation. Disease severity varies widely. Inheritance is X-linked, and syndrome occurs almost exclusively in males; female carriers may develop heart block. Type 2 muscle fibers predominate. Gene defect is in Xq28 region.