Scientists have discovered a genetic abnormality that apparently causes the most common adult form of muscular dystrophy, a discovery that could help research into developing a treatment. The finding also should allow better diagnosis of the inherited condition, called myotonic dystrophy, prenatally or before symptoms appear in later life, experts said. Myotonic dystrophy affects about one in every 7,000 to 8,000 people worldwide. It causes weakness and wasting of voluntary muscles. It also can lead to life-threatening irregularities in heartbeat. The new research is reported in today's issue of Nature by three international teams with members in five countries.