Myotonic muscular dystrophy (MMD) is a form of muscular dystrophy that affects muscles and many other organs in the body. Unlike some forms of muscular dystrophy, MMD often doesn’t become a problem until adulthood and usually allows people to walk and be fairly independent throughout their lives.
The infant form of MMD is more severe. It can occur in babies born to parents who have the adult form, even if the parents have very mild cases.
The word myotonic is the adjective for the word myotonia, an inability to relax muscles at will. Most commonly, myotonia makes it difficult to relax the fingers after a firm hand grip. People with adult-onset myotonic dystrophy may simply adjust to this problem, and not realize that slow muscle relaxation is abnormal.
The term muscular dystrophy means progressive muscle degeneration,with weakness and shrinkage of the muscle tissue. This muscle wasting generally presents much more of a problem to people with MMD than does the myotonia. Muscle pain also can occur in MMD.
Weakness and wasting of voluntary muscles in the face, neck and lower arms and legs are common in myotonic muscular dystrophy. Muscles between the ribs and those of the diaphragm, which moves up and down to allow inhalation and exhalation of air, also can be weakened.
MMD symptoms also can begin in children past infancy but not yet adolescents. Generally, the earlier MMD begins, the more severe the disease symptoms.
Myotonic muscular dystrophy often is known simply as myotonic dystrophy and occasionally is called Steinert disease, after the doctor who originally described the disorder in 1909. It’s also called by its Greek name dystrophia myotonica, and therefore sometimes is abbreviated “DM” rather than “MMD.”
Myotonia isn’t a feature of any other form of muscular dystrophy (although it occurs in other kinds of muscle diseases, where it can be severe). When a person suspected of having muscular dystrophy has myotonia, the diagnosis is likely to be MMD.
MMD varies greatly in severity, even within the same family. Not everyone has all the symptoms and not everyone has them to the same degree. For some people, symptoms are fairly mild even in middle age, while for others, the weakness and wasting are severely limiting to daily activities. For still others, the effects are somewhere in between.
There is, however, a distinct difference between the type that affects newborn infants — congenital MMD — and the type that begins in adolescence or adulthood — adult-onset MMD.
Infants with congenital MMD have severe muscle weakness, including weakening of the muscles that control breathing and swallowing. These problems can be life-threatening and need intensive care. Myotonia isn’t part of the picture in infants with MMD but may occur in later life.