Myotonic dystrophy is part of a group of inherited disorders called muscular dystrophies. It is the most common form of muscular dystrophy that begins in adulthood. It is a multi-system disorder that affects the skeletal muscles (the muscles that move the limbs and trunk) as well as smooth muscles (the muscles that control the digestive system) and cardiac muscles of the heart. Symptoms of myotonic dystrophy might include difficulty releasing one’s grip (myotonia), weakness of muscles in the hands and feet, difficulty swallowing and abnormal heart rhythms. Non-muscle symptoms may also include learning difficulties, daytime sleepiness, infertility and early cataracts. Any single individual is unlikely to have all or even most of these symptoms.
Signs and symptoms of myotonic dystrophy usually develop when a person is in his or her twenties or thirties. The severity of myotonic dystrophy varies widely among those who have it, even among family members. The weakness and muscle wasting that occurs slowly progress to the point of disability. Usually, disability does not become severe until fifteen to twenty years after the symptoms appear. The progression of muscle weakness is slower and is less serious in people who are older when the muscle weakness is first noticed.
Both Type 1 and Type 2 myotonic dystrophy are inherited in families in an autosomal dominant pattern. In autosomal dominant inheritance, having one copy of the altered (mutated) gene in each cell will cause the disorder. Both men and women are equally likely to be affected and to pass on the disorder. Usually a person who has myotonic dystrophy also has a one parent who has myotonic dystrophy. In families that have myotonic dystrophy, the altered gene is passed down from one generation to the next. The disorder may begin earlier in life and signs and symptoms become more severe. This is called anticipation. In Type 1 myotonic dystrophy, anticipation happens because there is an increase in the length of the unstable region in the DMPK gene (expansion). The cause of anticipation seen in families who have Type 2 myotonic dystrophy is not yet known.
The two major types of myotonic muscular dystrophy (DM) — DM1 and DM2 — are both caused by genetic defects. DM1, the most common type, results from an abnormal DNA expansion in the DMPK gene on chromosome 19. DM2 arises from an abnormal expansion of DNA in the ZNF9 gene (also called CNBP gene) on chromosome 3. Within DM1 there are additional subtypes, depending on a person’s age at onset of symptoms. The age of onset is roughly correlated with the size of the DNA expansion, with larger expansions associated with earlier disease onset. The subtypes of DM1 are:
DM2 — sometimes called PROMM (proximal myotonic myopathy) — has not been seen in a congenital-onset form and rarely begins in childhood. Therefore, it is not described in subtypes. DM2 tends to involve the proximal muscles (close to the center of the body) rather than the distal muscles (far from the center of the body) that are the first to be affected in DM1. In general, the disorder is not as severe as DM1. However, it may affect walking ability earlier than DM1, because it causes early weakening of the hip muscles. DM2 is rare compared to DM1.
Myotonic dystrophy affects at least 1 in 8,000 people worldwide. The prevalence of the two types of myotonic dystrophy varies among different geographic and ethnic populations. In most populations, type 1 appears to be more common than type 2. However, recent studies suggest that type 2 may be as common as type 1 among people in Germany and Finland.
A diagnosis of myotonic dystrophy may be suspected based upon a thorough clinical evaluation, a detailed patient and family history, and identification of characteristic physical findings. A family history of muscle weakness and myotonia is a strong indicator of a diagnosis for DM. Evaluation of a patient for myotonic dystrophy includes neurological examination, EMG assessment (that records electrical activity in skeletal (voluntary) muscles at rest and during muscle contraction) and obtaining serum for genetic testing. Molecular genetic testing can confirm a diagnosis of DM1 or DM2. Molecular genetic testing looks for changes or alterations in the DMPK gene known to cause DM1, or in the CNBP gene for DM2. However, this testing is available only as a diagnostic service at specialized laboratories.
No treatments currently exist that slow the progression of myotonic dystrophy, but symptomatic treatments are available. Managing the symptoms of this disease can reduce suffering and improve quality of life for patients. Ongoing monitoring can avert or reduce the complications seen at critical times. Symptomatic treatments include: