Myotonic muscular dystrophy is a common multi-system disorder that affects the skeletal muscles as well as smooth muscles 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. There are two known forms of this disease (Myotonic Dystrophy Type 1 and Myotonic Dystrophy Type 2). Both are caused by abnormal expansions of repeated areas of genes. In Myotonic Dystrophy Type 1, the repeat expansion enlarges with each generation, frequently leading to earlier onset and increased severity of symptoms with each affected generation. Myotonic Dystrophy Type 1 therefore frequently affects children in families with this disorder.
What causes this disease?
Type 1 MMD (MMD1) occurs when a gene on chromosome 19 called DMPK contains an abnormally expanded section. Type 2 MMD (MMD2) is caused by an abnormally expanded section in a gene on chromosome 3 called ZNF9. The expanded sections of DNA in these two genes appear to have many complex effects on various cellular processes.
Symptoms
MMD causes weakness of the voluntary muscles, although the degree of weakness and the muscles most affected vary greatly according to the type of MMD and the age of the person with the disorder. Myotonia, the inability to relax muscles at will, is another feature of MMD. For example, it may be difficult for someone with MMD to let go of someone's hand after shaking it. As the disease progresses, the heart can develop an abnormal rhythm and the heart muscle can weaken. The muscles used for breathing can weaken, causing inadequate breathing, particularly during sleep. In addition, in type 1 MMD, the involuntary muscles, such as those of the gastrointestinal tract, can be affected. Difficulty swallowing, constipation and gallstones can occur. In females, the muscles of the uterus can behave abnormally, leading to complications in pregnancy and labor. The development of cataracts (opaque spots in the lenses of the eyes) relatively early in life is another characteristic of MMD, in both type 1 and type 2. Overall intelligence is usually normal in people with MMD, but learning disabilities and an apathetic demeanor are common in the type 1 form. In congenital MMD1, which affects children from the time of birth, there can be serious impairment of cognitive functioning. These children also may have problems with speech, hearing and vision.
TreatmentsEfforts to find a cure for myotonic dystrophy are ongoing. At present treatment is symptom oriented. The increased muscle tone for which the disease is named is often not debilitating however the excess tone may respond to medication. Genetic counseling is important especially since prenatal diagnosis is possible. Ankle-foot-orthotics (AFOS) are used for foot drop, wrist braces can be recommended for wrist weakness. Cataracts are removed when symptomatic. Special attention to cardiac complications is warranted; recommendations have included yearly electrocardiograms. Because cardiac arrythmias have been detected in as many as 50% of those with myotonic dystrophy, holtor monitoring is an important diagnostic modality. Pulmonary hygiene includes breathing exercises and postural drainage. Genetic counseling is important especially since prenatal diagnosis is possible. Patients with myotonic dystrophy should be aware of a special vulnerability when undergoing anesthesia.
Sources
http://mda.org/disease/myotonic-muscular-dystrophy/overview
http://www.hopkinsmedicine.org/neurology_neurosurgery/specialty_areas/muscular_dystrophy/conditions/myotonic_muscular_dystrophy.html
http://medicine.yale.edu/neurology/divisions/neuromuscular/md.aspx
