Genetics Northern California

Myotonic Dystrophy

Myotonic dystrophy (DM) is the most common adult form of muscular dystrophy. It is a progressive muscular dystrophy that affects approximately 1 in 8000 individuals. Unlike other forms of muscular dystrophy, in this condition the muscle weakness is accompanied by myotonia (delayed relaxation of muscles after contraction) and by a variety of abnormalities in addition to those of muscle. The disorder is also known as Steinert's disease and dystrophia myotonica.

The primary symptoms of this disorder are myotonia and progressive muscle weakness. Multiple system involvement may include skeletal muscle and smooth muscle, as well as the eye, heart, endocrine system, and central nervous system. Mild DM is characterized by cataract and mild myotonia; life span is normal. Classical DM is characterized by muscle weakness and wasting, myotonia, cataract, and often by cardiac conduction abnormalities. Onset of symptoms is often in the 3rd or 4th decade. Adults may become physically disabled and may have a shortened life span. Congenital DM is evident at birth and is characterized by hypotonia and severe generalized weakness at birth, often with respiratory and feeding problems. Mental retardation is common in individuals with congenital DM. Prenatally affected pregnancies are often characterized by polyhydramnios and reduced fetal movement. The congenital form is most often observed in infants of mothers with DM.

Cause
DM is genetic condition inherited in an autosomal dominant manner. It is caused by  gene defect called a trinucleotide repeat (triplet repeat). A trinucleotide repeat is a sequence of three nucleotides repeated a number of times in a row within a gene (e.g., CTGCTGCTGCTG…). These repeated sequences often vary in length from person to person in the general population without causing any health problems. In the DM, the gene involved is called the DMPK (myotonin protein kinase) gene and is located on chromosome 19.  Within the DMPK gene there is a repeated DNA sequence of “CTG”. Usually a person has fewer than 37 copies of the repeated CTG code. When the repeat length exceeds 37 repeats, the gene is no longer able to work properly.  The total number of repeats a person has in the DMPK gene can help predict the severity of medical problems that may be seen, with higher numbers usually causing more problems. 

The size of the triplet repeat can increase when it is passed from a parent to child, resulting in “anticipation”. Anticipation is the tendency in certain genetic disorders for earlier age of onset and more severe involvement in successive generations. Anticipation is often observed in disorders resulting from trinucleotide repeat expansion mutations.

Diagnosis
DM is suspected in adults with the following:

  • Muscle weakness, especially of the distal leg, hand, neck, and face

  • Myotonia (sustained muscle contraction), which often manifests as the inability to quickly release a hand grip

  • Cataracts, which require slit lamp examination by an ophthalmologist for early detection

DM is suspected in newborns with some combination of hypotonia, facial muscle weakness, generalized weakness, club foot, and respiratory insufficiency or failure.

The diagnosis of DM is confirmed by detection of an expansion of the CTG trinucleotide repeat in the DMPK gene.  DNA-based testing is essentially 100% sensitive and is widely available.

Treatment/Services
Treatment is available, as needed, to manage many of the symptoms associated with this disease. Cataracts, diabetes mellitus, hypothyroidism, and sleep apnea are all amenable to specific treatment, just as they are in patients who do not have DM. However, no specific treatment exists for the progressive weakness that is responsible for most of the disability in patients with DM. Physiatrists can help evaluate patients regarding the need for ankle/foot, orthoses, wheelchairs, or other assistive devices.

As a general rule, it is recommended that an EKG be obtained once a year to detect asymptomatic cardiac conduction defects. Consultation with a cardiologist is recommended for patients with cardiac symptoms or an arrhythmia.

Baseline ophthalmologic examination, fasting serum glucose concentration, and TSH should be obtained on all patients. Measurement of fasting serum glucose concentration should be repeated yearly.

RESOURCES

Myotonic dystrophy - General information about myotonic dystrophy provided by the U.S. National Library of Helath.  Also includes links to related sites.

Facts About Myotonic Muscular Dystrophy - Detailed information about myotonic dystrophy from the Muscular Dystrophy Association (MDA).

Myotonic Dystrophy Foundation -  A patient advocacy group dedicated to leading and mobilizing resources toward effective management, treatment, and a cure for myotonic dystrophy. The website has information about DM, support networks, and current news about DM research.

International Myotonic Dystrophy Organization - Provides extensive information and resources on myotonic dystrophy.