Myotonic Dystrophy Steinert disease








The diagnosis is based on several criteria. First is the hereditary factor, when there are many cases present in the family. Second is the clinical factor, when subjects present characteristic signs of the disease. When these two factors are present the diagnosis is easy. Sometimes the diagnosis is more complex because certain subjects have very few signs of the disease while others have no signs at all.

Until 1992 the only tests that could help in the diagnosis were the electromyogram (EMG) and a muscle biopsy. An EMG consists in recording muscle activity. As for muscle biopsy, it can help in the diagnosis but cannot confirm it since no muscular anomaly is specific to Myotonic Dystrophy.

Since 1992, when in doubt, the diagnosis for Myotonic Dystrophy is based on genetic analysis. This is done by a simple blood test that enables a diagnosis in almost 100% of cases. It consists in isolating the blood DNA that contains the genetic information and to measure the number of repetitions.

In DM1, the genetic anomaly is an increase in CTG repetitions in the DM1 gene. Within the normal population, the number of repetitions is always inferior to 40, while in subjects with the disease the number of repetitions can be as high as 4000 to 5000.

In DM2, the genetic anomaly is an increase in the number of CCTG repetitions in the ZnF9 gene. In the normal population, the number of CCUG repetitions is inferior to 75 but in the disease, the number of repetitions can be up to 11 000 with an average of 5000 repetitions.

Beyond the diagnosis of the disease, genetic testing can give an idea of the extent of the disease, especially for the DM1 form. In general, the higher the number of repetitions, the worse the disease will be. There are of course numerous cases that do not follow this rule and thus do not make this a reliable tool for prognosis.

Who is genetic testing for? Of course, someone who has manifestations of the disease does not have to take a genetic test in order to be diagnosed. These tests are useful when the symptoms of the disease are minor or absent as well as for the diagnosis of certain congenital forms of the disease since the signs are not specific and sometimes the parents are not aware that they are carriers of the gene for this disease. Genetic testing is also used to determine whether individuals presenting no signs of the disease are carriers of the genetic anomaly. Many young people who know that the disease runs in their family but show no signs want to know if they carry the genetic anomaly in case they decide to have children. In Quebec, the bill or rights authorizes individuals aged 14 years and over to undergo these tests without parental consent.

Prenatal genetic tests are currently available and offered to couples in which one of the spouses is affected by Myotonic Dystrophy. These tests are done within the first few weeks of pregnancy and are designed to determine if the foetus carries the genetic anomaly since there is a 50-50 chance of this. If the test result is positive, the woman can chose to have the pregnancy terminated if this is what she desire.