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Myotonic dystrophy

Clinical types

Myotonia is a disorder of the cessation of movement - it is easy to induce it by asking the patient to grip tightly, whereupon it is difficult for him/her to release his or her grasp. There are 4 major types of clinical manifestation of myotonic dystrophy, characterized mainly by severity and age of onset - all result from a mutation in the same gene.
  • Congenital myotonic dystrophy - this disease is expressed from birth. It is a very severe type that results in death at a young age due to respiratory failure.

  • Infantile myotonic dystrophy - the onset of this disease is in early childhood. It is a severe type that manifests as significant muscle weakness and mental retardation.

  • Childhood myotonic dystrophy - the onset of this disease is relatively late. It is moderately severe and mental retardation is not always present.

  • Adult myotonic dystrophy - the onset is in adult life. This disease is relatively mild and manifests mainly as myotonia.

This condition must be differentiated from another disease called Thomsen-s congenital myotonia (myotonia congenital), featuring mainly myotonia without involvement of other body systems, and whose prognosis is better.

Clinical signs

These are in accordance with the severity; as described above - the most severe cases have marked myotonia or laxity of muscles.

Electromyograms (EMG) show a characteristic picture.

In the congenital type, respiratory failure occurs immediately after birth.

The diaphragm is elevated (see diaphragmatic hernia), mainly to the left.

The disease manifests in utero as infrequent fetal movements, based on the mother-s sensations.

Sometimes there is associated clubfoot or polyhydramnios. These children die in infancy.

In the childhood or infantile type, there is typically very severe muscle weakness, lack of facial expression, and mental retardation of varying degrees.

In the adult-onset type, the main manifestation is myotonia - the hand -jams- when performing such actions as closing the hand, and it is difficult to release the contraction. Typically, the facies are elongated and lack expression. There is also progressive muscular weakness, which is not always significant. There may also be cataracts in the eyes. Males may have reduced fertility with a reduced quantity of semen that worsens over time. In males it is often common to find characteristic balding at the front of the head and temples.

Inheritance

Specific properties of the myotonic dystrophy gene

The gene for the disease is known as DM, and it produces a protein called myotonin. The DM gene is composed of the bases of the genetic code that form the myotonin protein in the muscle. Within the sequence there is a region in which three bases of the genetic code, CTG, repeat a number of times - usually there are approximately 40 repeats (120 bases). Each person has two genes for myotonic dystrophy, but the presence of only one abnormal gene when the second one is normal is sufficient for the disease to be expressed clinically.

The mutation in this gene is in the region of the repeats. When the number of repeats in either the maternal or paternal gene exceeds 40, this is abnormal, but the severity of the disease increases when the number of repeats is in the hundreds. When the number of repeats exceeds 40, the region is unstable, and during the copying of genetic material from a mother with more than 40 repeats to a fetus, the number of repeats can multiply (expand) several times. This phenomenon is known as increased anticipation. For some reason, this occurs only if the mother has the mutation - if the father has the mutation and he transmits the affected allele to the fetus, the number of repeats does not expand and the fetus will have the same number and the same severity of the disease as the father. The greater the number of repeats, the more severe is the damage to the gene and the protein. In the congenital type, for example, the number of CTG repeats can reach several thousand - the damage here is very severe. A smaller number of repeats manifests as a milder disease. Therefore the same family can have members affected to differing degrees of severity - each with an abnormal number of repeats that is different from that of relatives. Members of later generations are usually more affected due to the increase in the number of repeats from generation to generation, transmitted by mothers to their offspring. This is therefore a mutation that worsens from generation to generation (dynamic mutation).

Penetrance

  • In myotonic dystrophy - depends on the severity of the mutation, i.e. the number of repeats.

  • In Thomsen-s congenital myotonia the expression is full.

Associated features that can be demonstrated in imaging tests in pregnancy

In myotonic dystrophy: on ultrasound scanning reduced fetal movements, polyhydramnios and clubfoot can be detected.

In all cases, genetic counseling is important before embarking on a pregnancy. Preliminary tests must be performed on family members, and the risk of recurrence determined based on the number of patients and their relationship to the affected individual. If molecular testing of the couple is required, it must be taken into account that even though this is relatively not a long process, it should nevertheless be commenced before pregnancy. Only after completing the molecular analysis before pregnancy can prenatal diagnoses be undertaken in subsequent pregnancies.

During the pregnancy, all the data must be assessed, including the results of additional tests performed during the pregnancy such as alpha-fetoprotein, nuchal translucency, molecular tests, etc.

In Thomsen-s congenital myotonia there are no prominent findings in imaging tests in pregnancy.

What is the risk of recurrence in a subsequent pregnancy?

In myotonic dystrophy - a patient who has one affected gene and one gene with a normal number of repeats has a 50% chance of transmitting the normal gene, in which case the fetus will be healthy. If the abnormal gene is transmitted, the outcome depends on the sex of the parent who carries the abnormal gene. If the gene with an abnormal number of repeats is transmitted by an affected mother to her offspring (whether male or female), there is a worsening of the severity of the disease, involving a transition to a more severe form (increased anticipation), possibly to the congenital/lethal form (see explanation in the description of the gene for the disease). This does not occur when the affected parent is the father.

For more distant relatives, the risk is determined within genetic counseling, based on the clinical picture, the pedigree, the results of tests conducted such as molecular testing, etc.

In Thomsen-s congenital myotonia - the risk of recurrence depends on whether it is the dominant or recessive type.

Molecular genetic information

The gene for the disease

DM, which produces a protein known as myotonin.

Location

On chromosome 9q.

Genetic testing

Diagnostic testing

A direct test can be performed to identify the mutation in the gene - this is usually an increase in the number of repeats in the DM gene. These are counted in a direct test and are demonstrable in patients using a relatively simple method. See: Testing the disease-causing gene for mutations that are common in a specific ethnic group - autosomal dominant diseases.

Carrier testing

Same as diagnostic testing.

Fetal testing

Same as diagnostic testing.

 
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