About Myotonic Dystrophy

About Myotonic Dystrophy

Myotonic dystrophy (often abbreviated as DM after its Latin name dystrophia myotonica) exists in two forms, usually referred to as type 1 or DM1 and type 2 or DM2. Both conditions are genetic disorders but each affects a different gene. Myotonic dystrophy is thought to affect at least 1 in 8,000 people worldwide.

DM1 - type 1 myotonic dystrophy

Type 1 myotonic dystrophy is also sometimes called Steinert's Disease. In common with other muscular dystrophies, it causes muscle wasting and weakness. However, it also affects many other systems in the body and symptoms can also include cataracts, heart conduction defects, endocrine changes including diabetes, and myotonia (prolonged muscle contractions or difficulty relaxing grip). This multi-systemic/multi-organ involvement is a particular feature of myotonic dystrophy, and in some cases, the non-muscle-related features of the condition - especially the heart problems - can be more of an issue for patients than the muscle weakness or myotonia.

The genetic change that causes the symptoms of myotonic dystrophy is present at birth, but depending on its severity, myotonic dystrophy symptoms may become noticeable at almost any age. The disease is often categorized according to age of onset.

Adult onset ("classical" form)

Less severe cases may not show any symptoms until adulthood. Adults who are moderately affected may have muscle weakness that gradually worsens with age, in addition to other symptoms such as excessive daytime fatigue, myotonia, diabetes, problems with breathing and swallowing, and sudden cardiac events. In mild cases muscle weakness will not usually have been a feature of early life, and adults with an extremely mild form of the condition may even have been active or athletic in their youth. Sometimes the diagnosis is made when the person concerned goes to the doctor for a symptom completely unrelated to muscle weakness, such as cataracts, or when a genetic test is done because another member of the family is more severely affected.

Congenital

Severe cases of myotonic dystrophy will be noticeable at birth, and babies with this "congenital" form may have breathing and swallowing problems and be quite severely ill, needing respiratory support and a feeding tube. Unlike some other forms of muscular dystrophy where infants may not achieve motor milestones, children with congenital myotonic dystrophy who overcome their initial breathing and feeding problems can make progress and often learn to walk, but as children they may have learning difficulties and other myotonic dystrophy symptoms.

Childhood onset

Children who have no problems at birth but who show symptoms in childhood may be described as having this form. Like the adult onset form it can cause muscle weakness and stiffness, but often it can also cause learning difficulties.

Management of myotonic dystrophy type 1

Although no 'cure' for myotonic dystrophy exists at present, adequate care and support can do a lot to help those affected. Many doctors are unfamiliar with the condition and this makes it particularly important that people who have myotonic dystrophy and their families are themselves aware of the problems they may face. Adults with myotonic dystrophy may not need to see a myotonic dystrophy specialist on a regular basis, but should receive follow-up as required. They should have treatment for any of the particular symptoms they do have - for example heart problems or diabetes - from a doctor who understands that these symptoms are related to their myotonic dystrophy. Whenever they see a new doctor they should explain that they have myotonic dystrophy. More information about management of myotonic dystrophy is available from the links on the left.

Genetics of myotonic dystrophy type 1

Myotonic dystrophy is inherited in an autosomal dominant pattern. It is caused when a certain segment of DNA at the end of a gene called the DMPK gene is abnormally repeated many times, forming an unstable region in the gene. This is called a triplet repeat expansion. Unaffected individuals have a small number of repeats (up to about 35) in this region, but in affected individuals the number can be much higher - rising to several thousand in congenitally affected children.

There is some correlation between number of repeats and severity of the disease, although individuals with a similar repeat count may be quite differently affected. There is also a correlation between number of repeats and age of onset, and a noticeable feature of myotonic dystrophy is that the number of repeats tends to increase in each generation - so a grandparent (slightly increased repeat count) might experience their first mild symptoms at age 60, while their children (moderately increased repeat count) notice symptoms at 30, and grandchildren (massively increased repeat count) may be congenitally affected. This phenomenon is known as "anticipation".  It does not always occur. The severe congenital form most often occurs when a mildly affected mother gives birth to a congenitally affected child due to a sudden large expansion in the repeat count.

Further information about DM symptoms and genetics can be found in the links on the left.

DM2 - type 2 myotonic dystrophy

Type 2 myotonic dystrophy, also sometimes called ‘PROMM’ (proximal myotonic myopathy) is caused by a mutation in the CNBP gene - a different gene to that causing type 1 myotonic dystrophy. Type 2 myotonic dystrophy does not have a congenital or childhood onset form - it is only found in adults, with an age of onset generally between 30 and 60 years. People with DM2 may not have myotonia and do not usually have the same kind of facial muscle weakness or swallowing problems that are often seen in DM1. Weakness usually affects the proximal muscles (those closest to the trunk), and for many patients this is the most disabling feature later in life. It is quite common for DM2 patients to experience muscle pain (myalgia). Only a minority have cataracts. In common with DM1 patients, some DM2 patients may experience heart conduction defects and diabetes.

Genetics of myotonic dystrophy type 2

Like DM1, DM2 is caused by a "repeat expansion" in a segment of DNA in the affected CNBP gene. It is also inherited in an autosomal dominant pattern. Unlike DM1, it appears that the feature of "anticipation" (repeat count increasing with each successive generation) is not a feature of DM2. Prevalence of DM2 appears to vary in different populations and a higher prevalence has been observed in German populations.

 
12 Apr 2017