Myotonic dystrophy type 2

Other Names: Dystrophia myotonica type 2; DM2; Proximal myotonic myopathy; PROMM; Myotonic myopathy, proximal; Ricker syndrome Myotonic dystrophy type 2, one of the two types of myotonic dystrophy, is an inherited muscular dystrophy that affects the muscles and other body systems (e.g., heart, eyes, and pancreas). It is characterized by prolonged muscle tensing (myotonia) as well as muscle weakness, pain, and stiffness.

Cause

Mutations in the CNBP gene cause myotonic dystrophy type 2. The exact function of this gene is not known. The protein made by the CNBP gene is mainly found in the heart and in skeletal muscles, where it probably helps regulate the function of other genes.

In people with myotonic dystrophy type 2, a short piece of DNA is abnormally repeated many times, forming an unstable area of the gene. The mutated gene makes an altered version of messenger RNA (mRNA), which is a copy of the gene that is normally used for protein production. The abnormal mRNA forms clumps inside the cell that interfere with the production of many proteins. These changes prevent cells in muscles and other tissues from functioning normally, leading to the signs and symptoms of myotonic dystrophy.

Inheritance

Autosomal dominant pattern, a 50/50 chance.

Myotonic dystrophy type 2 is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is enough to cause symptoms of the condition. In most cases, an affected person has one affected parent. As myotonic dystrophy is passed from one generation to the next, it generally begins earlier in life and signs and symptoms become more severe. This phenomenon is called anticipation. The cause of the anticipation in families with myotonic dystrophy type 2 is unknown.

Signs and symptoms

Myotonic dystrophy type 2 is characterized by progressive muscle wasting and weakness. Symptoms typically begin in a person's twenties. People with this condition often have prolonged muscle contractions (myotonia) and are not able to relax certain muscles after use. For example, a person may have difficulty releasing their grip on a doorknob or handle. Also, affected people may have slurred speech; temporary locking of their jaw; and muscle pain and weakness that mainly affects the neck, shoulders, elbows, and hips. Less common symptoms include abnormalities of the electrical signals that control the heartbeat (cardiac conduction defects); clouding of the lens in the eyes (cataracts); and diabetes. Males may experience balding and infertility. The severity of symptoms varies among affected people. Compared to myotonic dystrophy type 1, type 2 is milder and does not necessarily shorten a person's lifespan.

For most diseases, symptoms will vary from person to person. People with the same disease may not have all the symptoms listed. 80%-99% of people have these symptoms

• Cataract(Clouding of the lens of the eye)
• Myotonia

Diagnosis

Myotonic dystrophy is diagnosed by doing a physical exam. A physical exam can identify the typical pattern of muscle wasting and weakness and the presence of myotonia. A person with myotonic dystrophy may have a characteristic facial appearance of wasting and weakness of the jaw and neck muscles. Men may have frontal balding. There are several laboratory tests that can be used to clarify the clinical diagnosis of myotonic dystrophy. One test, called electromyography (EMG), involves inserting a small needle into the muscle. The electrical activity of the muscle is studied and usually shows characteristic patterns of muscle electrical discharge. The definitive test for myotonic dystrophy type 2 is a genetic test. For this test, certain cells within the blood are analyzed to identify a change (mutation) in the CNBP gene.

Treatment

There is currently no treatment available to stop or slow the progression of myotonic dystrophy type 2. Management options depend on the symptoms that each affected person has, and aim to treat each specific symptom. For example:

• Ankle-foot braces, wheelchairs, or other assistive devices may be used as needed for weakness
• Defibrillator placement may be needed for arrhythmias
• Cataracts can be removed for those with impaired vision
• Testosterone replacement therapy may be useful for hypogonadism in males
• Myotonia is usually mild and rarely requires treatment. Routine exercise appears to help with pain control, as well as with muscle strength and endurance. The effectiveness of most medications for pain management varies. Mexilitene, which is very effective for some forms of myotonia, has helped control muscle pain in some people with this condition. Other medications that have been used with some success include gabapentin, nonsteroidal anti-inflammatory drugs (NSAIDS), low-dose thyroid replacement, low-dose steroids, and tricyclic antidepressants. Cholesterol-lowering medications should be avoided when they are associated with increased weakness.
• There are steps a person can take to prevent some secondary complications. Anesthetic risk may be increased, so careful assessment of heart and respiratory function before and after surgery are recommended. Affected people should also have a yearly electrocardiogram or cardiac MRI to detect possible conduction defects or cardiomyopathy.

Prognosis

In general, people with myotonic dystrophy type 2 have a better long-term outlook (prognosis) than those with type 1. Symptoms are usually relatively mild. While the rate of progression can vary among affected people, symptoms generally progress slowly. While mobility may be impaired at an earlier age, the ability to walk is often retained until around 60 years of age.

The prognosis for affected people can depend on the extent of heart (cardiac) involvement. While definitive information is not available, it appears there is relatively little shortening of the lifespan in people with myotonic dystrophy type 2.

NIH genetic and rare disease info

• NIH info on Myotonic dystrophy type 2

Myotonic dystrophy type 2 is a rare disease.

Myotonic dystrophy type 2 Resources
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