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Myotonic Dystrophy (DM)


Doctors with experience in neuromuscular disorders often find it easy to diagnose type 1 myotonic dystrophy (DM1). Sometimes, just by looking at a person, asking a few questions and examining him or her, they're well on the way to suspecting DM1. For instance, teenagers and adults with DM1 (the most common type) usually have a characteristic long face with hollow temples, and males often have early balding. (See Signs and Symptoms.)

In type 2 myotonic dystrophy (DM2), these facial features aren't as obvious (also see Signs and Symptoms).

Many people with DM1 tell the doctor about recurring abdominal pain, constipation or obstetrical complications. Many also report that their parents had muscle problems.

Sometimes, an eye doctor will notice the particular kind of cataract found in DM and suspect the disease, referring the patient to a neurologist. Cataracts are common in both DM1 and DM2, often occurring by middle age.

Many people may not realize they have any trouble relaxing their grip, while others say they’ve had trouble letting go of a shovel, screwdriver or some other device, especially in cold weather. These symptoms are present in both types of DM.

The doctor may check for myotonia by lightly tapping the area just under the thumb with a rubber hammer. In most people, there is little or no response. In people with myotonia, there’s a swift contraction of the muscle, which takes several seconds to relax.

The doctor may want to do electrical testing of the muscles and nerves, using an electromyogram, or EMG. In this exam, small needles are inserted into muscles to measure their electrical activity. Myotonia produces a characteristic sound, often described as the noise made by a dive-bombing airplane.

These days, a doctor who suspects DM1 or DM2 is likely to move from the history and physical exam to a DNA test (genetic test) to confirm a diagnosis. The DNA test involves only a blood sample and, in almost all cases, can determine whether the family is affected by DM. For more on getting a definitive genetic diagnosis, see MDA Genetic Counseling Webinar Answers Key Questions and The Genie's Out of the Bottle: Genetic testing in the 21st century.

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