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Myotonic Dystrophy (DM)

Diagnosis

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 performing an examination, a doctor can be well on the way to suspecting DM1. For instance, teenagers and adults with DM1 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), facial weakness may occur in later age and is not as prominent as it is in DM1 patients (also see Signs and Symptoms).

Many patients with DM1 tell the doctor about recurring abdominal pain, constipation, or obstetrical complications.1 Many also report that their parents had muscle-related issues.

Sometimes, an eye doctor will notice the kind of cataract found in myotonic dystrophy (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 have 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.

A 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, the thumb will abduct and relax slowly.

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 presence of an expanded cytosine-thymine-guanine (CTG) repeats in the dystrophia myotonica protein kinase (DMPK) gene is the gold standard for the diagnosis of DM1. The presence of an expanded cytosine-cytosine-thymine-guanine (CCTG) repeats in the ZNF9 (also known as CNBP) gene is confirmatory for DM2 if a genetic test for DM1 was negative and the clinical suspicion of DM is high. The DNA test involves only a blood sample and, in almost all cases, can determine whether a 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.

A 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, when audio is amplified. Because genetic testing is the gold standard for confirming the diagnosis of DM1 and DM2, the diagnostic role of electromyography (EMG) is limited. However, it is still important in atypical cases where the detection of myotonia provides support for the diagnosis of DM. EMG is also used even when molecular testing for DM1 or DM2 is normal and other symptoms appear.

Slit lamp examination may reveal the characteristic posterior subcapsular cataracts (opaque cloudy areas that appear on the back surface of the eye’s lens), which are detectable as red and green iridescent opacities.

Electrocardiography (ECG) is critically important for the recognition and characterization of asymptomatic cardiac conduction defects that are often encountered in DM1 and DM2 and require constant monitoring.

The diagnostic role for muscle biopsy is limited, given the availability of genetic testing for DM1 or DM2. However, muscle biopsy may be useful to help distinguish DM2 from an inflammatory or metabolic myopathy when the condition cannot be differentiated by clinical presentation alone. Muscle biopsy may suggest the diagnosis of DM in atypical cases with minimal weakness, unexplained elevations in creatine kinase, and nonspecific EMG findings.

Brain MRI studies in patients with DM1 demonstrate abnormalities in the frontal and anterior portion of the temporal lobes. In patients with DM1, and to a limited degree in DM2, positron emission tomography (PET) studies demonstrate low blood flow of frontal and temporal lobes, a finding that might underlie the executive dysfunction seen primarily in DM1 and to a lesser extent in DM2.

References

  1. Rönnblom, A., Forsberg, H. & Danielsson, Å. Gastrointestinal symptoms in myotonic dystrophy. Scand. J. Gastroenterol. (1996). doi:10.3109/00365529609009145

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