Myotonic Dystrophy Testing & Diagnosis

Myotonic Dystrophy Testing & Diagnosis

Your path to a correct myotonic dystrophy (DM) diagnosis can be long and complex, as medical professionals see these cases so infrequently that they often aren’t familiar with DM. Also, because the symptoms of DM can mimic more common diseases, you may undergo months – or even years – of medical testing to rule out other potential causes. Once the disorder is suspected, it’s not difficult to get a DM diagnosis, but if it’s taken a long time for your diagnosis, you’re not alone. On average it takes more than six years to reach a diagnosis of DM1 and more than 10 years to confirm DM2.

What Initial Testing Looks Like

You’ll start with a complete family history and physical examination that includes a battery of medical tests, depending on the symptoms you’re experiencing. If you have a family history of myotonic dystrophy, or if your symptoms include key indicators such as early onset cataracts or difficulty relaxing a hand grip (myotonia), you may get a blood test to detect the abnormal mutation. Should your clinician’s findings not clearly diagnose DM, your care provider may also request electrodiagnostic testing. Electromyography (EMG) is a highly effective tool to identify the presence of myotonia in people with DM1 or DM2.

What Genetic Testing Looks Like

To absolutely confirm a diagnosis of DM1 or DM2, you’ll likely need genetic testing (also referred to as DNA testing). DNA, the genetic material in the nucleus of cells, is isolated from a sample of your blood or other tissue, and then analyzed to determine whether or not a specific mutation is present.

Genetic testing for DM1 uses standard DNA diagnostic protocols (“PCR” and “southern blot”) to confirm the presence of DM.

Genetic testing for the diagnosis of DM2 also uses standard DNA diagnostic protocols. However, in some cases the “repeat expansion” for DM2 may be too large for PCR testing. In those instances, southern blot techniques are used for diagnosis. 

Genetic testing can be beneficial in the following situations:

  • A confirmed diagnosis can eliminate the need for additional medical tests and reduce anxiety about the cause of symptoms.
  • People living with DM should be educated about the dangers of anesthesia and alert their doctors should they ever need surgery.
  • Couples can make family planning decisions based on their genetic risk. 
  • Families can opt for prenatal testing, where the DNA of the fetus is checked for the presence of the myotonic dystrophy mutation.
  • Mothers living with DM1 can have special monitoring during pregnancy and prepare for risks involved for a child born with congenital DM.

How Does Repeat Length Relate to the Severity of Myotonic Dystrophy?

It is important to understand how CTG repeat length is associated with the severity of myotonic dystrophy type 1. CTG is the type of trinucleotide repeat expansion found on the DPMK gene inherited by individuals with DM1. For individuals with myotonic dystrophy type 2, the expanded CCTG repeat is found in the CNBP gene.

  • Individuals with mild or late onset DM1 typically have a CTG length of 50-150, often with age of onset over 50 years old with symptoms such as mild cataracts and mild weakness.
  • Individuals with typical or adult onset DM1 have a CTG length of 150-1000, typically with age of onset in the teenage years and older, and symptoms such as early cataracts, weakness and myotonia.
  • Children with childhood onset DM have a CTG length of 600-1200, with age of onset between 1 and 10 years old, commonly exhibiting symptoms such as intellectual impairment and GI distress.
  • Babies with congenital onset DM have a CTG length of 800 or more, with age of onset at birth and exhibiting symptoms such as floppiness, breathing and feeding problems.

For individuals with adult-onset myotonic dystrophy type 2, in general, repeat lengths less than 28 are considered normal, while repeats greater than 75 up to 1,000 are associated with clinical symptoms such as myalgic pains, myotonia, hip and neck flexor muscle weakness, cataracts and cardiac arrhythmias.

For more information on the significance of CTG repeats, watch Dr. Darren Monckton's presentation Everything You Wanted to Know About CTG Repeats.

Why Does Myotonic Dystrophy Take So Long to Diagnose?

Myotonic dystrophy can take multiple forms that affect a broad spectrum of systems. You may visit several different specialists for disparate symptoms, such as an ophthalmologist for blurred vision, a gastroenterologist for stomach pain, and a cardiologist for an abnormal heartbeat. These individual physicians may not be aware of your full range of problems and therefore may not be able to put the pieces together for an accurate diagnosis.

The severity of symptoms can also vary greatly, even within the same family. Quite often individuals go to their primary care physician with a variety of complaints, some so general that the doctor never suspects any serious problem. As a result, a correct diagnosis may be delayed until the disease has progressed significantly.

And, further complicating or delaying diagnosis, you may not show obvious DM symptoms. There are several explanations as to why you might not show symptoms of the disease:

  • Symptoms may be so mild you don’t realize you have the disorder.
  • You may have a late onset form of the disease and do not yet exhibit symptoms.
  • You may carry a pre-mutation (a form of the mutation that is less extensive than that seen in those who display symptoms). If so, you are not likely to develop disease symptoms, but your children are at risk of inheriting the mutation and having the disorder.

I Have No Symptoms, Should I Test for DM?

Some people opt against testing when they have no symptoms. Problems that may arise from a diagnosis of DM include:

  • Difficulties in obtaining insurance, such as health, disability and life
  • Prejudice in the workplace or elsewhere
  • Impact of being diagnosed with a disorder when no cure or treatment capable of slowing the progression of myotonic dystrophy currently exists.

For more information on the pros and cons of testing, read our interview with Carly Siskind, MS, LCGC, senior genetic counselor on the Stanford University Neuromuscular Disorders Team.

Where to Test for Myotonic Dystrophy

There are a number of labs that conduct testing for DM, including academic or institutional settings such as Baylor College of Medicine, etc., and for-profit corporations such as Athena Diagnostics and MNG Laboratories. There are a number of labs in the U.S. that offer testing for DM. Prices can vary considerably depending on the lab and whether someone is being tested for DM1 or DM2. Do your research in advance, and call your insurance plan before testing to see if the insurance company will cover a particular lab.

Athena Diagnostics

Financial aid is available for families in need. Athena Diagnostics offers qualifying patients a substantial discount on its regular price for clinical laboratory services. If you cannot afford to pay for testing ordered from Athena, you may obtain a 75% discount off the list price, if you meet certain income guidelines.

For more information, call (800) 394-4493, and ask to speak with a Reimbursement Services Representative or visit their website.

MNG Laboratories

For patients presenting with ambiguous symptoms of muscular dystrophy, MNG offers a Comprehensive Muscular Dystrophy/Myopathy panel. This phenotype-driven panel includes coverage of over 400 genes with clinical relevance to myopathy and muscular dystrophy, single-exon resolution copy number analysis, and complimentary mtDNA sequencing with deletion analysis.

Click here for information on a DM1 specific test. Click here for information on a DM2 specific test.

For more information on MNG Laboratories, please call (678)225-0222 or visit their website.

Other Testing Resources

Find other genetic testing resources for myotonic dystrophy on the DNA Testing Guides website.

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