Genetics Northern California

Myotonic Dystrophy, Type 1

What is myotonic dystrophy, type 1 (DM1)?
How common is DM1?
What causes DM1?
How is DM1 diagnosed?
How is DM1 treated?
Where can I get more information about DM1?

What is myotonic dystrophy, type 1 (DM1)?
Myotonic dystrophy, type 1, (DM1) is the most common type of muscular dystrophy in adults. This condition is also known as Steinert's disease and dystrophia myotonica.  The main muscle symptoms are myotonia (muscles that do not relax normally) and muscle weakness that gets worse over time. Many other parts of the body can be affected. Medical concerns can include eye cataracts, heart rhythm problems, and hormone problems. Symptoms can begin at any age depending on the type of DM1. DM1 affects people differently, even within the same family. 

  • Mild DM1 usually includes mild myotonia and cataracts. Life span is normal. 
  • Classical DM1 includes muscle weakness, muscle loss, myotonia, cataracts, and heart problems. Adults may become physically disabled and may have a shortened life span. 
  • Congenital DM1 starts at birth with very weak muscle tone (hypotonia), plus breathing and feeding problems. A baby with this condition may also be born with a clubfoot. Slow development and intellectual disability are more common in children with congenital DM1. 

There is a second type of myotonic dystrophy, called myotonic dystrophy, type 2 (DM2). This is a different condition than DM1. DM2 is more likely to affect muscles close to the body, like the neck, shoulders, elbows, and hips. 

How common is DM1?
About 1 in 8000 people has DM1. It affects people worldwide and of all ethnicities.  

What causes DM1?
DM1 is inherited in an autosomal dominant manner. That means it can be passed from a parent to a child. It is caused by harmful changes in a gene called DMPK (myotonin protein kinase). The DMPK gene includes a part of the DNA code (“CTG”) that repeats many times. The number of repeats can vary from person to person without causing any health problems. Usually, a person has fewer than 37 copies of the repeated CTG code. When there are more than 50 CTG repeats, the gene no longer works right, causing DM1. The number of repeats a person has in the DMPK gene can help predict the severity of medical problems. More repeats usually cause more problems. The number of repeats can also increase when the gene is passed from parent to child. This can lead to congenital DM1, with symptoms that start earlier and are more severe. Congenital DM1 is more likely to happen when the faulty gene is inherited from a mother with DM1, versus a father.

How is DM1 diagnosed?
DM1 is suspected in adults with the following symptoms:

  • Muscle weakness, especially of the feet and lower legs, hands, neck, and face
  • Myotonia, which is often noticed when a person is not able to quickly release a hand grip
  • Cataracts, which may require an eye exam for early detection

Congenital DM1 is suspected in newborns with weak muscle tone, facial weakness, club foot, and breathing problems.

The diagnosis of DM1 is confirmed by DNA testing of the DMPK gene. This is usually done by a blood sample. DNA testing is very accurate and widely available.

How is myotonic dystrophy treated?
There is currently no cure for myotonic dystrophy.  Medical guidelines help diagnose and treat problems that are more common in DM1. Medical care includes regular doctor visits to check for breathing issues, heart symptoms, eye problems, hormone problems, and muscle changes. Treatment is generally based on a person’s symptoms. There is currently no treatment that slows the progress of muscle weakness over time, but assistive devices can help with symptoms. 

Where can I get more information about myotonic dystrophy?

Last reviewed: September 9, 2020
Reviewed by: Katie Braverman, MS, LCGC, Kimberly Barr, MS, LCCG, and Mark Lipson, MD