What's myotonic dystrophy type 1?
Myotonic dystrophy type 1 (DM1) is an autosomal dominant, multisystem muscular dystrophy that arises from an expanded CTG repeat in a non-coding (3' UTR) part of the DMPK gene. A separate disorder, myotonic dystrophy type 2 (DM2), arises from a different repeat (CCTG) in the CNBP gene, formerly called ZNF9, so DM1 and DM2 are distinct conditions with different genes. The DM1 repeat is unstable and tends to grow when passed to the next generation, which underlies the disease's strong genetic anticipation.
| Also indexed as | OMIM:160900, MONDO:0008056 |
|---|---|
| Features mapped | 13 |
| Treatments mapped | 7 |
| Published sources | 18 |
| Last reviewed | 2026-08-04 |
Signs and symptoms
Cataract
Cataracts (clouding of the lens of the eye) are a characteristic feature and can appear earlier than usual, sometimes leading to the diagnosis being recognized.
Intellectual disability
Myotonic dystrophy type 1 can affect learning and thinking, with the impact ranging from mild in adult-onset disease to significant in the congenital form present from birth. This is part of how the disease affects the brain, not only the muscles.
Excessive daytime somnolence
Myotonic dystrophy type 1 reaches well beyond the muscles. Along with slowly worsening muscle weakness and stiffness, it affects many body systems, which is why symptoms such as ongoing daytime sleepiness can appear as part of the condition.
First degree atrioventricular block
Conduction problems in the heart, such as first-degree atrioventricular block (a slowing of the electrical signal between the upper and lower chambers), are the most common cardiac manifestation of myotonic dystrophy type 1.
Atrial fibrillation
Myotonic dystrophy type 1 is associated with abnormal heart rhythms, including atrial fibrillation, an irregular and often rapid rhythm of the heart's upper chambers.
Autosomal dominant inheritance
The condition is autosomal dominant, meaning a single altered copy of the DMPK gene is enough to cause it, and an affected parent has a chance of passing it to each child.
Myotonia
Myotonia is a delay in muscle relaxation after the muscle contracts, so a tightened muscle is slow to let go.
Muscle weakness
Myotonic dystrophy type 1 causes muscle weakness that gets worse over time, and the muscles farther from the center of the body (such as the hands and lower legs) tend to weaken more.
Hypogonadism
Myotonic dystrophy type 1 can disrupt the body's hormone-producing (endocrine) glands. This is part of why hormone-related changes are seen in the condition, and managing these hormonal and metabolic effects is an important part of care.
Frontal balding
Hair loss at the front of the scalp can occur in myotonic dystrophy type 1, and in the milder late-onset form it may appear along with cataract.
Cardiac conduction defect
In myotonic dystrophy type 1 the heart's electrical signaling can become faulty, leading to conduction problems and irregular heartbeats. These cardiac complications are a major cause of death in DM1.
Insulin resistance
Myotonic dystrophy type 1 commonly causes insulin resistance, meaning the body's cells respond poorly to insulin so blood sugar is harder to control. It is a core part of the metabolic picture in DM1 and a step on the path toward type 2 diabetes.
Type II diabetes mellitus
People with myotonic dystrophy type 1 have a higher rate of type 2 diabetes, a condition of persistently high blood sugar. In DM1 it adds to the overall clinical burden and is linked to poorer outcomes, so blood sugar is worth monitoring over time.
How it is diagnosed
Myotonic dystrophy 1
Diagnosed using: DMPK CTG repeat genetic test.
“Genetic test is routinely used in diagnostic procedure for DM1 for symptomatic, asymptomatic, and prenatal…”
Treatment and management
What the research describes, not a recommendation. Treatment decisions belong with your clinician.
This covers treatments that appear in the published research mapped here. Investigational and experimental therapies are not included, so their absence is a boundary of this map, not a sign they do not exist.
mexiletine
Mexiletine is a medication used to ease myotonia, the delayed muscle relaxation, in myotonic dystrophy. It is considered a first-line treatment for the myotonia of myotonic dystrophy, though it treats the symptom and does not change the underlying disease. Because of possible effects on the heart, cardiac assessment is needed before and during treatment.
Used to help with: Myotonia.
“…can be prescribed for the symptomatic treatment of myotonia of the skeletal muscles in adult patients with myotonic dystrophy under a compassionate use programme.”
mexiletine
Mexiletine is a medication used to ease myotonia, the delayed muscle relaxation, in myotonic dystrophy. It is considered a first-line treatment for the myotonia of myotonic dystrophy, though it treats the symptom and does not change the underlying disease. Because of possible effects on the heart, cardiac assessment is needed before and during treatment.
Used to help with: Myotonic dystrophy 1.
“Mexiletine is the only pharmacological agent approved for the symptomatic treatment of myotonia in adult patients with NDM and is considered to be the first-line treatment for…”
mexiletine for myotonia
Mexiletine is a medicine that can ease myotonia, the muscle stiffness and difficulty relaxing a muscle after using it, in adults with myotonic dystrophy. It treats this symptom rather than the underlying condition, and because it can affect the heart's rhythm, heart checks are done before and during treatment.
Used to help with: Myotonia.
“…can be prescribed for the symptomatic treatment of myotonia of the skeletal muscles in adult patients with myotonic…”
implantable cardioverter-defibrillator or pacemaker
Because myotonic dystrophy type 1 can disturb the heart's electrical signals, some people need a small implanted device, a pacemaker or an implantable cardioverter-defibrillator (ICD), to keep the heartbeat steady and guard against dangerous rhythms. These devices are a main option for the heart problems the condition can cause.
Used to help with: Myotonic dystrophy 1.
“…limited therapeutic options beyond implantable cardioverter-defibrillator (ICD)/pacemaker…”
physical therapy
Physical therapy is one of the core ways to manage myotonic dystrophy type 1. Used alongside medicines for muscle stiffness and careful heart monitoring, it helps people keep moving and maintain function as the condition progresses.
Used to help with: Myotonic dystrophy 1.
“Effective strategies to manage symptoms remain crucial, such as physical therapy, medications for myotonia and diligent cardiac care.”
supportive and palliative care
There is no cure for myotonic dystrophy type 1 yet, so ongoing supportive care is central. It focuses on managing complications across the many body systems the condition affects and on helping people stay as independent as possible.
Used to help with: Myotonic dystrophy 1.
“…supportive and palliative care remains essential, with a focus on addressing multisystemic complications and maintaining functional…”
metformin
Metformin, a medicine already used for other conditions, is being studied as a possible treatment for people with myotonic dystrophy type 1 who have heart complications. At this stage it is a candidate under investigation rather than an established therapy for the condition.
Used to help with: Myotonic dystrophy 1.
“…repurposed small-molecule drugs, such as vorinostat, tideglusib, and metformin, could serve as potential therapeutic agents for DM1 patients with cardiac…”
What changes how it shows up
Carrying the genetic change is not the whole story. The factors below are described in the research mapped here as changing whether, or how strongly, the condition appears. They modulate how the genotype is expressed; they do not, on their own, cause or cure it.
anticipation
In myotonic dystrophy type 1 the condition can begin earlier and run a more severe course in each successive generation of a family, a pattern called anticipation.
Described as modulating: Myotonic dystrophy 1.
“The meiotic instability of the (CTG)n repeats leads to genetic anticipation where increased size of DM1 mutation and a more severe phenotype have been reported in affected individuals across generations.”
maternal transmission
The most severe, congenital form of myotonic dystrophy type 1 is strongly biased toward being passed on from the mother.
Described as modulating: Myotonic dystrophy 1.
“…an earlier age of onset and more severe clinical course in subsequent generations, and exclusively maternal transmission of the most severe congenital type.”
DMPK CTG repeat expansion size
Myotonic dystrophy type 1 is caused by an expanded run of CTG repeats in the DMPK gene, and the number of repeats broadly tracks with how early the disease starts and how severe it is, though it does not predict the course precisely for any one person.
Described as modulating: Myotonic dystrophy 1.
“The number of CTG repeats broadly correlates with both the age-at-onset and overall severity of the…”
How to read the evidence labels
Where this comes from
This guide is built from 18 published source(s). Every claim above links back to one of them. Click any source ID to read the original on PubMed.
Take it further
Printed, source-linked documents built from this condition's graph — ready to bring to an appointment or attach to a coverage request. Every claim carries its published source, the same as this guide.