Myotonic dystrophy is the most common form of adult-onset muscular dystrophy, affecting roughly 10 in every 100,000 people worldwide. It’s a genetic condition defined by progressive muscle weakness and myotonia, a prolonged stiffness after a muscle contracts. Unlike many conditions people associate with “muscular dystrophy,” it reaches far beyond muscles, affecting the heart, eyes, brain, digestive system, and hormones.
The Two Types
There are two distinct forms. Type 1 (DM1), sometimes called Steinert disease, is the more common and generally more severe. It causes facial weakness, drooping eyelids, and weakness that starts in the hands, forearms, and lower legs before eventually spreading to larger muscle groups closer to the trunk. Many people with DM1 eventually need a wheelchair as the disease progresses.
Type 2 (DM2) tends to appear later in life, typically between a person’s 40s and 60s. It primarily affects the thighs, hips, and neck flexors rather than the hands and face. Muscle wasting is milder in DM2, and some people even develop enlarged calf muscles rather than wasted ones. One of the most significant differences: DM2 never presents at birth. A severe congenital form exists only in DM1.
What Causes It
Both types are inherited in an autosomal dominant pattern, meaning you only need one copy of the mutated gene (from one parent) to develop the disease. In DM1, the problem is a repeating segment of DNA in the DMPK gene. Healthy individuals have 5 to 37 copies of a three-letter DNA sequence (CTG). People with 38 to 49 repeats carry a “premutation” and typically have no symptoms, but their children are at risk. Between 50 and roughly 150 repeats can produce mild disease. Classic DM1 involves 50 to 1,000 repeats, and the most severe congenital form usually involves more than 1,000.
A critical feature of this mutation is anticipation: the repeat tends to expand from one generation to the next. A parent with mild symptoms and a moderate repeat count can have a child with a much larger expansion and far more severe disease.
How the Mutation Damages Cells
What makes myotonic dystrophy unusual among genetic diseases is that the DNA mutation doesn’t produce a broken protein. Instead, it produces toxic RNA. When the expanded repeat is copied into RNA, those long, repetitive RNA strands fold into abnormal structures and get stuck inside the cell nucleus. There, they trap essential proteins that normally regulate how other genes are processed.
The result is a chain reaction. Proteins that should be guiding adult gene expression get locked away, while proteins that promote fetal gene patterns become overactive. Cells essentially start producing versions of proteins that belong in a developing fetus rather than an adult. One well-understood example involves a chloride channel in muscle cells. The toxic RNA causes cells to produce a fetal version of this channel that gets destroyed before it can function. Without working chloride channels in the muscle membrane, electrical signals linger too long, and muscles can’t relax properly. That’s myotonia.
Symptoms Beyond Muscle Weakness
Myotonic dystrophy is a whole-body disease, and the non-muscle symptoms are often what bring people to a doctor first.
Cataracts are one of the earliest signs, sometimes appearing well before noticeable muscle problems. In DM1, these cataracts have a distinctive “Christmas tree” appearance under an eye exam, with tiny iridescent specks scattered through the lens. When someone under 55 develops cataracts, especially with a family history of early cataracts, it can be the clue that leads to a myotonic dystrophy diagnosis.
Heart problems affect over 50% of people with DM1. The electrical system of the heart is particularly vulnerable, leading to abnormal rhythms and conduction delays that can worsen gradually over years. The annual rate of sudden cardiac death is estimated at about 1%, making regular heart monitoring essential. Current guidelines recommend an electrocardiogram at diagnosis and annually afterward, with 24-hour heart rhythm monitoring at least once a year as well.
The brain is also affected. Excessive daytime sleepiness is extremely common and often debilitating, separate from poor nighttime sleep. Many people experience cognitive changes, apathy, and difficulty with motivation that can be mistaken for depression or laziness. Gastrointestinal problems, including bloating, constipation, diarrhea, and abdominal pain, are frequent. Insulin resistance and thyroid disorders add to the metabolic burden. Fertility issues and pregnancy complications are common in women with DM1.
The Congenital Form
Congenital myotonic dystrophy (CDM) is the most severe presentation, appearing at birth almost exclusively in babies who inherit the expanded gene from their mother. These newborns are profoundly floppy, with little muscle tone and weak reflexes. Ironically, the hallmark myotonia of the disease is absent in infancy.
Breathing problems occur in about half of affected newborns and are the leading cause of death in the neonatal period. Babies who survive the first weeks face significant developmental challenges. Cognitive impairment is one of the most consistent features, with IQ scores typically ranging from 40 to 80 (average is 100). Between 50% and 60% of children with CDM have intellectual disability, and rates of autism, ADHD, and other behavioral conditions are elevated.
Diagnosis
A doctor may suspect myotonic dystrophy based on the combination of grip myotonia (difficulty releasing a handshake, for example), characteristic facial weakness, and early cataracts. But the definitive diagnosis comes from genetic testing, which identifies and measures the size of the repeat expansion. Several laboratory techniques can detect the mutation, and results typically confirm the diagnosis with high certainty. The size of the repeat also gives some indication of likely severity, though it’s not a perfect predictor for any individual.
Treatment and Management
There is no cure for myotonic dystrophy, and no treatment slows the underlying genetic process. Management focuses on each symptom individually.
For myotonia itself, mexiletine (a sodium channel blocker) is considered the first-choice medication. It reduces the muscle stiffness that makes it hard to release objects or start walking after sitting. About 20% of people taking it experience stomach irritation, which can often be managed by taking the medication with food.
Cardiac care involves regular monitoring and, when conduction abnormalities progress, pacemakers or implantable defibrillators. Cataracts are treated with standard surgical removal when they impair vision. Excessive sleepiness may be managed with stimulant medications. Physical therapy helps maintain mobility, and ankle braces can compensate for foot drop caused by lower leg weakness. One important practical note: people with myotonic dystrophy have increased sensitivity to anesthesia, making it critical to inform surgical teams about the diagnosis before any procedure.
Life Expectancy
In adult-onset DM1, life expectancy is meaningfully reduced. A large study of Dutch patients found a median survival of 60 years for men and 59 years for women. By age 65, only 18% of patients were still alive, compared to 78% in the general population. The two leading causes of death, each responsible for roughly 30% of cases, were pneumonia (often from weakened breathing muscles and difficulty clearing the airway) and cardiac arrhythmias.
DM2 generally follows a milder course with less impact on lifespan, though it still causes significant disability through muscle pain, weakness, and the same multisystem complications seen in DM1. The wide variability in repeat size, age of onset, and organ involvement means that two people with the same diagnosis can have very different experiences with the disease.