Myotonic Dystrophy

What causes myotonic dystrophy

Both types are caused by an expanded repeat in a gene:

• DM1: an expanded CTG repeat in the DMPK gene on chromosome 19
• DM2: an expanded CCTG repeat in the CNBP gene on chromosome 3

The expanded genetic material forms toxic RNA that traps proteins needed to process many other genes, affecting muscles, heart, brain and other tissues.

It is autosomal dominant: each child of an affected parent has a 50% chance of inheriting it. In DM1, the repeat often grows when passed on, especially from mothers, so children can be affected more severely and earlier than their parents. This is called anticipation.

DM1 vs DM2

Symptoms of DM1

Muscles

• myotonia: difficulty releasing a grip, opening hands after a fist, or stiffness after rest that improves with repeated movement ("warm-up")
• weakness of the face (long, thin face, drooping eyelids), jaw, neck, hands, forearms and lower legs (foot drop)
• muscle wasting
• slurred speech and difficulty swallowing

Other organs

• heart: conduction block and arrhythmias such as heart block and atrial fibrillation, and sudden cardiac death risk
• eyes: early cataracts, often before 50
• breathing: weak breathing muscles, sleep apnea, and pneumonia from aspiration
• brain: excessive daytime sleepiness, fatigue, apathy, and difficulties with planning and social cognition
• hormones: insulin resistance and diabetes, low testosterone and infertility in men, thyroid problems
• gut: constipation, diarrhea, gallstones and swallowing difficulties
• skin and hair: frontal balding, and benign skin tumors (pilomatricomas)

Congenital DM1

Babies born with DM1, usually to affected mothers, can have severe low muscle tone, breathing and feeding difficulties, clubfoot and later intellectual disability. Childhood-onset DM1 mainly causes learning and behavior difficulties before muscle symptoms.

Diagnosis

• genetic testing: the definitive test, measuring the repeat expansion in DMPK or CNBP
• EMG: shows myotonic discharges ("dive-bomber" sounds)
• CK: normal or mildly raised
• ECG and heart monitoring: baseline at diagnosis
• slit-lamp eye exam for characteristic cataracts
• lung function tests and sleep studies

Management

There is no disease-modifying treatment yet, though clinical trials of targeted therapies are under way. Good care focuses on monitoring and early treatment of complications:

• heart: ECG at least yearly, Holter monitoring and referral to a cardiologist. Pacemakers or implantable defibrillators prevent sudden death in people with conduction disease.
• breathing: lung function tests, sleep studies, and noninvasive ventilation (BiPAP) at night when needed.
• anesthesia: higher risk of breathing complications after surgery and sensitivity to sedatives. The care team should follow specific anesthesia guidance.
• myotonia: often mild and does not need treatment. Mexiletine can help troublesome myotonia, with heart monitoring.
• sleepiness: sometimes treated with modafinil.
• physical and occupational therapy: ankle braces for foot drop, mobility aids and energy conservation.
• speech and swallowing therapy.
• eyes: cataract surgery when vision is affected, and eyelid surgery for severe drooping.
• hormones: diabetes and thyroid screening, and testosterone assessment.
• genetic counseling and reproductive options such as preimplantation genetic testing.

Outlook

Myotonic dystrophy varies widely, even within families. Many people with adult-onset DM1 or DM2 work and live independently for decades. Life expectancy can be reduced in DM1, mainly from breathing and heart complications, which regular monitoring and timely treatment help prevent.

Frequently asked questions

What is myotonic dystrophy?

Myotonic dystrophy is an inherited condition that causes progressive muscle weakness and myotonia, difficulty relaxing muscles after use. It also affects the heart, eyes, breathing, brain, hormones and digestive system. It is the most common adult-onset muscular dystrophy.

What is the difference between DM1 and DM2?

DM1 is caused by a change in the DMPK gene, can start at any age including birth, and mainly weakens hand, forearm, face and lower leg muscles. DM2 is caused by a CNBP gene change, starts in adulthood, mainly affects the hips and thighs, and is usually milder.

What does myotonia feel like?

Myotonia is a delay in relaxing a muscle after contracting it. People notice they cannot quickly let go of a handshake, a door handle or a tool, or that their jaw or tongue feels stiff. It usually improves with repeated movements and is worse in the cold.

How does myotonic dystrophy affect the heart?

It can damage the heart's electrical conduction system, causing heart block and arrhythmias that can lead to fainting or sudden death. Regular ECGs and cardiology review allow early pacemaker or defibrillator placement, which greatly reduces this risk.

Is myotonic dystrophy inherited?

Yes. It is autosomal dominant, so each child of an affected parent has a 50% chance of inheriting it. In DM1, symptoms can be more severe in each generation, especially when inherited from the mother. Genetic counseling is recommended for families.