Myotonic Muscular Dystrophy

MMD varies widely from person to person, even in the same family.

• Some people have no symptoms, or the symptoms are so mild the person doesn’t know they have disease. They might learn they have MMD only if they have genetic testing after a relative is diagnosed.
• In other people, the symptoms are easier to notice. They get worse over time, usually slowly. MMD may change your child’s abilities and lead to a range of health concerns, from mild to serious.
• The most severe MMD, which starts at birth or soon after (congenital MMD), can be life threatening. Newborns with symptoms that may be caused by MMD typically need neonatal intensive care.

MMD happens because of a problem in the gene DMPK on chromosome 19 or the gene CNBP (also known as ZNF9) on chromosome 3.

Certain building blocks of DNA in these genes repeat many more times than normal. This prevents cells in your child’s muscles and other tissues from working the way they should.

When a child has MMD, it means an abnormal gene passed to them from one of their parents. Often, the parent had mild or no symptoms and did not know they carried the gene.

If the problem is in the gene DMPK, each time the gene is passed to a new generation, the repeated stretch of building blocks expands – sometimes dramatically. (This is called anticipation.) This may lead to more severe symptoms that start earlier in life.

A parent with MMD has a 50% chance of passing the gene to each child they have.

There are 2 main types of MMD, based on which gene is abnormal:

• MMD1, also called type 1: DMPK is abnormal. Symptoms can start at any age. Usually they start in adolescents or young adults.
• MMD2, also called type 2: CNBP (ZNF9) is abnormal. This type begins later, usually between 30 and 60 years old.

Doctors often group children with MMD1 based on the age when symptoms started.

• Congenital MMD: Started at or near birth.
• Childhood-onset MMD: Started after infancy but before adolescence. This type is sometimes called juvenile-onset MMD.
• Adult-onset MMD: Started in adolescence or adulthood. This type is sometimes called classic MMD.

Myotonic muscular dystrophy is also called:

• Myotonic dystrophy
• Dystrophia myotonica, or DM (DM1 for type 1 and DM2 for type 2)

Each child with MMD is unique. It’s hard for doctors to predict how MMD will affect your child. In general, the older your child is when symptoms begin, the milder the effects and the more slowly the disease worsens.

MMD does not always cause serious disability, but it can. Usually children with more serious disability start having symptoms in early childhood.

There is no cure yet for this disease, but treatment to deal with symptoms and prevent or manage complications can make a big difference in your child’s quality of life. Together, you and your child’s healthcare team can set goals and make care choices to help your child learn and grow to their fullest.

Babies with MMD symptoms at or soon after birth may have:

• Serious trouble breathing due to weak breathing muscles
• Serious muscle weakness and lack of muscle tone, which may make them seem floppy
• Difficulty sucking and swallowing
• Clubfoot or tight ankles
• Impaired cognitive development
• Problems with hearing, vision and speech
• Developmental delays 

Breathing and muscle movement tend to get better in the first couple of years. Most babies learn to sit, stand and walk. These milestones might take longer to reach than average.

When they reach adolescence, children with congenital MMD start to have symptoms of adult-onset MMD.

If symptoms begin during childhood but after infancy, the first symptoms are usually:

• Intellectual disability, like struggling to plan ahead, make decisions, process visual or spatial information or pay attention
• Learning disabilities

Your child might also:

• Limit or avoid interacting with people or behave differently around people out of concern about being judged (avoidant personality)
• Seem to lack emotion or interest (apathetic personality), which might come from trouble organizing and processing information and making decisions, rather than not caring

Over time, your child will start having muscle problems and other symptoms of adult-onset MMD.

If symptoms begin during adolescence or young adulthood, the first thing you may notice is weakness in skeletal muscle that slowly gets worse.

In MMD1, weakness usually starts in:

• The face and neck, which can cause a weak smile, drooping eyelids and trouble swallowing
• The fingers, hands and forearms, which can cause weak grip
• The feet and lower legs, which can cause foot drop

Over time, weakness can spread to other muscle groups, like the upper legs and breathing muscles.

Other symptoms or health effects of adult-onset MMD1 may include:

• Myotonia
• Shrinking muscles (wasting)
• Problems with muscles in the digestive system, which can lead to choking, getting food or liquid in the airways (aspirating), gallstones, constipation, diarrhea or pain
• Not breathing well enough, especially during sleep
• Arrhythmia and cardiomyopathy
• Cataracts
• Cognitive impairment or learning disabilities
• Avoidant personality or apathetic personality
• Being unusually sleepy during the day (which may have to do with weakness, poor breathing, effects of MMD on the brain or other factors)
• Hormone problems, like hypothyroidism, insulin resistance, diabetes or, in males, hypogonadism that leads to infertility

Some of these effects, like cardiomyopathy and cataracts, are not likely to affect your child for many years, if at all.

MMD can affect muscles of the uterus, so girls and women with MMD need special care if they become pregnant.

The Neuromuscular Program team includes experts from Rehabilitation Medicine, Endocrinology, Neurology, Pulmonary and Sleep Medicine and the Heart Center, as well as a dietitian, genetic counselor and social worker.

Based on your child’s needs, we involve specialists from other clinics and programs around Seattle Children’s, such as:

• Gastroenterology for digestive concerns
• Neurodevelopmental to check for learning disabilities or challenges
• Speech and Language Services if your child has trouble swallowing or communicating
• Orthopedics if surgery on muscles, tendons or bones would help your child
• Ophthalmology to evaluate and treat eye problems

Newborns with congenital MMD are treated in our Level IV neonatal intensive care unit (NICU) by doctors who understand this form of the disease.

Common treatments to maintain your child’s ability to move and do the things they want to do include:

• Physical therapy to maintain muscle tone and improve range of motion
• Occupational therapy to help your child with activities like dressing and using a computer keyboard
• Exercise and stretching to maintain your child’s strength and endurance, improve tight muscles and joints (contractures) and control pain
• Splints or braces for your child’s hands or feet to provide support so your child can do more on their own
• Medicine to relieve myotonia or pain in skeletal muscle
• Devices that help your child get around, such as a walker or wheelchair, or that help them with daily tasks, such as a special chair for sitting in the shower
• Surgery to release contractures (tight muscles and joints) or help your child walk

If your child’s diaphragm and other breathing muscles are weak, we will evaluate how well their lungs work, with a focus on preventing and treating respiratory infections.

Infections can become serious if weak breathing muscles make it hard for your child to cough out phlegm. A device called a cough-assist machine can help your child get a deeper breath and then clear their airways. A respiratory therapist sets up the machine and teaches you how to use it.

Some babies born with congenital MMD may need to be on a machine to help with breathing at first in our NICU until they can breathe well on their own. This could mean continuous positive airway pressure (CPAP), bilevel positive airway pressure (Bi-PAP) or a ventilator.

Some people with MMD need CPAP or Bi-PAP to help with breathing during sleep.

Your child will have a yearly evaluation to check for problems with electrical signals that control their heart rate and rhythm (cardiac conduction defects). Seattle Children’s Arrhythmia Program provides comprehensive evaluation and treatment, including medicines, pacemaker implantation and defibrillator implantation.

A neuropsychologist can evaluate your child’s behavior and cognitive function, including reasoning, memory, attention and learning and recommend treatments or services your child might need, such as:

• Medicines for alertness and attention
• Special education strategies
• Counseling

Children who have cognitive impairment can learn with teaching and tools matched to their abilities and needs.

Medicines may help treat a range of other MMD effects, including:

• Insulin resistance
• Hypothyroidism
• Hypogonadism in males
• Constipation

A dietitian will work with you and your child to:

• Make sure your child gets the right amount of calories to have energy, maintain strength and be as healthy and active as possible
• Prevent or manage constipation, diarrhea or gallstones

Babies born with congenital MMD may need a nasogastric tube, gastrostomy tube, intravenous (IV) feeding or a special bottle to get all the nutrients they need.

A social worker focuses on supporting your child and family and helps with coping, social relationships, behavior and emotions. Along with your child’s doctors, nurses and therapists, the social worker will connect you with helpful resources at Seattle Children’s and in the community.

• We have been named an MDA Care Center by the Muscular Dystrophy Association.
• Our dedicated team – doctors, nurse practitioners, therapists, dietitians, genetic counselors and social workers – has expertise and experience in diagnosing and treating MMD. Many healthcare providers in the community are not as familiar with MMD because they rarely see children who have it.
• Seattle Children’s brings together pediatric specialists from Rehabilitation Medicine, Neurology, Pulmonary and Sleep Medicine and the Heart Center to evaluate your child’s unique needs.

• We design and provide care to help your child and family manage MMD and to give your child the best possible function and quality of life now and as they grow.
• Seattle Children’s provides a full range of treatments, from medicines, physical therapy and movement aids to mechanical ventilation, heart care and orthopedic surgery.

• In the Neuromuscular Program, your child may see their entire team in 1 room on a single day, making the process easier on you and your child.
• To make sure your child has a full evaluation and receives complete, coordinated care, we will involve experts from across Seattle Children’s.
• Based on your child’s needs, your team may include experts from Orthopedics, Orthotics, Speech and Language Services, Neurodevelopmental Clinic and other specialties and programs.