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Congenital myasthenic syndrome is a group of conditions where physical activity leads to muscle weakness. It happens when your nerve and muscle cells don’t communicate as expected. Treatment is available, but there’s no cure for this genetic condition.
Congenital myasthenic syndrome (CMS) is a group of conditions that cause muscle weakness that gets worse with physical activity (exertion). “Congenital” means the condition is present at birth. It most commonly affects your facial muscles, like the ones you use to chew, swallow and blink. But it can also affect the skeletal muscles you use for movement. Symptoms can be mild or severe. Severe cases may be life-threatening, especially if CMS affects the muscles that help you breathe.
When you exercise, your muscles might feel tired when you complete the activity. This is an expected response. If you have CMS, any effort you make during physical exertion can cause symptoms like muscle weakness. This makes the activity more difficult and sometimes unsafe. When the activity stops, you’ll likely feel better.
There are multiple types of congenital myasthenic syndromes. Healthcare providers group them into categories based on what part of the neuromuscular junction the condition affects. This is where your nerve and muscle cells meet. CMS affects how your nerve cells communicate with your muscle cells at this junction. Types include:
Healthcare providers further identify these types by the specific cellular structures or components affected at the juncture, like the basal lamina or acetylcholine receptor, for example.
The prevalence of congenital myasthenic syndrome isn’t well known. One U.K. study estimated that CMS affects about 9 children (under age 18) per 1 million people.
Congenital myasthenic syndrome and myasthenia gravis are similar conditions that cause weakness due to problems where nerve cells and muscle cells meet (neuromuscular junction). CMS is a genetic condition caused by a genetic change. Myasthenia gravis is an autoimmune condition caused by your immune system mistakenly attacking this area of your body.
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The signs and symptoms of congenital myasthenic syndrome vary based on type but could include:
Most congenital myasthenic syndrome symptoms begin in early childhood. A sign that your child might have CMS is delayed motor skills development (like crawling and walking). Some symptoms of CMS start later, for example, during adolescence or adulthood.
A genetic change (mutation) causes congenital myasthenic syndrome. Some of the gene changes responsible include, but aren’t limited to, the following:
These genes provide instructions for your cells to produce proteins that help signals pass between cells at the neuromuscular junction. Signaling is how your nerves and muscles communicate across a bridge (the neuromuscular junction). When your nerve cells signal to your muscle cells, your nerves tell your muscles to move.
If a genetic mutation happens, your cells don’t have the instructions they need to send signals or messages across the bridge. This leads to symptoms of CMS.
Yes, you can inherit congenital myasthenic syndrome. It usually happens in an autosomal recessive pattern. This means that both biological parents need to have the gene and pass it on to their child for the child to develop the condition. Some CMS types pass in an autosomal dominant pattern, where only one biological parent needs to pass the gene onto their child for them to inherit the condition.
Some gene changes happen randomly, without a history of the condition in a person’s biological family history.
Anyone can develop congenital myasthenic syndrome. You’re more at risk if it runs in your biological family history.
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Congenital myasthenic syndrome complications may include:
A healthcare provider will diagnose congenital myasthenic syndrome after a physical and neurological exam. They’ll review your symptoms and take a complete medical history. If they suspect CMS, they may ask you to perform a physical activity under their supervision to observe how your body reacts, like walking up stairs, for example. They may offer tests to rule out conditions with similar symptoms, like:
Genetic testing helps your healthcare provider detect the gene change that’s causing your symptoms. Your provider may draw a small sample of your blood to examine your DNA. The gene change can help your provider detect what type of CMS you have and where in your neuromuscular junction there’s an issue.
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There isn’t a cure for CMS, but treatment is available to help you manage your symptoms. Medications are usually helpful to maintain or boost your muscle function. Your provider might recommend:
While physical activity can make your symptoms worse, your healthcare provider may suggest meeting with a physical therapist to help you find light activities and exercises that are safe for you to maintain good health and prevent muscle stiffness.
You may need to use assistive devices like a wheelchair to navigate and prevent injury.
The side effects vary for each type of medication but may include:
Talk to your provider about the possible side effects before starting a medication to make an informed decision about your health.
There’s no known way to prevent congenital myasthenic syndrome. If you plan on expanding your family, you can learn more about your risk of having a child with a genetic condition by talking to a healthcare provider about genetic testing.
If you anticipate taking a new medication, always talk to your provider about having CMS first. To prevent worsening symptoms, your healthcare provider may recommend alternatives if you need to take the following medications:
Congenital myasthenic syndrome symptoms range from mild to severe. It may have very little effect on your life, while others may experience life-threatening symptoms like breathing difficulties. Your provider can give you the best outlook for your situation.
Congenital myasthenic syndrome may or may not affect your life expectancy. If you have mild symptoms, CMS won’t have a major effect on your overall health. CMS can impact your life expectancy if symptoms affect the muscles that regulate your breathing. Your care team will help you manage your symptoms to prevent life-threatening complications.
Visit a healthcare provider if you or your child experience symptoms of congenital myasthenic syndrome when participating in physical activities, like muscle weakness. Let your child’s provider know if your child is unable to eat or if they miss developmental milestones for their age.
Contact 911 or your local emergency services number immediately if your child has trouble breathing or their skin, lips or nails turn pale or blueish gray (cyanosis).
Congenital myasthenic syndrome (CMS) symptoms are different for each person. It can interfere with your daily routine and make things a little more challenging, like using the stairs or exercising. Your healthcare provider may recommend you use assistive devices like a wheelchair to prevent falls or injury.
Children diagnosed with the condition may miss developmental milestones for their age, especially motor skills, like taking their first steps. If you notice your child needs more time than other children to reach growth goals, let their healthcare provider know.
After a diagnosis, you’ll likely have several questions. Your care team is available to help you better understand how this condition affects you and what treatment options are available.
Last reviewed on 02/15/2024.
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Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy
