What Is Congenital Myasthenic Syndrome?

Last updated: August 2022

Myasthenia gravis (MG) is a disease that causes weakness and fatigue in muscles that you voluntarily control. It is an autoimmune disease. This means that your immune system mistakes normal, healthy parts of your nerves for foreign invaders and attacks them.1

These attacks are what cause the breakdown in communication between your muscles and your brain. However, MG is not the only disorder that can cause this type of muscle weakness. Congenital myasthenic syndromes (CMSs) are similar to MG. CMSs cause a faulty connection between your muscles and your nervous system.1-5

But CMSs do not work in the same way as MG. CMSs are genetic conditions, not autoimmune ones. Because they have different causes, the 2 disorders have differences that affect your treatment and care.2-5

What is CMS?

CMSs are a group of disorders that affect the connection between your muscles and your brain. They are hereditary, which means that they are caused by genes passed from parents to their child. The different genes involved in CMSs are what cause their different severity, symptoms, and response to treatment.2,4

CMS causes muscular weakness just like MG does. But where this weakness is can change depending on the type of CMS you have (called a CMS subtype). One person with CMS could have weak facial muscles or trouble swallowing. Another could have issues walking or using their arms.4

Researchers estimate that about 5,000 people in the United States have some type of CMS. Depending on your CMS subtype, symptoms can range from minor to more severe. CMS first appears during the first 24 months of a child’s life, and symptoms are usually stable throughout a person’s lifetime.2,4

If your baby has muscle weakness but tests negative for MG they may have CMS. Your doctor will look at your family history and do genetic tests to confirm whether your child has CMS and – if so – what subtype.5

How are the subtypes different?

There are 2 different ways CMS can be passed down from a parent to their child. Most types of CMS are passed down through autosomal recessive inheritance. This means that both parents had to give their child a copy of the gene for CMS. Less commonly, CMS can be passed down through autosomal dominant inheritance, which means only one parent had to give their child the gene for CMS.3,4

The types of CMS are distinguished from one another based on how the connection between your muscles and your nervous system is broken. This connection can be faulty on the muscular side, on the nervous system side, or somewhere else along the path from muscles to brain. This can cause differences in:4

  • Severity
  • Your age when symptoms start
  • Areas where you feel muscle weakness

Doctors have found that at least 32 different genes could be responsible for someone’s CMS. These different genes are each associated with distinct CMS subtypes and respond differently to treatment. Genetic testing can confirm which subtype you have and how best to treat it.4

How are CMS and MG different?

The main difference between CMS and MG is that MG is an autoimmune disorder, while CMS is genetic. However, there are other differences as well.1,2

CMS symptoms usually start much earlier in life than MG. MG typically develops in adults, and CMS starts when someone is very young – in newborns and infants less than 2 years old.2,3

Treatment between the 2 is different as well. People with CMS and people with MG can both take pyridostigmine. But people with MG can take immunosuppressant drugs to treat their symptoms. Because CMS is genetic, not autoimmune, you cannot take immunosuppressants or use immunotherapy to treat its symptoms.2

People with CMG can also take medicine not prescribed to people with MG. Some drugs that people with CMS can take include:2

  • Ephedrine
  • 3,4-DAP
  • Quinidine
  • Fluoxetine
  • A variety of other drugs, depending on the type of CMS

A drug that could help one type of CMS could make another worse. Make sure to talk with your care team before starting any course of treatment or taking medicine, including over-the-counter drugs.4

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