Congenital Myasthenic Syndromes

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.

Congenital myasthenic syndrome types

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:

• Presynaptic congenital myasthenic syndrome occurs in the nerve cell.
• Synaptic congenital myasthenic syndrome occurs in the area between the nerve cell and the muscle cell.
• Postsynaptic congenital myasthenic syndrome occurs in the muscle cell.

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.

How common is congenital myasthenic syndrome?

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.

What’s the difference between congenital myasthenic syndrome and myasthenia gravis?

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.