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What Causes Myasthenia Gravis?

Reviewed by: HU Medical Review Board | Last reviewed: August 2020

Myasthenia gravis (MG) is a disease that causes certain muscles to become weak. The muscles involved are called voluntary muscles, or the ones a person can control. Normally, when you think about moving a muscle, the movement is triggered by an electrical pulse from a nerve that travels to a muscle. With myasthenia gravis, there is a problem with the communication between nerves and muscles.1-3

The muscles most often involved in MG include muscles that control:1-3

  • Arm and leg movement
  • Breathing
  • Eye movement and eyelids
  • Speech, chewing, and swallowing

MG is an autoimmune disease. This means that the body’s immune system can no longer tell the difference between healthy cells and invaders like viruses, fungi, or bacteria. Because it cannot tell the difference, the body begins to attack healthy spots on the muscle that receive communication signals from nerves. This causes the muscles to become weak and tire easily.

Two things that go wrong in the body can lead to myasthenia gravis: antibodies and the thymus gland.2

Antibodies that cause MG

Antibodies are chemicals the body’s immune system makes to kill germs. In myasthenia gravis, antibodies begin to block, change, or destroy places where nerves and muscles communicate. This area is called the neuromuscular junction.1-3

A neuromuscular junction includes spots on the muscle that "read" the nerve signals, telling it to move. These spots are called receptors because they receive nerve signals. In a person with MG, when the nerves send out their message using a chemical called acetylcholine, the neuromuscular junction cannot “read” the signal.

Most people with MG have problems with acetylcholine receptors (AChR). However, other antibodies may cause MG, such as muscle-specific kinase (MuSK) and low-density lipoprotein receptor-related protein 4 (LRP4).2

Diagram of a neuromuscular junction with and without myasthenia gravis.

Some people with myasthenia gravis will have AChR, MuSK, or LRP4 antibodies in their blood, and high levels of these antibodies will show up in blood tests. The most common antibody found in blood is AChR. However, not everyone with MG will show high numbers of these antibodies in their blood.2

The thymus gland

The thymus gland is a small gland located in the upper chest beneath the sternum. It helps control the immune system, and when it malfunctions it may cause myasthenia gravis. Many people with MG have a large or overactive thymus gland. Some even develop tumors on their thymus gland. These tumors are called thymomas. These tumors can be harmless, but also can turn into cancer or cause lasting health issues.1-3

Doctors believe that the thymus gland in people with MG may cause the immune system to go into overdrive. However, doctors do not understand the exact role the thymus gland plays.

It may take some time to be diagnosed with myasthenia gravis. While treatments may help improve the symptoms of MG, many find living with a chronic condition to be unpredictable, as well as physically and emotionally challenging.

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