What is myasthenia gravis?

Myasthenia gravis (MG) is a neuromuscular disease, meaning that it affects the muscles and the nerves that control them. It is caused by a disorder in the immune system that causes the body to attack the area of the muscles where the nerves connect to them.

The immune system is the body’s natural defense against disease. Normally, when bacteria or other foreign substances enter the body, the immune system produces molecules called antibodies that attack the bacteria.

In people with myasthenia gravis, the immune system produces abnormal antibodies that prevent the muscles from receiving signals from the nerves that tell them when to relax or contract. This causes muscle weakness with symptoms that can include in double vision or blurred vision (eye muscle weakness), drooping eyelids (eyelid muscle weakness), difficulty with speaking and swallowing (throat muscle weakness) and weakness of the limbs.

When the immune system acts against healthy tissue by mistake, it is called an autoimmune disorder, with “auto” meaning “self. So myasthenia gravis is a neuromuscular autoimmune disease.

Myasthenia gravis is most common in young women and older men, but people of any age or either sex can get it.

What causes myasthenia gravis?

Scientists do not completely understand what triggers the autoimmune reaction in myasthenia gravis, but they do know that the thymus gland plays a role in the disease.

The thymus is a small gland that lies in the front part of the chest, beneath the breastbone, and extends into the lower part of the neck. It is most important early in life during immune system development.

A baby’s thymus gland weighs between .7 and 1.1 oz. The gland continues to grow and by puberty weighs 1.1 to 1.8 oz. The thymus gland’s job is thought to be completed by puberty, and after that it decreases in size. Over time, fat replaces portions of the gland. In older people the thymus weighs only .1 to .5 oz.

Tumors of the thymus gland are called thymomas. Around 10-15 percent of people with myasthenia gravis have a thymoma Another 60%, however, will have other abnormalities of the gland including thymic hyperplasia (an enlarged gland).

The original association between the thymus gland and myasthenia gravis was made back in the early 1900’s when surgeons observed that removal of a thymoma resulted in the improvement in the patient’s myasthenia gravis. Ultimately surgeons began removing of the thymus gland in myasthenic patients without thymic tumors and a similar response was noted.

Research into the causes and treatments of myasthenia gravis will help scientists learn more about the role of the thymus in the disease.

How is myasthenia gravis treated?

The key to treatment of myasthenia gravis begins with an accurate diagnosis. The evaluation is usually directed by a Neurologist and can involve blood tests, nerve testing and tests involving administration of medicines in order to differentiate myasthenia gravis from other disease of muscles and nerves.

Once the diagnosis has been confirmed, a treatment plan is developed with the goal of reducing the number of antibodies causing the disease and/or improving the communication between the nerves and muscles. The ultimate results is improving muscle strength. Medical treatment options include:

  • Medicines that suppress antibody production or improve nerve signal transmission
  • Plasmapheresis, a procedure that removes antibodies from the blood
  • High-dose intravenous immune globulin, the infusion of normal antibodies from donated blood to temporarily modify the immune system

Surgical treatment is thymectomy, removal of the thymus gland. This is the treatment for patients with thymomas, but is also considered for patients with MG who do not have thymomas.

At Cleveland Clinic, the Thoracic Surgeons are part of the treatment team evaluating patients and identifying the most appropriate combination of therapies for each individual.

Last reviewed by a Cleveland Clinic medical professional on 05/14/2019.

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