Myasthenia Gravis


Also known as: MG
Last Review Date: August 1, 2018


What is myasthenia gravis?

Myasthenia gravis (MG) is a chronic autoimmune disease that affects skeletal muscle strength by impeding the communication between nerves and muscles. MG is typically first noticed when it causes weakness in eye muscles and symptoms such as a drooping eyelid and/or double vision. This is often referred to as ocular MG. From the eye muscles, it can spread over time to facial and neck muscles, causing weakness, slurred speech, difficulty chewing and swallowing, and/or difficulty breathing, and from the head and neck to other parts of the body, resulting in generalized MG. Muscle weakness will vary over time; it tends to worsen with sustained effort and improves with rest.

Body movements, including those as small as keeping the head upright and eyes open, are normally carried out by a coordinated series of muscle contractions. These muscle contractions are initiated by chemical nerve signals. On a microscopic level, a nerve impulse travels to a nerve ending and acetylcholine, a neurotransmitter, is released. This chemical travels from the nerve ending to a muscle fibre across a tiny gap that exists at the "neuromuscular junction" and it binds to one of many acetylcholine receptors on the muscle fibre. This binding activates the receptor and initiates muscle contraction.

With MG, the body's immune system produces proteins (autoantibodies) that target a person's own acetylcholine receptors and block or destroy them. This inhibits the receipt of acetylcholine signals and causes weakness and rapid muscle fatigue.

In Australia, MG is estimated to affect about 1 person in 10,000. Anyone can develop MG, but it is most frequently diagnosed in men over 60 years of age and in women under 40. During pregnancy, a woman with MG may pass acetylcholine receptor antibodies to her fetus. This can cause a newborn to have symptoms of MG, but the symptoms typically resolve within 2-3 months of birth.

The cause of MG is not known, but about 75% of those affected have an abnormally large thymus gland and some develop thymomas (generally benign tumours of the thymus). The thymus is a small gland located behind the upper breastbone at the base of the neck. It plays an active role in the immune system through young adulthood, but normally begins to shrink after puberty and is essentially nonfunctional in adults. The relationship between MG and the thymus is not fully understood, but some think that the thymus plays an indirect role in triggering acetylcholine receptor antibody production.

Those who have MG are at an increased risk of also developing other autoimmune disorders, such as systemic lupus erythematosusrheumatoid arthritis, or autoimmune thyroid disease.

A small percentage of those with MG have an affected family member, but most do not. Congenital myasthenic syndrome is an inherited condition that causes symptoms similar to MG, but it is not an autoimmune disorder. Another condition, Lambert-Eaton Myasthenic Syndrome, can also cause similar symptoms, but it involves interference with the release of acetylcholine by the nerve ending, not acetylcholine receptor activity.


Signs and symptoms

A person with myasthenia gravis (MG) may experience a variety of symptoms. These may vary from day to day, worsen after activity and later in the day, and improve with rest. Symptoms may stay confined to the muscles around the eyes (ocular MG), but in most people they will extend to the face, neck, and other parts of the body within about a year (generalised MG). MG symptoms tend to worsen for several years and then stabilize. Some people may go through extended periods of remission.

Symptoms may include:

  • Drooping eyelid
  • Double vision
  • Decreased eye movement control
  • Difficulty swallowing, chewing, with choking, drooling and gagging
  • Slurred speech
  • Weak neck muscles
  • Trouble holding up head
  • Difficulty breathing
  • Difficulty walking and an altered gait
  • Specific muscle weakness but normal feelings/sensations

When MG affects the muscles that control breathing, it can cause a medical emergency called a myasthenic crisis that often requires hospitalisation and may require the temporary use of a respirator to assist ventilation.

Heat, stress, illness, and certain medications can cause symptoms to worsen.


Tests 

The goals with testing are to diagnose myasthenia gravis (MG), distinguish it from other conditions with similar symptoms, and to guide treatment. Some tests may be performed to monitor a person's health status over time.

Laboratory tests
Laboratory testing may include the measurement of one or more autoantibodies:

  • Acetylcholine receptor Ab (ACHR)—found in up to 90% of those with generalised MG and about 50% of those with ocular MG; this is the primary blood test for MG. There are three types of ACHR antibodies: binding (most frequently tested), blocking, and modulating.
  • Anti-MuSK (muscle-specific kinase) antibodies—found in up to 70% of those who are negative for AchR antibodies and have generalized MG.
  • Anti-Striated muscle antibody test—found in about 80% of those with MG who have an enlarged thymus gland and indicate a significantly increased likelihood of the person having a thymoma.
Other testing that may sometimes be ordered: Non-laboratory tests
  • Tensilon test (acetylcholinesterase test using edrophonium)—a person is given a drug intravenously to see if muscle strength improves for a few minutes (for more on this, see MedlinePlus Medical Encyclopedia: Tensilon test).
  • Repetitive nerve stimulation and/or single-fibre electromyography—to evaluate neuromuscular response in a specific area.
  • A chest CT scan (computed tomography)—to detect an enlarged thymus gland or thymoma.
  • A brain and eye orbit MRI (magnetic resonance imaging)—this is not routine but can help rule out other causes of eye-related symptoms.

Treatment

There is no way to prevent or cure myasthenia gravis (MG), but its symptoms can be managed and most of those affected can lead normal or near-normal lives. The most significant complication, a myasthenic crisis that affects a person's ability to breathe, must be treated as medical emergency and frequently requires hospitalisation.

Management of MG may include:

  • Acetylcholine esterase (AChE) inhibitors/Cholinesterase inhibitors, drugs that improve nerve and muscle communication and improve muscle strength. These are the primary treatment for MG.
  • Thymectomy, the removal of the thymus gland, may be performed when a thymoma is present. This can reduce symptoms and in some cases may resolve them.

Other types of treatments are available to manage MG. For additional details on these, visit the Treatment for MG page on the Myasthenia Gravis Foundation of America web site.

Newborns that acquire MG symptoms from their mothers must be closely monitored for a couple of weeks until the influence of the mothers' antibodies subsides.

People's treatment needs will vary over time. Those affected should work closely with their health care provider to determine the treatments appropriate for their changing health status.
 


Related pages

On this site
Tests: Acetylcholine receptor Ab, Rheumatoid Factor, CCP Antibody, ANA, Autoantibodies, Thyroid Antibodies
Conditions: Rheumatoid Arthritis, Systemic Lupus Erythematosus, Autoimmune Disorders, Graves Disease

Elsewhere on the web
Better Health Channel - Myasthenia gravis
The Australian Myasthenic Association
MedlinePlus Medical Encyclopedia: Myasthenia gravis  
Muscular Dystrophy Association: Facts About Myasthenia Gravis, Lambert-Eaton Myasthenic Syndrome & Congenital Myasthenic Syndromes  
Muscular Dystrophy Association: Myasthenia Gravis



 

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