Understanding Myasthenia Gravis (MG)

Overview of Myasthenia Gravis

Hospital Patient
Hospital Patient. EdStock / Staff / Getty Images

Myasthenia gravis, also referred to as MG, is the most prevalent of neuromuscular junction disorders, affecting about 150 to 200 people per million. The disease causes muscle weakness that can sometimes be life-threatening, but fortunately, it's also one of the best understood of all neurological disorders, and some very successful treatments are readily available.

Symptoms of Myasthenia Gravis

Myasthenia gravis causes true muscle fatigue — more than just a feeling of tiredness, which everyone experiences from time to time.

 True muscle fatigue occurs when, following exertion, the muscles simply lose their ability to move with their normal strength — or sometimes even to move at all, no matter how hard someone tries. Also, usually only some muscles are weakened in myasthenia gravis, as opposed to all at once.

Over half of people with myasthenia gravis first have symptoms of double vision (diplopia) due to weakness of the eye muscles. Often the eyelids begin to droop as well (ptosis), which may make it difficult to see properly. About 15 percent have weakness of the muscles needed to speak and talk properly. This is especially dangerous, as without these muscles, a person may not be able to protect his or her airway, and may need to be intubated. Initially, though, involvement of the facial muscles may just limit someone's ability to smile. The voice may sound more nasal as the muscles of the pharynx weaken.

The weakness of myasthenia gravis also frequently involves the neck and limbs.

Typically, the weakness is worse in the muscles closer to the torso, and weakness is generally worse in the arms as opposed to the legs, but exceptions do occur. If the neck is weak, someone may have a hard time holding his head upright.

Weakness of the muscles required to breathe are the most serious consequences of myasthenia gravis and may require treatment in an intensive care unit.

This is known as a myasthenia crisis.

The disease tends to be most common in young women and men over the age of 50. In general, symptoms peak a few years after they start. Initially, the weakness may be transient, with long periods between episodes of weakness. In those patients who first have eye muscle weakness, the disease sometimes never progresses to involve other muscle groups. Symptoms either stabilize, from which they may be worsened by some medications or infections, or sometimes they spontaneously go away on their own.

Causes of Myasthenia Gravis

Myasthenia gravis occurs due to the presence of abnormal antibodies against components of the muscle. The most common antibody is against the acetylcholine receptor at the neuromuscular junction, where the muscle receives signals from the nerve that it's time to contract. The nerve does this by releasing the neurotransmitter acetylcholine into the area between the muscle and nerve (the synapse). If the neurotransmitter makes contact with a receptor, the signal to contract is given. The antibodies in myasthenia gravis cause the receptor to be taken into the muscle cell, where it's no longer available to receive the signal of acetylcholine.

While the most common type of antibody is the acetylcholine receptor antibody (AChR antibody), 10 to 20 percent of people with myasthenia gravis do not have these antibodies when checked in the lab. Of these, between 40 to 70 percent of Caucasians will have antibodies to muscle-specific receptor tyrosine kin (MuSK). This number may be lower in Asian populations.

In patients without either AChR or MuSK antibodies, the disease is still thought to be autoimmune, but less is known about the antibodies involved.

The Thymus in Myasthenia Gravis

The thymus is an organ involved with the immune system found near the bottom of the throat.

Sixty to 70 percent of people with myasthenia gravis due to AChR antibodies have a large thymus (thymic hyperplasia), and 10 to 12 percent have a tumor of the thymus (thymoma). Sometimes the disease improves after the thymus is removed during a procedure called a thymectomy. This has led many to suspect that the thymus is tightly related to myasthenia gravis. Still, not all people with a thymoma get myasthenia, and vice versa, suggesting that other factors, including genetics, are likely involved.  

In short, myasthenia gravis is not common overall, but should still be considered whenever someone is truly weak — especially if the weakness markedly worsens with repeated use of the affected muscles. Neurologists have a good understanding of how most forms of myasthenia gravis work, and will often be able to help once the correct diagnosis is reached.

Treating Myasthenia Gravis

Patients with myasthenia gravis generally can control their disease with treatments currently available. Several approaches can help reduce and improve muscle weakness. These include medications such as anticholinesterase agents, which increase muscle strength; and immunosuppressive drugs, which also suppress production of certain antibodies and improve muscle strength. 

Some patients who undergo thymectomy, the surgical removal of the thymus gland (which often is abnormal in individuals with myasthenia gravis), experience a reduction in symptoms and may even cure others.

Other therapies used to treat myasthenia gravis include plasmapheresis and high-dose intravenous immune globulin. Plasmapheresis is a procedure that involves separating plasma from the cells (thereby removing the antibodies present in the plasma that attack the immune system) and replacing it with a substitute. High-dose intravenous immune globulin infuses antibodies from donated blood to temporarily modify the immune system 

Prognosis for Patients With Myasthenia Gravis 

Most patients with myasthenia can significantly reduce their symptoms with currently available treatments and achieve a normal quality of life. For some patients, the disease may even go into remission and they can discontinue treatment. About 50 percent of patients who undergo a thymectomy will attain complete remission. In very few cases, myasthenia gravis may result in respiratory failure, which requires immediate emergency medical care.


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AH Ropper, MA Samuels. Adams and Victor's Principles of Neurology, 9th ed: The McGraw-Hill Companies, Inc., 2009.

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