Myasthenia gravis (MG) is a chronic disease characterized by weakness and rapid fatigue of the voluntary muscles. The weakness is due to a breakdown in communication between a nerve ending and its adjoining muscle fiber. The onset of MG can be sudden, with severe and generalized muscle weakness, but more often its symptoms in the early stages are subtle and variable, making it difficult to diagnose correctly. MG affects people of all ages and either sex, but it more affects young women and old men.

MG is a disorder of the neuromuscular junction - the space between the nerve and muscle. Neurotransmitters are released from the nerve in response to an electrical signal and cross this space to activate muscle movements. MG is an autoimmune disorder, meaning the body’s immune system actually attacks itself. As a result, in MG, the receptors to these neurotransmitters (in particular, acetylcholine) are reduced in number and function. Thus, the muscle never receives appropriate signals and patients naturally experience muscle weakness. ‘Generalized’ MG can affect muscles of the eyes (resulting in double vision or drooping of the eyelid), pharynx (difficulty swallowing and speaking clearly), chest wall (shortness of breath), and arms and legs (weakness and easy fatigability). A more focal form of MG known as ‘ocular MG’, can present with visual symptoms alone. However, a significant percentage of patients with ocular MG can progress to the generalized form. MG tends to have a fluctuating course, but can be worsened by exertions, a variety of infections, certain medications, co-existent illnesses, and pregnancy. MG may be associated with other autoimmune disorders, most commonly of the thyroid and gastrointestinal tract. It may also be associated with a tumor of the thymus gland (called a thymoma).

What are the symptoms of Myasthenia Gravis (MG)?

The first noticeable symptom of MG is frequently weakness of the eye muscles. The disease may remain confined to this area of the body, or it may progress to muscles involving swallowing, chewing, speech or limb movement. Symptoms vary among patients, but can include:

  • A drooping of one or both the eyelids (ptosis)
  • Blurred or double vision
  • Unstable or "waddling" gait
  • Weakness in the arms or legs
  • Difficulty swallowing
  • Difficulty breathing
  • Difficulty speaking clearly

Once symptoms begin, muscle weakness may worsen over days or weeks. Symptoms may remain constant, may progress or may fluctuate from hour to hour or day to day. Weakness tends to worsen with exercise and at the end of the day. When MG affects a patient’s ability to swallow or breathe, the disease can become life-threatening if not treated immediately. However, the disease is seldom fatal if managed properly.

What are the causes of Myasthenia Gravis (MG)?

MG develops when antibodies are produced against the region of the muscle that connects with a nerve ending. For muscle to contract, the neurotransmitter chemical acetylcholine must attach to the muscle membrane after its release from the nerve ending. But this is prevented by the antibody’s attack on the muscle’s acetylcholine receptors. The result is muscle weakness.

Doctors have found that the thymus gland appears to trigger this autoimmune response by encouraging production of acetylcholine receptor antibodies.

Located behind the breastbone, the thymus gland is part of the body’s immune system, producing white blood cells called lymphocytes, which are responsible for immune responses to bacteria, toxins and viruses. Normally large in infancy, the thymus gland shrinks in size so that in the normal adult it is hardly functional. For patients who have MG, however, the thymus gland can have an excess number of cells (hyperplasia), which may contribute to the autoimmune response.

Reviewed by a Cleveland Clinic medical professional.

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