Myasthenia Gravis

Home » Myasthenia Gravis » Overview, Types, Incidence and Prevalence, Causes and Risk Factors

Overview

Myasthenia gravis (MG) is a chronic autoimmune disorder that results in progressive skeletal muscle weakness. Skeletal muscles are primarily muscle fibers that contain bands or striations (striated muscles) that are connected to bone. MG causes rapid fatigue (fatigability) and loss of strength upon exertion that improves after rest.

In early stages, myasthenia gravis primarily affects muscles that control eye movement (extraocular muscles) and those that control facial expression, chewing, and swallowing. If untreated, the disorder may affect muscles that control breathing (respiration), causing acute respiratory failure.

Types
Myasthenia gravis can be classified according to which skeletal muscles are affected. Within a year of onset, approximately 85–90% of patients develop generalized myasthenia gravis, which is characterized by weakness in the trunk, arms, and legs.

About 10–15% of patients have weakness only in muscles that control eye movement. This type is called ocular myasthenia gravis.

Other types of MG include congenital, which is an inherited condition caused by genetic defect, and transient neonatal, which occurs in infants born to mothers who have MG. Congenital MG develops at or shortly after birth and causes generalized symptoms.

Transient neonatal MG is a temporary condition that develops in 10–20% of infants born to mothers who have MG. Transient neonatal MG is caused by circulation of the mother's antibodies through the placenta and it lasts as long as the mother's antibodies remain in the infant (usually a few weeks after birth).

Incidence and Prevalence
Myasthenia gravis affects approximately 2 out of every 100,000 people and can occur at any age. It is most common in women between the ages of 18 and 25. In men, the condition usually develops between 60 and 80 years of age.

Causes and Risk Factors

MG usually is caused by a malfunction of the immune system. The causative factor is unknown, but the disorder may have a genetic link. Causes include a genetic defect, which results in congenital MG, and the circulation of maternal antibodies through the placenta, which result in transient neonatal MG.

Acetylcholine (ACh) is a neurotransmitter that is involved in the transfer of information to muscle tissue. In myasthenia gravis, cells that bind to other cells to neutralize or destroy them (called antibodies) destroy acetylcholine receptor sites (AChR) in areas of muscle tissue that receive nerve impulses (called neuromuscular junctions), preventing nerve impulses from reaching the muscles. This results in weakness and rapid fatigue in affected muscles.

MG may be associated with other autoimmune diseases. Patients with family members who suffer from disorders such as rheumatoid arthritis, scleroderma, and lupus may have an increased risk for myasthenia gravis.

The thymus is an organ that produces cells involved in immune responses. It is located below the larynx and above the heart. Approximately 15% of MG patients have a tumor of the thymus (thymoma) and 60–80% have abnormal enlargement (hyperplasia) of the thymus.

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