Overview of Myasthenia Gravis
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 of Myasthenia Gravispr> Myasthenia gravis can be classified according to which skeletal muscles are affected. Within a year of onset, approximately 8590% of patients develop generalized myasthenia gravis, which is characterized by weakness in the trunk, arms, and legs.
About 1015% 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 1020% 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 of Myasthenia Gravis
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.