There are several types of myasthenia gravis (MG), an autoimmune disease that occurs when the immune system produces self-reactive antibodies that wrongly target and attack certain proteins, such as acetylcholine receptors, that are needed for nerve-muscle communication.
All result in muscle weakness and fatigue. But the different types of MG depend on the time of disease onset, the specific cause of neuromuscular dysfunction, and the muscle groups affected.
Some patients also may be classified into specific disease subtypes.
Congenital myasthenia gravis
Congenital MG is estimated to occur in 2–12 people per million, and is typically inherited instead of being caused by a malfunction of the body’s immune system. This specific type of MG occurs due to genetic defects in genes, typically inherited from both parents, that are involved in nerve-muscle communication.
There are several types of congenital MG, depending on the type of genetic defect a patient harbors and at which point of the nerve-muscle communication signaling cascade there are impairments. Each specific type may be associated with a different set of symptoms, degree of severity, and treatments.
Ocular myasthenia gravis
In 10–40% of MG patients, symptoms of muscle weakness and fatigue are restricted to the muscles that control eye and eyelid movements. Patients with this type of MG, called ocular MG, tend to experience eyelid droopiness and double vision as their eyes and eyelid muscles progressively lose strength.
While many patients with other forms of MG first experience symptoms in these muscles before the disease progresses and spreads to other muscle groups, in those with ocular MG, symptoms of muscle weakness and fatigue do not spread to other muscles in the body.
Generalized myasthenia gravis
About 50–80% of patients with eye symptoms develop generalized MG. This specific type of the disease is characterized by widespread muscle weakness and fatigue that is not confined to eye and eyelid muscles, or any other specific muscle groups. It may spread to muscles in the face, as well as those in the limbs.
In approximately 10% of these patients, muscle weakness spreads to respiratory muscles or those of the throat and jaw. This can make breathing fairly difficult and lead to potentially life-threatening episodes known as myasthenic crises.
Generalized MG is more common in women than in men, particularly in those younger than 40.
Transient neonatal myasthenia gravis
This type of MG is reported to be rare and thought to occur in 10–15% of infants born to mothers with the disease.
During pregnancy, the mother’s self-reactive antibodies can be passed on to the unborn child, resulting in the destruction of acetylcholine receptors in the fetus.
Newborns typically develop symptoms of transient neonatal MG about 48 hours after birth. Symptoms may include overall muscle weakness, impaired sucking and swallowing, a weak cry, and respiratory insufficiency. While serious, these symptoms usually disappear within a few days or weeks.
Juvenile myasthenia gravis
In juvenile MG, symptoms begin before the onset of puberty. In its most benign form, juvenile MG is confined to eye muscles, but severe cases involving other muscle groups have been reported. Symptoms can range from clumsiness to swallowing difficulties to tiring easily.
Young patients are less likely to test positive for the acetylcholine receptor self-reactive antibodies that characterize generalized or ocular MG in adults. Juvenile MG patients are also more likely to achieve remission than adult patients, but periods of remission can give way to relapses.
Many of these patients have an abnormal growth of the thymus — a gland that is part of the immune system and thought to trigger or help maintain the production of the harmful self-reactive antibodies that impair nerve-muscle communication in MG. As such, a possible treatment is surgically removing this gland, a procedure called thymectomy.
Last updated: August 5, 2021
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