Myasthenia gravis, a neuromuscular disease characterized by weakness and fatigue, is typically divided into five types: congenital myasthenia gravis, generalized myasthenia gravis, ocular myasthenia gravis, transient neonatal myasthenia gravis, and juvenile myasthenia gravis, depending on time of disease onset, the cause of the neuromuscular dysfunction, and the muscle groups affected.
Some patients may be classified as having multiple types.
Congenital myasthenia gravis
Congenital myasthenia gravis is defined by its cause: a genetic defect typically inherited from both parents, rather than an autoimmune disease in which immune system attacks damage communication between nerves and muscles — the cause of the other forms of this disease.
Congenital myasthenia gravis appears to result from changes in genes involved in neuro-muscular communication or communication between nerve and muscle cells. In some cases, genes encoding for the acetylcholine receptor itself may be affected.
The various types of congenital myasthenia gravis are defined by the location and type of genetic defect that causes poor neuromuscular signaling.
Ocular myasthenia gravis
In ocular myasthenia gravis, the muscles that move the eyes and control the eyelids are weakened and easily fatigued. Symptoms include drooping eyelids and double vision.
While many patients with other forms of myasthenia gravis first experience symptoms in the muscles around the eyes as well, patients with ocular myasthenia gravis only have symptoms in these muscles, and the weakness and fatigue do not spread to other muscle groups.
Generalized myasthenia gravis
In patients with generalized myasthenia gravis, muscle weakness and fatigue are not confined to the eye muscles or other specific muscle groups. It may spread to the facial muscles as well as the limbs.
In approximately 10 percent of these patients, muscle weakness spreads to the respiratory muscles or muscles of the throat and jaw. This can make breathing quite difficult, and can be extremely serious. Such episodes are called a “myasthenic crisis.”
Transient neonatal myasthenia gravis
Infants born to mothers with myasthenia gravis may develop symptoms of the disease about 48 hours after birth.
Symptoms can include impaired sucking and swallowing, a weak cry, and respiratory insufficiency. While serious, the symptoms typically disappear within days or weeks.
Juvenile myasthenia gravis
In juvenile myasthenia gravis, symptoms begin before the onset of puberty.
Juvenile myasthenia gravis in its most benign form is confined to the eye muscles, but severe cases involving other muscle groups are known. Symptoms can range from clumsiness to difficulty in swallowing and tiring easily.
Young patients are less likely to test positive for the acetylcholine receptor antibodies that characterize generalized or ocular myasthenia gravis in adults. Juvenile myasthenia gravis patients are also more likely to achieve remission than adult patients, but periods of remission can give way to relapses.
Because many of these patients have thymic dysplasia, or abnormal growth, a possible treatment is surgically removing the thymus.
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