What Is Myasthenia Gravis?
Myasthenia gravis (MG) is a neuromuscular condition triggered by an autoimmune response. The disease occurs when the body’s immune system mistakenly attacks certain proteins, breaking down normal communication between nerve cells and muscles, which causes muscle weakness and fatigue.
The disorder most commonly affects women under the age of 40 and men older than 60, but it can occur at any age, including during childhood.
Types of MG
There are several types of MG, ranging from a localized form limited to eye muscles, called ocular MG, to a more severe and widespread form, called generalized MG, in which many muscles, including those that control breathing, are affected.
Depending on the time of onset, the disease may also be classified as transient neonatal MG, when it arises in infants of mothers with the disease, and juvenile MG, when it manifests before puberty.
Some children are born with a distinct type of MG, called congenital MG, a rare hereditary form caused by genetic mutations and not by an autoimmune response.
Causes of MG
In MG, the communication between nerve cells and muscles is interrupted at the neuromuscular junction — the place where nerve cell endings connect with the muscles they control. Normally, nerve cell endings release a neurotransmitter, or signaling molecule, called acetylcholine, which binds to acetylcholine receptors found on the surface of muscle cells, causing them to contract.
In most people with MG, antibodies, which normally help fight off infections and threats, mistakenly destroy, damage, or block acetylcholine receptors. With fewer receptors available, muscles cannot contract properly and become weaker.
However, in some patients the disease is caused by the immune system wrongly attacking other proteins, such as muscle-specific kinase, which is important for maintaining the function of the neuromuscular junction.
The specific cause of the abnormal autoimmune response in MG patients is unknown. However, researchers believe that the thymus gland, which is part of the immune system, may help trigger or sustain the production of the harmful antibodies that disrupt nerve-muscle communication.
Symptoms of MG
Muscle weakness in eye and eyelid muscles and double vision are usually among the first symptoms of MG. In some people, difficulty swallowing and talking are the first manifestations of the disease.
Although the disorder usually becomes apparent during adulthood, the onset of symptoms can occur at any age. However, it most commonly manifests in women when they are in their 20s or 30s and in men when they are in their 50s or 60s. Symptoms can also appear suddenly and are often not immediately associated with MG.
Diagnosis of MG
Several tests can be performed to identify and confirm the diagnosis of MG. These include blood tests to detect the presence of the harmful antibodies that cause the disease, specialized tests to assess muscle and nerve function, and imaging scans to evaluate the size and shape of the thymus gland.
Physicians may also use the ice pack and edrophonium tests to confirm the diagnosis of MG. In both tests, a temporary relief in eye and eyelid muscle weakness — following the placement of a bag filled with ice or an injection of edrophonium chloride — may indicate the presence of MG.
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