Myasthenia gravis (or MG) is a chronic autoimmune disease that negatively impacts skeletal muscle strength by obstructing the communication between muscles and nerves. Typically, MG is first noticed when it brings about weakness in eye muscles and certain symptoms such as double vision and/or drooping eyelid. “Ocular MG” is the collective term for this condition. From the eye muscles, it could spread over time to the neck and facial muscles, causing difficulty chewing and swallowing, slurred speech, and/or difficulty breathing, and weakness from the head and neck to other body parts, which results in generalized MG. Over time, muscle weakness will vary, and it tends to deteriorate with sustained effort but improves with rest.
All body movements, even those as small as keeping the eyes open and head upright, are usually performed by a coordinated series of muscle contractions. Typically, the initiation of these muscle contractions is done by chemical nerve signals. A nerve impulse travels to a specific nerve ending (on a microscopic level), and a neurotransmitter called acetylcholine is discharged. The chemical travels from that nerve ending to a muscle through a tiny gap found at the “neuromuscular junction.” Then, it adheres to one of many acetylcholine receptors located on the muscle fiber. So, this binding triggers the receptor and causes muscle contraction.
With myasthenia gravis, the body’s immune system releases proteins (autoantibodies) that target an individual’s acetylcholine receptors and eradicate or block them (acetylcholine receptor antibodies). In turn, this causes the receipt of acetylcholine signals and inhibits weakness and quick muscle fatigue.
Generally speaking, in the United States, MG is approximated to affect around 5-20 people out of 100,000. Although anyone can develop MG, it is most commonly diagnosed in men aged 60 years and older and also in adult women younger than 40 years old. A woman with MG might pass acetylcholine receptor antibodies to her fetus during pregnancy. This could cause a newborn to experience symptoms of MG; however, the symptoms usually disappear within 2-3 months of birth.
What Are the Causes of MG?
It is not known what the cause of MG is; however, approximately 75% of those affected consist of an unusually big thymus gland, and certain people develop thymomas (basically benign tumors of the thymus). Found behind the upper breastbone, the thymus is a relatively small gland. Through young adulthood, it plays a significant role in the immune system but usually starts to shrink after puberty. It is not as active in adults. It’s not fully understood what the relationship between the thymus and MG is. Still, some think that the thymus has an indirect role in activating the production of acetylcholine receptor antibodies.
A fraction of those with MG also have an affected family member, but a majority of them do not. The condition known as congenital myasthenic syndrome has symptoms similar to MG, but it is inherited and isn’t an autoimmune disorder. Lambert-Eaton Myasthenic Syndrome is another condition that causes similar symptoms, but it entails an interference with the production of acetylcholine by the nerve ending and not the acetylcholine receptor activity.
What Are the Symptoms and Signs of MG?
An individual with myasthenia gravis (MG) might exhibit a wide range of symptoms. These might vary daily, deteriorate after activity and much later in the day; they may improve with rest. MG symptoms might stay confined to the muscles surrounding the eyes – this condition is called ocular MG. In most people, however, the symptoms will extend to the neck, face, and other body parts within about 12 months (generalized MG). In most cases, MG symptoms deteriorate for a few years then stabilize. Some people may have remission over prolonged periods.
Some of the symptoms might include:
- Specific muscle weakness but usual feelings/sensations
- Difficulty walking and an altered gait
- Trouble holding up the head
- Weak neck muscles
- Slurred speech
- Difficulty chewing, swallowing, with choking, gagging and drooling
- Decreased eye movement control
- Double vision
- Drooping eyelid
If MG negatively impacts the muscles that control breathing, it could result in a medical emergency known as a myasthenic crisis that usually necessitates hospitalization and might also dictate the temporary use of a respirator to help in ventilation. Illness, stress, heat, and certain medications can result in degrading symptoms.
Laboratory Tests for MG
The primary objectives of testing are to diagnose Myasthenia Gravis, differentiate it from other conditions that have similar symptoms, as well as to guide treatment. Also, some tests might be carried out to track an individual’s health status over time.
In general, laboratory testing might involve measuring one or more autoantibodies:
- Anti-MuSK (or muscle-specific kinase) antibodies are found in approximately 50-70% of those who have a negative status for AChR antibodies and also have generalized MG.
- AChR (or acetylcholine receptor) antibodies are found in up to 90% of those people with generalized MG and approximately 50% of those with ocular MG. AChR is MG’s primary blood test. There are three kinds of AChR antibodies: blocking, binding (most frequently tested), and modulating.
- Anti-striated muscle antibodies are found in approximately 80% of those people with MG who also have an expanded thymus gland. The presence of these antibodies shows a significantly higher chance of the individual having a thymoma (usually benign tumor in the thymus).
Some other tests that might sometimes be performed are:
- CCP (or cyclic citrullinated peptide) antibody testing might be used to eliminate rheumatoid arthritis.
- RF (or rheumatoid factor) could be used to rule out or detect rheumatoid arthritis.
- Thyroid panels and thyroid antibodies are used to exclude Graves disease, which is an autoimmune condition related to hyperthyroidism.
- ANA (or antinuclear antibody) might be used to exclude other autoimmune disorders like lupus.
Non-laboratory tests for MG
Especially when laboratory test results are inconclusive, one or more of the following evaluations may be performed to help diagnosis.
- Acetylcholinesterase test using edrophonium (or tensilon test) involves the intravenous introduction of a drug to determine whether muscle strength improves for a few minutes.
- Single-fiber electromyography and/or repetitive nerve stimulation is performed to assess neuromuscular response is a particular area of the body.
- Commuted tomography (or CT) of the chest area is used to identify thymoma or an enlarged thymus gland.
- An eye orbit or brain MRI (or magnetic resonance imaging) is not performed routinely but could assist in eliminating other causes of eye-related symptoms.
Treatment for Myasthenia Gravis
There is no known way to prevent MG or to cure it. However, MG symptoms may be managed so that most people who are afflicted can lead fairly normal lives. That being said, if a person’s MG develops a myasthenic crisis affecting their ability to breathe, it must be treated as a medical emergency and often leads to admission to a hospital.
Managing Myasthenia Gravis
AchE (or acetylcholine esterase) inhibitors / cholinesterase inhibitors are drugs that will improve nerve and muscle communication and improve muscle strength by increasing the amount of acetylcholine at the neuromuscular junction. These inhibitors constitute the primary treatment for managing MG.
A thymectomy (thymus gland removal) may be performed when the patient has a thymoma. This procedure may reduce a person’s symptoms and, in some instances, resolve them.
Suppressing a person’s immune response and/or removing or blocking antibodies in the blood are other types of treatments to manage MG.
Newborns that get MG symptoms from their mother need to be monitored closely for a couple of weeks until the influence of the mother’s antibodies goes away.
Personal treatment methods and requirements will change over time.
People with MG need to work with the healthcare provider to stay on top of the right treatments for their changing health status.