Muscle weakness caused by myasthenia gravis worsens as the affected muscle is used repeatedly. Because symptoms usually improve with rest, your muscle weakness may come and go. However, myasthenia gravis symptoms tend to progress over time, usually reaching their worst within a few years after the onset of the disease.
Although myasthenia gravis can affect any of the muscles that you control voluntarily, certain muscle groups are more commonly affected than others.
In more than half the people who develop myasthenia gravis, their first signs and symptoms involve eye problems, such as:
- Drooping of one or both eyelids (ptosis).
- Double vision (diplopia), which may be horizontal or vertical, and improves or resolves when one eye is closed.
Face and throat muscles
In about 15 percent of people with myasthenia gravis, the first symptoms involve face and throat muscles, which can cause:
- Altered speaking. Your speech may sound very soft or nasal, depending upon which muscles have been affected.
- Difficulty swallowing. You may choke very easily, which makes it difficult to eat, drink or take pills. In some cases, liquids you're trying to swallow may come out your nose.
- Problems chewing. The muscles used for chewing may wear out halfway through a meal, particularly if you've been eating something hard to chew, such as steak.
- Limited facial expressions. Your family members may comment that you've "lost your smile" if the muscles that control your facial expressions have been affected.
Neck and limb muscles
Myasthenia gravis can cause weakness in your neck, arms and legs, but this usually happens along with muscle weakness in other parts of your body, such as your eyes, face or throat.
The disorder usually affects arms more often than legs. However, if it affects your legs, you may waddle when you walk. If your neck is weak, it may be hard to hold up your head.
When to see a doctor
Talk to your doctor if you have difficulty:
- Using your arms or hands
- Holding up your head
Your nerves communicate with your muscles by releasing chemicals (neurotransmitters) that fit precisely into receptor sites on the muscle cells at the nerve-muscular junction.
In myasthenia gravis, your immune system produces antibodies that block or destroy many of your muscles' receptor sites for a neurotransmitter called acetylcholine (as-uh-teel-KOH-leen). With fewer receptor sites available, your muscles receive fewer nerve signals, resulting in weakness.
Antibodies may also block the function of a protein called a muscle-specific receptor tyrosine kinase (TIE-roh-seen KIE-nays). This protein is involved in forming the nerve-muscular junction. When antibodies block the function of this protein, it may lead to myasthenia gravis. Research continues to study how the antibodies inhibiting this protein are related to the development of myasthenia gravis.
Researchers believe that the thymus gland, a part of your immune system situated in the upper chest beneath your breastbone, may trigger or maintain the production of the antibodies that block acetylcholine.
Large in infancy, the thymus is small in healthy adults. In some adults with myasthenia gravis, however, the thymus is abnormally large. Some people with myasthenia gravis also have tumors of the thymus (thymomas). Usually, thymomas aren't cancerous (malignant).
Some people may have myasthenia gravis that isn't caused by antibodies blocking acetylcholine or the muscle-specific receptor tyrosine kinase. This type of myasthenia gravis is called antibody-negative myasthenia gravis. Antibodies against another protein, called lipoprotein-related protein 4, may play a part in the development of this condition.
Genetic factors also may be associated with myasthenia gravis.
Rarely, mothers with myasthenia gravis have children who are born with myasthenia gravis (neonatal myasthenia gravis). If treated promptly, children generally recover within two months after birth.
Some children are born with a rare, hereditary form of myasthenia, called congenital myasthenic syndrome.
Factors that can worsen myasthenia gravis
- Extreme heat
- Some medications — such as beta blockers, quinidine gluconate, quinidine sulfate, quinine (Qualaquin), phenytoin (Dilantin), certain anesthetics and some antibiotics
Complications of myasthenia gravis are treatable, but some can be life-threatening.
Myasthenic crisis is a life-threatening condition that occurs when the muscles that control breathing become too weak to do their jobs. Emergency treatment is needed to provide mechanical assistance with breathing. Medications and blood-filtering therapies help people to again breathe on their own.
About 15 percent of people with myasthenia gravis have a tumor in their thymus, a gland under the breastbone that is involved with the immune system. Most of these tumors, called thymomas, aren't cancerous (malignant).
People with myasthenia gravis are more likely to have the following conditions:
- Underactive or overactive thyroid. The thyroid gland, which is in the neck, secretes hormones that regulate your metabolism. If your thyroid is underactive, you may have difficulties dealing with cold, weight gain and other issues. An overactive thyroid can cause difficulties dealing with heat, weight loss and other issues.
- Autoimmune conditions. People with myasthenia gravis may be more likely to have autoimmune conditions, such as rheumatoid arthritis or lupus.