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Myasthenia gravis (MG) develops in about 14 to 20 people per 100,000. In people above age 50 years, MG is more common in men, but below age 40, females are more frequently affected, often with the disease starting in the teens through 20s. There is no effect on fertility, so MG can develop during pregnancy.
What is MG? It is a disorder that affects the neuromuscular junction, the interface between nerves and muscles where nerve impulses are transformed into muscle movement. Neuromuscular junctions contain different types of receptors. These are proteins that respond to a special chemical called “acetylcholine” that is released from nerve cells to control the corresponding muscle cells. There are different types of acetylcholine receptors. MG results from attack of the immune system on a type of acetylcholine receptor called the nicotinic receptor. The result of the problem with this type of receptor is muscle weakness that gets progressively worse, but that usually is most severe in small muscles, such as those that control movements of the eye. Usually, the symptoms are first visible in these small muscles, meaning that there are visual effects, such as double vision. After a while, the disease can affect any muscle, including those that affect the mouth and throat and also the muscles of breathing.
As for diagnosing MG, your history and physical examination provide the doctor with the most important clues about whether you may suffer from MG. The history can reveal double vision and drooping eyelids, trouble chewing and swallowing, and limb weakness developing over many months or years. Typically, weakness is worse toward the end of the day as you get tired and improves on awakening in morning, or after rest. A person with MG may also report drooling and choking. Speech may be slurred and have a nasal tone. Eye movement problems and weakness of facial muscles are common first symptoms, because this involves small muscles. Women who have weakness in neck muscles and muscles of the shoulder girdle may have breathing trouble.
Further testing includes blood tests for antibodies against acetylcholine receptors and antibodies against an enzyme called tyrosine kinase. Your doctor, or usually a neurologist, will test muscle function with a technique called electromyography (EMG). Your chest will be imaged with computed tomography (CT), or, during pregnancy, more likely magnetic resonance imaging (MRI) to look for a type of tumor called a thymoma. This tumor is the cause of about 15 percent of MG cases. Thyroid also will be tested and you will be given what is called the edrophonium test. In this test, you are injected with edrophonium, a drug that slows the breakdown of acetylcholine in the neuromuscular junction. If symptoms then improve, this suggests MG and this can lead to a diagnosis.
MG symptoms worsen during pregnancy in about 40 percent of cases, but do not change in about 30 percent of such women. In close to 30 percent of pregnant women with MG, the condition improves. Even with MG symptoms, women with MG can experience a normal pregnancy. Many can have vaginal delivery at term and without induction, as long as the MG is under control with medications. MG is usually treated effectively with cholinesterase inhibitors (ChE inhibitors), the most common being pyridostigmine and neostigmine, which are not harmful to the fetus. Other drugs include agents that inhibit of the immune system, such as corticosteroids, which are considered relatively safe during pregnancy. However, certain other types of immune-inhibitors may cause birth defects, such as azathioprine (AZA) and rituximab. Also, a drug called mycophenolate mofetil (MFM), given for mild cases of MG, is thought to carry a high risk of birth defects, so it should be avoided. On the other hand, an immune inhibitor, called cyclosporine is thought to be fairly safe during pregnancy.
About 10-20 percent of newborns of MG mothers develop a temporary myasthenic weakness, generally about 2-4 days after birth. This can interfere with sucking, weak crying, cause some eyelid drooping, and sometimes cause breathing problems. To avoid complications, you should give birth at a medical center that has advanced neonatal support facilities.
When it comes to breastfeeding, the same MG drugs that are safe for the fetus during pregnancy are safe for the nursing infant. Thus, ChE inhibitors and corticosteroids are safe, except that with corticosteroids some doctors advise discarding breast milk that has accumulated in the hours following steroid administration and then building up a new batch of milk to use for the actual nursing. You may prefer to use infant formula instead.
