Myasthenia gravis is an autoimmune disease that affects the contact point between nerves and muscles. For some unknown reason, the body's immune system, which normally helps fight infections, attacks the acetylcholine receptors found on muscles.
These receptors normally receive a chemical called acetylcholine, which is released by the nerves at the neuromuscular junction (the point of contact between nerve and muscle) and signals the muscles to contract. Once these receptors are damaged or processes involved with the receptors are altered, the muscles cannot respond to the nerve signals and the muscles become weak.
Myasthenia gravis is almost twice as common in women as in men. It is most commonly diagnosed in women under 40 and men over 50 but it can occur at any age. Children are very unlikely to suffer from this disease. People from all continents are equally susceptible.
A closely related disease called congenital myasthenic syndrome is genetically inherited. Myasthenia gravis itself probably has a genetic component, but the children of people with this condition are only slightly more likely than average to suffer from autoimmune disease.
Myasthenia gravis is a serious condition, but not usually life-threatening. Most people with myasthenia gravis have a life expectancy that is the same as for someone without it. People affected with myasthenia gravis have reduced physical activities, reduced quality of life, and spend more days being sick. In severe cases, the chest muscles can become so weak that patients have trouble breathing on their own and may require a ventilator for a few days to a few weeks.
The exact cause of myasthenia gravis is not known. One theory is that the condition may be triggered by a virus or other infection that has a similar structure to a part of the acetylcholine receptor (found in the neuromuscular junction and needed for normal function). The antibodies that the immune system produces to fight the virus then also mistakenly attack the receptors.
What we do know is that certain antibodies have been found in the blood of people with myasthenia gravis that are built to attack acetylcholine receptors.
Sometimes women with myasthenia gravis who give birth transmit these particular antibodies to their babies, causing muscle weakness in the infant. This is called neonatal myasthenia and occurs in about 12% of babies whose mother has myasthenia gravis. It is different from congenital myasthenia because the infant gets better in a few weeks as the antibody blood levels go down.
Some people with myasthenia gravis do not have antibodies to acetylcholine receptors. Many of these people, though, have antibodies to an enzyme involved in acetylcholine receptor function.
The other abnormality that often shows up in people with myasthenia gravis is an overactive, overlarge, or otherwise malfunctioning thymus. The thymus is a gland located where the neck joins the chest. It is important for the normal development of the immune system. Normally, the thymus is larger in childhood than in adulthood and becomes inactive around puberty. In people with myasthenia gravis, however, it often keeps going into adulthood.
Some people with myasthenia gravis turn out to have a thymoma (a tumour in the thymus). This is a type of cancer, but very few people die of thymomas (see "Treatment and Prevention").
Along with infection, symptoms may be triggered by surgery or medications.
Symptoms and Complications
The most common symptoms of myasthenia gravis are double vision (diplopia), drooping eyelids (ptosis), and muscle weakness that usually gets worse after exercise or at the end of the day and improves with rest.
The muscles around the eyes are particularly likely to be affected by myasthenia gravis, and eye problems are the first sign of the disease in about 40% of cases. Eventually, 85% of people with myasthenia gravis will experience eye symptoms. About 15% of people experience symptoms only around the eyes and nowhere else – this is called ocular myasthenia gravis. Those who do get symptoms elsewhere will usually develop them within the first 3 years.
Other common symptoms of this disease include:
- blurred vision
- difficulty chewing or swallowing
- inability to stand up without using hands
- unstable or unusual gait
- weakness in the arms, legs, hands, and fingers
- weakness in the muscles of facial expression
Many people find that their symptoms rise and fall in intensity many times in the course of a single day. Often, they follow a similar pattern each day, with muscle fatigue at its worst in the evening.
People who find it temporarily impossible to eat may have to be admitted to hospital and be fed intravenously. Of more immediate danger is when the disease causes difficulty in breathing. This is called a myasthenic crisis, and it's responsible for most of the small number of deaths due to myasthenia gravis. People with breathing problems must go to a hospital immediately.
Making the Diagnosis
Your doctor will ask you questions about your symptoms and perform a physical exam. To help diagnose myasthenia gravis, your doctor may order blood tests to detect antibodies that may be affecting the function of acetylcholine receptors.
Your doctor may use the ice pack test as part of the physical exam if your eyelids are noticeably drooping. An ice pack is placed on the closed lid for 2 minutes, and if the drooping improves after the test, you're more likely be diagnosed with myasthenia gravis.
Another important diagnostic test is called electromyography (EMG) and single-fibre EMG. These tests are used to determine whether the nerve-to-muscle contact is working properly.
A chest X-ray or CAT scan of the chest may be done to look for a thymoma. Breathing tests called pulmonary function tests may also be done.
Treatment and Prevention
Myasthenia gravis can be managed well with treatment. A treatment plan may involve one or more of the following:
Medications: Anticholinesterase-inhibiting medications like pyridostigmine and neostigmine control symptoms. Corticosteroids (e.g., prednisone) and immunosuppressants (e.g., azathioprine, cyclosporine, tacrolimus, mycophenolate mofetil, cyclophosphamide) can slow or stop the disease process. Although these medications don't cure it, they are the mainstay of long-term management.
Plasmapheresis: Large amounts of blood are removed, then put in a centrifuge to separate the red and white cells from the serum, the liquid that contains the immune antibodies which are doing the damage. The red and white cells are then replaced, along with donated or synthetic serum.
Immunoglobulin: The fact that this treatment works suggests that a weakened immune system is part of the cause of myasthenia gravis. Immunoglobulin therapy involves injections of blood serum from a person without autoimmune disease. It lowers the numbers of tissue-attacking antibodies in the bloodstream of the person with myasthenia gravis.
Monoclonal Antibodies: Monoclonal antibodies are a treatment option for people who have a disease that has not responded to other therapies. The medication eculizumab is a manufactured antibody which helps suppress specific parts of the immune system.
Thymectomy: The thymus isn't essential in adults and it can be surgically removed without negative effects. Symptoms will improve in about 70% of people who have a thymectomy, and some of these people may be cured. Many others can reduce the dose of their medication(s). In fact, the cure rate in myasthenia gravis patients with thymomas is about the same as in people with an apparently healthy thymus or a malfunctioning but non-cancerous thymus.
Some people diagnosed with myasthenia gravis are diagnosed with thymoma at the same time. Fortunately, thymomas are slow-growing tumours, and while they're not actually benign (noncancerous), they have a low chance of spreading in any given year. Most thymomas are found while they're still localized in the thymus, and removing that organ usually provides a complete and lasting cure for both the myasthenia and the cancer. In fact, myasthenia gravis is often a lifesaver, because it alerts doctors to a tumour while there's still plenty of time to do something about it.
Nothing can prevent this disease, since we don't know what causes it. However, serious complications can be prevented if people with myasthenia gravis go to the hospital if they ever have difficulty breathing. Up to 20% of people with this disease will face a myasthenic crisis at some point.
It is also recommended to get a seasonal influenza vaccination for all people receiving immunosuppressive therapy, and for those with neurologic conditions such as myasthenia gravis. Studies have shown that people who received the influenza vaccine had a lower incidence of symptom aggravation compared to those who had an influenza-like illness.