Myasthenia gravis (MG) is a persistent, long term autoimmune disease that involves the voluntary body muscles, particularly those used for movement of the eyes, mouth, throat, arms and legs. An autoimmune disease arises when the body's immune system malfunctions and attacks healthy tissues. MG is a rare disorder that can occur at any age and affects about 20 people in 100,000 population in the United States. Autoimmune juvenile myasthenia gravis represents 10 to 15 percent of the diagnosed cases in North America.
There are two clinical types of MG, generalized and ocular.
- Often, in ocular MG, the muscle weakness first occurs in the eyelids muscle and other extraocular muscles, i.e., muscles that control the eye's movement.
- In generalized MG, the muscle weakness affects the ocular muscles and a variable mix of the respiratory and limb muscles.
Causes & Risk Factors
MG is the most common primary neuromuscular transmission condition, but its exact cause is not well understood. Researchers suspect that disruption or dysfunction in the communication between nerve cells and muscles causes MG. The impairment prevents essential muscle contractions leading to muscle weakness. Nerves communicate with the muscles through neurotransmitters. These are chemicals that fit exactly into receptors on the muscle cells called acetylcholine, situated at the muscular-nerve junction.
Researchers believe that antibodies released by the immune system in response to bacteria or viruses mistakenly destroy or block many of the acetylcholine. The damage causes weakness because it reduces the flow of nerve signals to the muscles. Antibodies can also inhibit a muscle-specific receptor tyrosine kinase activity. It’s a protein involved in forming the muscular-nerve junction and its blockage leads to MG. Antibodies are proteins that the immune system normally deploys to attack harmful, foreign substances in the body.
- Congenital myasthenic syndrome is a rare hereditary form of the disease
- Sporadically, neonatal myasthenia gravis can affect children of mothers with MG
- The thymus gland occasionally plays a role in abnormal autoimmune response leading to the development of MG
- Antibody-negative MG can occur due to antibodies activity against a protein known as lipoprotein-related protein 4
Signs & Symptoms
Usually, muscle weakness due to MG worsens with the muscle's use, and the symptoms improve with rest. Thus, muscle weakness can come and go, but the symptoms usually progress over time and peak within several years after the disease onset. In over half of MG patients, the initial signs and symptoms involve eye disorders like drooping of one or both eyelids (ptosis) and double vision (diplopia). The diplopia may be vertical or horizontal and improves or clears up when one eye is closed.
Other signs and symptoms include:
- Hoarse voice
- Trouble talking
- Weak arms, legs, or neck
- Problems chewing and swallowing
- Problems walking upstairs or lifting objects
- Difficulty making facial expressions (Facial paralysis)
- Shortness of breath and occasionally serious breathing difficulties
Typically, the degree of muscle weakness varies from day to day, with the severity of the symptoms increasing over time if left untreated.
Factors that can aggravate the symptoms include:
- Menstrual periods
- Some medications, e.g., certain anesthetics, some antibiotics, quinine, etc.
The doctor will take a detailed medical history and symptoms of the patient and conduct a complete physical exam and various specialized tests. Common diagnostic studies include the Tensilon (edrophonium) test and neostigmine test, where the doctor observes the patient's response to an intravenously administered drug, either edrophonium or neostigmine.
Other tests may include:
- Genetic tests
- Nerve conduction studies
- Blood testing for antibodies related to MG
- Electromyogram (EMG) to measure the electrical activity of a muscle
- Imaging studies, such as computed tomography (CT) scanning or magnetic resonance imaging (MRI) to rule out a tumor
MG is incurable, and treatment is supportive to address the symptoms and control the immune system's activity. The treatment approach depends on the specifics of each patient.
The doctor may prescribe steroids, anticholinesterase medicines, or immunosuppressive drugs, i.e., medicines that suppress the immune system's response. Intravenous immune globulin (IVIG), a donor blood product that affects the creation and function of antibodies, can also be used to treat autoimmune MG. Medical therapy helps lower the abnormal immune response seen in MG.
The doctor may recommend patching therapy to treat diplopia.
Thymus gland removal (thymectomy) reduces symptoms in more than 70 percent of patients without thymus cancer. The procedure is thought to work by altering the immune system response leading to improved muscle function.
Plasma exchange (plasmapheresis) is a short-term treatment that helps extreme MG weakness or before surgical treatment. It's a procedure that removes irregular antibodies from the blood and replaces them with healthy antibodies from donated blood, leading to improved muscle strength.
Adequate rest can help to minimize muscle weakness. Patients should also avoid stress and heat exposure because both can aggravate symptoms.
Prognosis & Long-term outlook
MG is a lifelong medical disorder, and the patients are at an increased risk of developing other autoimmune diseases like lupus and rheumatoid arthritis. Early detection and proper treatment are crucial to managing the condition. It can limit disease progression in many patients.
The long-term outlook for MG depends on many factors. Some patients may only experience mild symptoms, while others eventually become wheelchair users.
Prevention & Follow Up
MG is not preventable. Follow up is essential for monitoring the disease status and minimizing the severity.