Diagnosing Myasthenia Gravis
Myasthenia gravis is a chronic condition that impairs the connection between the nerves and skeletal muscles, causing progressive muscle weakness. It is an autoimmune disease, which means that the immune system, which normally protects you from foreign organisms such as bacteria or viruses, mistakenly attacks another part of the body.
In myasthenia gravis, the immune system interferes with signals sent by the peripheral nerves—those outside of the brain and spinal cord—to muscles throughout the body. People with this condition have muscle weakness that increases during activity and improves after rest. Myasthenia gravis can occur at any age but often affects women younger than age 40 and men older than age 60.
The condition occurs when communication between the nerves and muscles is disrupted at an area called the neuromuscular junction—the space where nerves connect to the muscles they control. Normally, when impulses travel down a nerve from the brain, the nerve endings release a chemical called acetylcholine. Acetylcholine travels from the neuromuscular junction and binds to acetylcholine receptors in the muscles, generating a muscle contraction.
In myasthenia gravis, antibodies prevent muscle contraction. Antibodies are proteins produced by the immune system to attack foreign matter such as viruses. In this condition, antibodies attack the neuromuscular junction, causing inflammation at the neuromuscular junction, and destroy the acetylcholine receptor sites, preventing muscle contraction and causing weakness.
Muscles most often affected by myasthenia gravis include those that control eye and eyelid movement, facial expression, chewing, talking, and swallowing. The muscles that control breathing and neck and limb movement may also weaken. Symptoms vary depending on the muscle groups affected. Myasthenia gravis of the eye and facial muscles, for example, can cause drooping of one or both eyelids, blurred or double vision, a change in facial expression, difficulty swallowing, or slurred speech. The muscle weakness can fluctuate, so that someone with myasthenia gravis may feel stronger in the morning and worse in the evening.
To determine if you have myasthenia gravis, NYU Langone specialists take a thorough medical history and perform both a physical and neurological exam, looking for impairment of eye movements and other muscle weakness. If your doctor suspects myasthenia gravis, several tests are available to confirm the diagnosis, including the following.
A blood test to look for the presence of muscle-weakening antibodies can help confirm a diagnosis. Most people with myasthenia gravis have elevated levels of antibodies that attack acetylcholine receptor sites in the neuromuscular junction and prevent muscles from contracting.
A CT scan is a type of X-ray that creates very detailed images of your tissues and internal organs. CT scans can identify or rule out an enlarged thymus gland or tumors of the thymus gland, called thymoma.
The thymus gland is located beneath the breastbone in the chest and is responsible for immune system development. As a person ages, the thymus gets smaller and is replaced by fat, but in people with myasthenia gravis, the thymus enlarges. The relationship between the thymus and myasthenia gravis is unclear, but doctors believe the thymus may give incorrect instructions to developing immune cells, leading to the autoimmune attack that triggers the condition.
NYU Langone doctors may perform electromyography (EMG) testing of the muscle and nerve connection to help confirm a diagnosis of myasthenia gravis, even when blood test results are negative. An EMG test shows how the nerves and muscles work together by measuring the electrical impulses along nerves, nerve roots, and muscular tissue.
To perform this test, the doctor inserts a tiny needle—a conductor for electrical current called an electrode—through the skin and into the muscle. This allows your doctor to measure the amount of electricity generated by muscle cells when they become activated by a nerve impulse. In people with myasthenia gravis, the muscle fibers do not respond as well to repeated electrical stimulation as normally functioning muscles do.
MRI scans use a magnetic field and sound waves to create two- or three-dimensional pictures of the inside of the body. A doctor may order an MRI scan of the head to better view the brain and area around the eyes. This provides a clear picture of other potential causes of symptoms, such as a mass compressing cranial nerves or a lesion on the brain stem.
Whether you’re seeking a diagnosis of myasthenia gravis or have been living with the condition for many years, NYU Langone doctors can discuss treatment options, including medication, plasma exchange (also called plasmapheresis), and immunotherapy. You and your physician determine the best treatment for you based on your symptoms, the severity of your muscle weakness, the muscles affected, your age, and other medical conditions you might have.