What Is Myasthenia Gravis?
Myasthenia gravis is an
autoimmune disease that affects the transmission of signals from nerves to muscles. The name myasthenia gravis comes from Greek and Latin words meaning "grave muscle weakness." However, most cases of this condition are not as "grave" as the name implies, and most people who have it can expect to live normal or nearly normal lives. The hallmark of myasthenia gravis is muscle weakness that increases during activity and improves after rest. In most cases, the condition affects the muscles that control eye and eyelid movement. This is called ocular myasthenia gravis.
Myasthenia gravis may also affect the muscles that control:
- Facial expressions
- Chewing
- Talking
- Swallowing
- Breathing
- Neck and limb movements.
When it affects more than just the eye muscles, it is called generalized myasthenia gravis.
Understanding Nerves and Muscles
Normally when impulses travel down the nerves, the nerve endings release a neurotransmitter substance called acetylcholine. Acetylcholine travels through the neuromuscular junction and binds to acetylcholine receptors, which are activated and generate a muscle
contraction.
Causes of Myasthenia Gravis
In myasthenia gravis, antibodies block, alter, or destroy the receptors for acetylcholine at the neuromuscular junction. This prevents the muscle
contraction from occurring. Myasthenia gravis is considered an
autoimmune disease, in which the body's immune system turns against itself and attacks its own tissues. Research scientists are still trying to understand why the immune system attacks the acetylcholine receptors.
It is estimated that the number of people who are affected by myasthenia gravis is between 5 to 14 people out of 100,000. The condition occurs in men and women of all ethnic groups, and although myasthenia gravis most commonly affects women under 40 and men over 60, it can occur at any age.
The condition is not directly inherited, nor is it contagious. However, if a woman with myasthenia gravis becomes pregnant, the baby may acquire antibodies from the mother and have
myasthenia gravis symptoms for a few weeks or months after birth. This is called neonatal myasthenia. In rare cases, myasthenia gravis is caused by a defective gene and appears in infants born to mothers who do not have myasthenia gravis. This type of myasthenia gravis is called congenital myasthenia.
Myasthenia Gravis and the Thymus Gland
The thymus gland, which is found in the upper chest area beneath the breastbone, is a part of the body's normal immune system. The thymus gland is abnormal in most adults with myasthenia gravis. Some people with myasthenia gravis develop thymomas, which are tumors of the thymus gland. Generally, thymomas are benign, but they can become malignant (cancerous). The relationship between the thymus gland and myasthenia gravis is not yet fully understood.
The hallmark of myasthenia gravis is muscle weakness that increases during periods of activity and improves after periods of rest. Although symptoms may affect any voluntary muscle, they usually affect the muscles that control eye and eyelid movement, facial expression, and swallowing. Symptoms of myasthenia gravis may appear suddenly and may not be immediately recognizable as symptoms of myasthenia gravis.
Diagnosing Myasthenia Gravis
Weakness is a common symptom of many disorders, which is why myasthenia gravis is often misdiagnosed in people who experience mild weakness or in people whose weakness is restricted to only a few muscles. Therefore, a delay in diagnosis of one or two years is not unusual in cases of myasthenia gravis.
In order to make a diagnosis, doctors will need to:
- Review the patient's medical history
- Review the patient's physical and neurological examinations
- Look for symptoms such as impairment of eye movements or muscle weakness.
Tests that are used to confirm the diagnosis of myasthenia gravis include the following:
- Blood test
- Edrophonium test
- Nerve conduction study
- Single fiber electromyography (EMG)
- Computed tomography (CT) scan
- Pulmonary function tests (PFT).
Current treatment for myasthenia gravis may include:
- Medications
- Thymectomy
- Plasmapheresis or plasma exchange
- High-dose intravenous immune globulin.
Medications
Medications used for myasthenia gravis treatment are meant to:
- Improve neuromuscular transmission
- Increase muscle strength
- Suppress the production of abnormal antibodies.
It is important to note that medications must be used with careful medical follow-up because they may cause major side effects.
Thymectomy
A thymectomy is the surgical removal of the thymus gland, which is usually abnormal in patients with myasthenia gravis. A thymectomy may:
- Improve symptoms
- Cure myasthenia gravis
- Re-balance the immune system.
Plasmapheresis or Plasma Exchange
A plasmapheresis is a procedure in which blood is separated into cells and plasma (liquid). The plasma is removed and replaced with fresh frozen plasma, a blood product called albumin, and/or a plasma substitute. The procedure is often referred to as plasma exchange.
High-Dose Intravenous Immune Globulin
In some
autoimmune diseases such as myasthenia gravis, one particular type of antibody is produced in large numbers and is attacking the patient's own healthy tissue. In these cases, high-dose immune globulin can be given to suppress the immune system.
Prognosis for Myasthenia Gravis
With treatment, the outlook for most patients with myasthenia gravis is good, and patients can expect to lead normal or nearly normal lives. However, in rare cases, the severe weakness of myasthenia gravis may cause respiratory failure, which will require immediate emergency medical care.
