What is myasthenia gravis?
Myasthenia gravis is a chronic autoimmune neuromuscular disease characterized by varying degrees of weakness of the skeletal (voluntary) muscles of the body. The name myasthenia gravis, which is Latin and Greek in origin, literally means “grave muscle weakness.” With current therapies, however, most cases of myasthenia gravis are not as “grave” as the name implies. In fact, most individuals with myasthenia gravis have a normal life expectancy.
The hallmark of myasthenia gravis is muscle weakness that increases during periods of activity and improves after periods of rest. Certain muscles such as those that control eye and eyelid movement, facial expression, chewing, talking, and swallowing are often, but not always, involved in the disorder. The muscles that control breathing and neck and limb movements may also be affected.
What causes myasthenia gravis?
Myasthenia gravis is caused by a defect in the transmission of nerve impulses to muscles. It occurs when normal communication between the nerve and muscle is interrupted at the neuromuscular junction—the place where nerve cells connect with the muscles they control. Normally when impulses travel down the nerve, the nerve endings release a neurotransmitter substance called acetylcholine. Acetylcholine travels from the neuromuscular junction and binds to acetylcholine receptors which are activated and generate a muscle contraction.
In myasthenia gravis, antibodies block, alter, or destroy the receptors for acetylcholine at the neuromuscular junction, which prevents the muscle contraction from occurring. These antibodies are produced by the body’s own immune system. Myasthenia gravis is an autoimmune disease because the immune system—which normally protects the body from foreign organisms—mistakenly attacks itself.
What is the role of the thymus gland in myasthenia gravis?
The thymus gland, which lies in the chest area beneath the breastbone, plays an important role in the development of the immune system in early life. Its cells form a part of the body’s normal immune system. The gland is somewhat large in infants, grows gradually until puberty, and then gets smaller and is replaced by fat with age. In adults with myasthenia gravis, the thymus gland remains large and is abnormal. It contains certain clusters of immune cells indicative of lymphoid hyperplasia—a condition usually found only in the spleen and lymph nodes during an active immune response. Some individuals with myasthenia gravis develop thymomas (tumors of the thymus gland). Thymomas are generally benign, but they can become malignant.
The relationship between the thymus gland and myasthenia gravis is not yet fully understood. Scientists believe the thymus gland may give incorrect instructions to developing immune cells, ultimately resulting in autoimmunity and the production of the acetylcholine receptor antibodies, thereby setting the stage for the attack on neuromuscular transmission.
What are the symptoms of myasthenia gravis?
Although myasthenia gravis may affect any voluntary muscle, muscles that control eye and eyelid movement, facial expression, and swallowing are most frequently affected. The onset of the disorder may be sudden and symptoms often are not immediately recognized as myasthenia gravis.
In most cases, the first noticeable symptom is weakness of the eye muscles. In others, difficulty in swallowing and slurred speech may be the first signs. The degree of muscle weakness involved in myasthenia gravis varies greatly among individuals, ranging from a localized form limited to eye muscles (ocular myasthenia), to a severe or generalized form in which many muscles—sometimes including those that control breathing—are affected. Symptoms, which vary in type and severity, may include a drooping of one or both eyelids (ptosis), blurred or double vision (diplopia) due to weakness of the muscles that control eye movements, unstable or waddling gait, a change in facial expression, difficulty in swallowing, shortness of breath, impaired speech (dysarthria), and weakness in the arms, hands, fingers, legs, and neck.
Who gets myasthenia gravis?
Myasthenia gravis occurs in all ethnic groups and both genders. It most commonly affects young adult women (under 40) and older men (over 60), but it can occur at any age.
In neonatal myasthenia, the fetus may acquire immune proteins (antibodies) from a mother affected with myasthenia gravis. Generally, cases of neonatal myasthenia gravis are temporary and the child’s symptoms usually disappear within 2-3 months after birth. Other children develop myasthenia gravis indistinguishable from adults. Myasthenia gravis in juveniles is uncommon.
Myasthenia gravis is not directly inherited nor is it contagious. Occasionally, the disease may occur in more than one member of the same family.
Rarely, children may show signs of congenital myasthenia or congenital myasthenic syndrome. These are not autoimmune disorders, but are caused by defective genes that produce abnormal proteins instead of those which normally would produce acetylcholine, acetylcholinesterase (the enzyme that breaks down acetylcholine), or the acetylcholine receptor and other proteins present along the muscle membrane.