Causes And Symptoms Of Myasthenia Gravis

What is this condition?

Myasthenia gravis produces sporadic bur progressive weakness and abnormal fatigue of skeletal muscles, which is exacerbated by exercise and repeated movement but improved by anticholinesterase drugs. Usually, this disorder affects muscles controlled by the cranial nerves (face, lips, tongue, neck, and throat), but it can affect any muscle group.

Myasthenia gravis follows an unpredictable course of recurring flare-ups and periodic remissions. There's no known cure. However, drug treatment has improved the prognosis and allows people to lead relatively normal lives except during flare-ups. When the disease involves the respiratory system, it may be life-threatening.
What causes it?

Myasthenia gravis causes a failure in transmission of nerve impulses at the neuromuscular junction. Theoretically, such impairment may result from an autoimmune response, or dysfunctional neurotransmitters activity.

Myasthenia gravis strikes 1 in 25,000 people of all age-groups, but incidence peaks between ages 20 and 40. It is three times more common in women than in men, but after age 40, it appears equally in both sexes. About 20% of infants born to myasthenia mothers have transient (or occasionally persistent) myasthenia. This disease may coexist with immune system and thyroid disorders; about 15% of people with myasthenia gravis have thymoma (a tumor formed by thymus gland tissue). Remissions occur in about 25% of people with this disease.
What are its symptoms?

The dominant symptoms of myasthenia gravis are skeletal muscle weakness and fatigability. In the early stages, easy fatigability of certain muscles may appear with no other findings. Later, it may be severe enough to cause paralysis. Typically, the muscles are strongest in the morning but weaken throughout the day, especially after exercise. Short rest periods temporarily restore muscle function. Muscle weakness is progressive; eventually some muscles may lose function entirely. Resulting symptoms depend on the muscle group affected; they become more intense during menstrual periods and after emotional stress, prolonged exposure to sunlight or cold, or infections.

Onset may be sudden or insidious. In many people, weak eye closure, drooping eyelids, and double vision are the first signs that something is wrong. People with myasthenia gravis usually have blank and expressionless faces and a nasal voice. They experience frequent nasal regurgitation of fluids and have difficulty chewing and swallowing. Because of this, they often worry about choking. They may also have difficulty breathing. Because their eyelids droop, they may have to tilt their heads back to see. Their neck muscles may become too weak to support their heads without bobbing.

People with myasthenic crisis (sudden development of respiratory distress) are predisposed to pneumonia and other respiratory tract infections. This situation may be severe enough to require an emergency airway and mechanical ventilation.
How is it diagnosed?

Muscle fatigue that improves with rest strongly suggests a diagnosis of myasthenia gravis. Tests for this neurologic condition record the effect of exercise and subsequent rest on muscle weakness. Electromyography, with repeated nerve stimulation, may help confirm this diagnosis.

The classic proof of myasthenia gravis is improved muscle function after an intravenous injection of Tensilon or Prostigmin. In people with myasthenia gravis, muscle function improves within 30 to 60 seconds after drug administration and lasts up to 30 minutes. However, long-standing eye muscle dysfunction may fail to respond to such testing. This test can differentiate a myasthenic crisis from a cholinergic crisis (caused by acetylcholine overactivity at the neuromuscular junction). Evaluation should rule out thyroid disease and thymoma.
How is it treated?

Treatment aims to minimize symptoms. Anticholinesterase drugs, such as Prostigmin and Mestinon, counteract fatigue and muscle weakness and allow about 80% of normal muscle function. However, these drugs become less effective as the disease worsens. Corticosteroids may relieve symptoms. Plasmapheresis (filtering of disease elements from the plasma) is used in severe flare-ups.

People with thymomas require removal of the thymus gland, which may cause remission in some cases of adult-onset myasthenia gravis. Acute flare-ups that cause severe respiratory distress require emergency treatment. Tracheotomy, positive-pressure ventilation, and vigorous suctioning to remove secretions usually produce improvement in a few days. Because anticholinesterase drugs aren't effective in myasthenic crisis, they're stopped until respiratory function improves. Myasthenic crisis requires immediate hospitalization and vigorous respiratory support.