Myasthenia gravis
Myasthenia gravis: Excerpt from Professional Guide to Diseases (Eighth Edition)
Myasthenia gravis produces sporadic but progressive weakness and abnormal fatigability of striated (skeletal) muscles, exacerbated by exercise and repeated movement, but improved by anticholinesterase drugs. Usually, this disorder affects muscles innervated 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 exacerbations and periodic remissions. There’s no known cure. Drug treatment has improved prognosis and allows patients to lead relatively normal lives except during exacerbations. When the disease involves the respiratory system, it may be life threatening.
Causes and incidence
Myasthenia gravis causes a failure in transmission of nerve impulses at the neuromuscular junction. Theoretically, such impairment may result from an autoimmune response, ineffective acetylcholine release, or inadequate muscle fiber response to acetylcholine. (See Impaired transmission in myasthenia gravis.)
Myasthenia gravis affects 3 of every 10,000 people at any age, but it’s more common in young women and older men. About 20% of neonates born to mothers with myasthenia gravis have transient (or occasionally persistent) myasthenia. This disease may coexist with immunologic and thyroid disorders; about 15% of patients with myasthenia gravis have thymomas. Remissions occur in about 25% of patients.
Signs and 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, myasthenic 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; more and more muscles become weak, and eventually some muscles may lose function entirely. Resulting symptoms depend on the muscle group affected; they become more intense during menses and after emotional stress, prolonged exposure to sunlight or cold, or infections.
Onset may be sudden or insidious. In many patients, weak eye closure, ptosis, and diplopia are the first signs that something is wrong. Patients with myasthenia gravis usually have blank, expressionless faces and nasal vocal tones. They experience frequent nasal regurgitation of fluids and have difficulty chewing and swallowing. Because of this, they usually worry about choking. Their eyelids droop (ptosis), and they may have to tilt their heads back to see. Their neck muscles may become too weak to support their heads without bobbing.
In patients with weakened respiratory muscles, decreased tidal volume and vital capacity make breathing difficult and predispose to pneumonia and other respiratory tract infections. Respiratory muscle weakness (myasthenic crisis) may be severe enough to require an emergency airway and mechanical ventilation.
Diagnosis
Repeated muscle use over a very short time that fatigues and then improves with rest 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 neural stimulation, may help confirm this diagnosis. Acetylcholine receptor antibodies may be present in the blood.
Confirming diagnosis The classic proof of myasthenia gravis is improved muscle function after an I.V. injection of edrophonium or neostigmine (anticholinesterase drugs).
In patients with myasthenia gravis, muscle function improves within 30 to 60 seconds and lasts up to 30 minutes. Long-standing ocular 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). The acetylcholine receptor antibody titer may be elevated in generalized myasthenia. Evaluation should rule out thyroid disease and thymoma.
Treatment
Treatment is symptomatic. Anticholinesterase drugs, such as neostigmine and pyridostigmine, 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. Immunosuppressants are also used. Plasmapheresis is used in severe myasthenic exacerbation.
Patients with thymomas require thymectomy, which may cause remission in some cases of adult-onset myasthenia. Acute exacerbations that cause severe respiratory distress necessitate 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.
Special considerations
Careful baseline assessment, early recognition and treatment of potential crises, supportive measures, and thorough patient teaching can minimize exacerbations and complications. Continuity of care is essential.
❑Establish an accurate neurologic and respiratory baseline. Thereafter, monitor tidal volume and vital capacity regularly. The patient may need a ventilator and frequent suctioning to remove accumulating secretions.
❑Be alert for signs of an impending crisis (increased muscle weakness, respiratory distress, and difficulty in talking or chewing).
❑To prevent relapses, adhere closely to the ordered drug administration schedule. Be prepared to give atropine for anticholinesterase overdose or toxicity.
❑Plan exercise, meals, patient care, and activities to make the most of energy peaks. For example, give medication 20 to 30 minutes before meals to facilitate chewing or swallowing. Allow the patient to participate in his care.
❑When swallowing is difficult, give soft, solid foods instead of liquids to lessen the risk of choking.
❑Patient teaching is essential because myasthenia gravis is usually a lifelong condition. Help the patient plan daily activities to coincide with energy peaks. Stress the need for frequent rest periods throughout the day. Emphasize that periodic remissions, exacerbations, and day-to-day fluctuations are common.
❑Teach the patient how to recognize adverse effects and signs of toxicity of anticholinesterase drugs (headaches, weakness, sweating, abdominal cramps, nausea, vomiting, diarrhea, excessive salivation, and bronchospasm) and corticosteroids (euphoria, insomnia, edema, and increased appetite).
❑Warn the patient to avoid strenuous exercise, stress, infection, and needless exposure to the sun or cold. All of these things may worsen signs and symptoms.
❑For more information and an opportunity to meet other myasthenia gravis patients who lead full, productive lives, refer the patient to the Myasthenia Gravis Foundation.
Pictures
Book Source Details
- Book Title: Professional Guide to Diseases (Eighth Edition)
- Author(s): Springhouse
- Year of Publication: 2005
- Copyright Details: Professional Guide to Diseases (Eighth Edition), Copyright © 2005 Lippincott Williams & Wilkins.
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Copyright notice for book excerpts: Copyright © 2008 Lippincott Williams & Wilkins. All rights reserved.
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