Myasthenia gravis
Myasthenia gravis: Excerpt from Handbook of Diseases
Myasthenia gravis produces sporadic but progressive weakness and abnormal fatigability of striated (skeletal) muscles, which are exacerbated by exercise and repeated movement but improved by anticholinesterase therapy. 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 periodic exacerbations and remissions. (See Coping with lifelong myasthenia gravis.) There is no known cure. Drug treatment has improved the 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
Myasthenia gravis causes a failure in transmission of nerve impulses at the neuromuscular junction. Such impairment may result from an autoimmune response, ineffective acetylcholine release, or inadequate muscle fiber response to acetylcholine.
Myasthenia gravis affects 3 in 10,000 people at any age and is more common in young women and older men.
This disease may coexist with immune and thyroid disorders; about 15% of myasthenic patients have thymomas. Remissions occur in about 25% of patients.
gender influence Pregnancy is possible for female patients with myasthenia gravis, although they must be closely supervised. About 20% of infants born to myasthenic mothers have transient, or occasionally persistent myasthenia. The infant appears temporarily weak and may require medications for a few weeks after birth. Usually the baby doesn’t develop the disorder, but he must receive follow-up attention.
Signs and symptoms
The dominant symptoms of myasthenia gravis are skeletal muscle weakness and fatigability. In the early stages, easy fatigability of certain muscles, notably the eye and eyelid muscles and muscles involving swallowing and talking, 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.
Progressive muscle weakness
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.
Myasthenic patients may have hoarseness or a changing voice because of muscle weakness. They experience difficulty chewing and swallowing and are prone to choking. Eyelids droop and may impair vision. Neck muscles may become too weak to support the head without bobbing.
In patients with weakened respiratory muscles, decreased tidal volume and vital capacity make breathing difficult and predispose them 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
Muscle fatigability 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 neural stimulation, may help confirm this diagnosis.
The classic proof of myasthenia gravis is improved muscle function after an I.V. injection of edrophonium or neostigmine in the Tensilon test. In myasthenic patients, 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. Other autoimmune disorders, such as rheumatoid arthritis, lupus erythematosus, and polymyositis, are commonly associated with myasthenia gravis.
Treatment
Treatment for myasthenia gravis is aimed at relieving symptoms. Anticholinesterases, such as neostigmine and pyridostigmine, improve communication between nerve and muscle, counteract fatigue and muscle weakness, and allow about 80% of normal muscle function. However, these drugs become less effective as the disease worsens. Decreasing the immune response toward acetylcholine receptors at the neuromuscular junction is the goal of immunosuppressant therapy. Corticosteroids, azathioprine, cyclosporine, and cyclophosphamide are used in a progressive fashion (when the previous drug response is poor, the next one is used). To suppress the immune system during acute relapses, gamma globulin may also be used. Plasmapheresis is used to treat severe exacerbations or to quickly improve symptoms (for example, preoperatively).
Patients with thymomas require a 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 anticholinesterases aren’t effective in patients with 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, difficulty in talking or chewing).
❑ Space administration of drugs evenly, and give them on time to prevent relapses. 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 self-care.
❑ When swallowing is difficult, give soft, solid foods instead of liquids to lessen the risk of choking.
❑ After a severe exacerbation, try to increase social activity as soon as possible.
❑ Patient teaching is essential because myasthenia gravis is usually a lifelong condition.
CLINICAL TIP: Teach the patient to avoid or closely monitor the effects of certain drugs. Curare-like drugs, local anesthetics, common cold products, tonic water and antiarrhythmics containing quinine, aminoglycoside antibiotics, tetracyclines, morphine sulfate, beta-adrenergic blockers, and calcium channel blockers may worsen muscle weakness by impairing the transmission of impulses across the neuromuscular junction.
❑ An eye patch may be recommended if double vision is troublesome.
❑ The patient should avoid stress and excessive heat exposure because they may cause worsening of symptoms.
Pictures
Book Source Details
- Book Title: Handbook of Diseases
- Author(s): Springhouse
- Year of Publication: 2003
- Copyright Details: Handbook of Diseases, Copyright © 2003 Lippincott Williams & Wilkins.
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Copyright notice for book excerpts: Copyright © 2008 Lippincott Williams & Wilkins. All rights reserved.
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More About This Book:
Title: Handbook of Diseases
Authors: Springhouse
Publisher: Lippincott Williams & Wilkins
Copyright: 2003
ISBN: 1-58255-266-5
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