Myasthenia Gravis



Myasthenia Gravis



Chapter Objectives


After reading this chapter, will be able to:


• List the anatomic alterations of the lungs associated with myasthenia gravis.


• Describe the causes of myasthenia gravis.


• List the cardiopulmonary clinical manifestations associated with myasthenia gravis.


• Describe the general management of myasthenia gravis.


• Describe the clinical strategies and rationales of the SOAPs presented in the case study.


• Define key terms and complete self-assesment questions at the end of the chapter and on Evolve.



Key Terms

























Anatomic Alterations of the Lungs Associated with Myasthenia Gravis


Myasthenia gravis is a chronic disorder of the neuromuscular junction that interferes with the chemical transmission of acetylcholine (ACh) between the axonal terminal and the receptor sites of voluntary muscles (see Figure 29-1). It is characterized by fatigue and weakness, with improvement following rest. Because the disorder affects only the myoneural (motor) junction, sensory function is not lost.


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FIGURE 29-1 Myasthenia gravis, a disorder of the neuromuscular junction that interferes with the chemical transmission of acetylcholine. AB, Antibody; ACh, acetylcholine; AT, axonal terminal; D, dendrite; MF, muscle fiber; MNF, myelinated nerve fiber; MRS, muscle receptor site; V, vesicle. Note that the antibodies have a physical structure similar to that of ACh, which permits them to connect to (and block ACh from) the muscle receptor sites. Inset, Atelectasis, a common secondary anatomic alteration of the lungs.

The abnormal weakness may be confined to an isolated group of muscles (e.g., the drooping of one or both eyelids), or it may manifest as a generalized weakness that in severe cases includes the diaphragm. When the diaphragm is involved, ventilatory failure can develop. If the ventilatory failure is not properly managed, mucous accumulation with airway obstruction, alveolar consolidation, and atelectasis may develop.


The major pathologic or structural changes of the lungs associated with the ventilatory failure that may accompany myasthenia gravis are as follows:




Etiology and Epidemiology


The cause of myasthenia gravis appears to be related to ACh receptor antibodies (the IgG antibodies) that block the nerve impulse transmissions at the neuromuscular junction. It is believed that the IgG antibodies disrupt the chemical transmission of ACh at the neuromuscular junction by (1) blocking the ACh from the receptor sites of the muscular cell, (2) accelerating the breakdown of ACh, and (3) destroying the receptor sites (see Figure 29-1). Receptor-binding antibodies are present in 85% to 90% of persons with myasthenia gravis. Although the specific events that activate the formation of the antibodies remain unclear, the thymus gland is almost always abnormal; it is generally presumed that the antibodies arise within the thymus or in related tissue.


About 30,000 people in the United States are affected by myasthenia gravis. It is most common in young women and older men. The disease usually has a peak age of onset in females of 15 to 35 years, compared with 40 to 70 years in males. The clinical manifestations associated with myasthenia gravis are often provoked by emotional upset, physical stress, exposure to extreme temperature changes, febrile illness, and pregnancy. Death caused by myasthenia gravis is possible, especially during the first few years after onset. After the disease has been in progress for 10 years, however, death from myasthenia gravis is rare.



Screening and Diagnosis


Screening methods and tests used to diagnosis myasthenia gravis include (1) the clinical history, (2) neurologic examination, (3) electromyography, (4) blood analysis, (5) edrophonium (Enlon) test, (6) ice pack test, (7) sleep test, and (8) computed tomography (CT) or magnetic resonance imaging (MRI) of the thymus.



Clinical Presentation


The hallmark of myasthenia gravis is chronic muscle fatigue. The muscles become progressively weaker during periods of activity and improve after periods of rest. Signs and symptoms include facial muscle weakness; ptosis (drooping of one or both eyelids); diplopia (double vision); difficulty in breathing, speaking, chewing, and swallowing; unstable gait; and weakness in arms, hands, fingers, legs, and neck brought on by repetitive motions. The muscles that control the eyes, eyelids, face, and throat are especially susceptible and are usually affected first. The respiratory muscles of the diaphragm and chest wall can become weak and impair the patient’s ventilation. Impairment in deep breathing and coughing predispose the patient to excessive bronchial secretions, atelectasis, and pneumonia.


The signs and symptoms of myasthenia gravis during the early stages are often elusive. The onset can be subtle, intermittent, or sudden and rapid. The patient may (1) demonstrate normal health for weeks or months at a time, (2) show signs of weakness only late in the day or evening, or (3) develop a sudden and transient generalized weakness that includes the diaphragm. Because of this last characteristic, ventilatory failure is always a sinister possibility. In most cases, the first noticeable symptom is weakness of the eye muscles (droopy eyelids) and a change in the patient’s facial expressions. As the disorder becomes more generalized, weakness develops in the arms and legs. The muscle weakness is usually more pronounced in the proximal parts of the extremities. The patient has difficulty in climbing stairs, lifting objects, maintaining balance, and walking. In severe cases the weakness of the upper limbs may be such that the hand cannot be lifted to the mouth. Muscle atrophy or pain is rare. Tendon reflexes almost always remain intact.







Ice Pack Test


The ice pack test (Figure 29-2) is a very simple, safe, and reliable procedure for diagnosing myasthenia gravis in patients who have ptosis (droopy eye). In addition, the ice pack test does not require special medications or expensive equipment and is free of adverse effects. The test consists of the application of an ice pack to the patient’s symptomatic eye for 3 to 5 minutes. The test is considered positive for myasthenia gravis when there is improvement of the ptosis (an increase of at least 2 mm in the palpebral fissure from before to after the test).


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FIGURE 29-2 Ice pack test. A, Myasthenia gravis in patient who has ptosis (droopy left eye). B, Same patient after 5-minute application of an ice pack. Note the patient’s left eye lid is no longer droopy.

A major disadvantage of the ice pack test is that it is useful only when ptosis is present. Even though the symptoms associated with diplopia (double vision) may also improve with the ice pack test, the reliability of the ice pack test in patients with diplopia without ptosis is usually questionable because the patient’s personal impression of the diplopia is subjective. Therefore caution should be exercised in patients with isolated diplopia without ptosis. The ice pack test may be especially useful in patients in whom the edrophonium test is contraindicated by either cardiac status or age.




Computed Tomography or Magnetic Resonance Imaging


A CT or MRI scan may be used to identify an abnormal thymus gland or the presence of a thymoma (a usually benign tumor of the thymus gland that may be associated with myasthenia gravis). A thymectomy has been shown to reduce symptoms in more than 70% of patients with myasthenia gravis. In fact, a thymectomy may be recommended even when there is no tumor. The removal of the thymus seems to improve the condition in many patients.



image OVERVIEW of the Cardiopulmonary Clinical Manifestations Associated with Myasthenia Gravis

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Jun 11, 2016 | Posted by in RESPIRATORY | Comments Off on Myasthenia Gravis

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