Myasthenia Gravis

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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.

image
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).

image
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

The following clinical manifestations result from the pathologic mechanisms caused (or activated) by Atelectasis (see Figure 9-8), Alveolar Consolidation (see Figure 9-9), and Excessive Bronchial Secretions (see Figure 9-12)—the major anatomic alterations of the lungs associated with myasthenia gravis (when ventilatory failure is not properly managed) (see Figure 29-1).

CLINICAL DATA OBTAINED FROM LABORATORY TESTS AND SPECIAL

Pulmonary Function Test Findings* (Restrictive Lung Pathology)

FORCED EXPIRATORY FLOW RATE FINDINGS

FVC FEVT FEV1/FVC ratio FEF25%-75%
↓ N or ↓ N or ↑ N or ↓
FEF50% FEF200-1200 PEFR MVV
N or ↓ N or ↓ N or ↓ N or ↓

image

LUNG VOLUME AND CAPACITY FINDINGS

VT IRV ERV RV  
↓ ↓ ↓ ↓  
VC IC FRC TLC RV/TLC ratio
↓ ↓ ↓ ↓ N

image

NEGATIVE INSPIRATORY FORCE (NIF) ↓

*Progressive worsening of these values is key to anticipating the onset of ventilatory failure.

Arterial Blood Gases

Oxygenation Indices*

*C(a-image)O2, Arterial-venous oxygen difference; DO2, total oxygen delivery; O2ER, oxygen extraction ratio; image, pulmonary shunt fraction; image, mixed venous oxygen saturation; image, oxygen consumption.

General Management of Myasthenia Gravis

In the past, many patients with myasthenia gravis died within the first few years of diagnosis of the disease. Today, a number of therapeutic measures provide most patients with marked relief of symptoms and allow them to live a normal life. Frequent measurements of the patient’s vital capacity (VC), negative inspiratory force (NIF), blood pressure, oxygen saturation, and arterial blood gases should be performed. Mechanical ventilation should be initiated when the clinical data demonstrate impending or acute ventilatory failure.

Good clinical indicators of acute ventilatory failure include the following:

The Bronchopulmonary Hygiene Protocol and Lung Expansion Therapy Protocol should be instituted to prevent mucous accumulation, airway obstruction, alveolar consolidation, and atelectasis. During a myasthenic crisis, the treatment modalities described in the following paragraphs also may be used.

Respiratory Care Treatment Protocols

Oxygen Therapy Protocol

Oxygen therapy is used to treat hypoxemia, decrease the work of breathing, and decrease myocardial work. Because of the hypoxemia that may develop in myasthenia gravis, supplemental oxygen may be required. However, because of the alveolar consolidation and atelectasis associated with myasthenia gravis, capillary shunting may be present. Hypoxemia caused by capillary shunting is refractory to oxygen therapy (see Oxygen Therapy Protocol, Protocol 9-1).

CASE STUDY

Myasthenia Gravis

Admitting History

A 35-year-old Spanish-American woman was a schoolteacher with a 3-year-old son and an unemployed husband who was still “finding his real place in life.” The woman was a high achiever. She had recently received her doctoral degree in education, but she continued to work in the classroom with the grade-school children she loved so much. She was named Teacher of the Year in the large city where she lived. Her colleagues at school considered her a nonstop worker. She had never smoked.

At home, she was always on the move. She had just finished remodeling her kitchen and two bathrooms. She also did her own backyard landscaping on the weekends, a job she particularly enjoyed. She read and played with her son whenever they had time together. Although she enjoyed cooking (a skill she learned from her mother), she did not like to shop for groceries. Fortunately, this was a chore that her husband enjoyed.

Three weeks before the current admission, the woman noticed that her eyes “felt tired.” She began to experience slight double vision. Thinking that she was working too hard, she slowed down a bit and went to bed earlier for about a week. However, she progressively felt weaker. Her legs quickly became tired, and she began having trouble chewing her food. Concerned, the woman finally went to see her doctor. After reviewing the woman’s recent history and performing a careful physical examination, the physician admitted her to the hospital for further evaluation and treatment.

Over the next 48 hours, the woman’s physical status declined progressively. At the patient’s bedside, an ice pack test was positive for myasthenia gravis when her ptosis improved by 5 mm. She also indicated that her diplopia was better for about 10 minutes after the test. After the administration of edrophonium, her muscle strength increased significantly for about 10 minutes. Electromyography disclosed extensive muscle involvement and a high degree of fatigability in all the affected muscles. A diagnosis of myasthenia gravis was recorded in the patient’s chart.

The woman began to choke and aspirate food during meals, and a nasogastric feeding tube was inserted. Her speech became more and more slurred. Both her upper eyelids drooped, and she was unable to hold her head off her pillow on request. The respiratory therapists who monitored her vital capacity, negative inspiratory pressure, pulse oximetry, and arterial blood gas values (ABGs) reported a progressive worsening in all parameters.

When the woman’s ABGs were pH 7.32, Paco2 51, image 23, and Pao2 59 (on room air), the respiratory therapist called the physician and reported an assessment of acute ventilatory failure. The doctor had the patient transferred to the intensive care unit, intubated (No. 7 endotracheal tube with a tube length charted at 23 cm at the lip), and placed on a mechanical ventilator. The initial ventilator settings were as follows: intermittent mechanical ventilation (IMV) mode, 10 breaths/min, tidal volume 0.6 L, Fio2 0.5, and positive end-expiratory pressure (PEEP) of 7 cm H2O.

Approximately 25 minutes after the patient was placed on the ventilator, she appeared agitated. No spontaneous ventilations were seen. Her vital signs were as follows: blood pressure 132/86, heart rate 90 bpm, and rectal temperature 38° C (100.5° F). A portable chest x-ray film had been taken, but the image was still being processed. Normal vesicular breath sounds were auscultated over the right lung, and diminished-to-absent breath sounds were auscultated over the left lung. On an Fio2 of 0.5, her ABG values were as follows: pH 7.28, Paco2 64, image 26, and Pao2 52. Her oxygen saturation measured by pulse oximetry (Spo2) was 80%. On the basis of these clinical data, the following SOAP was recorded.

Respiratory Assessment and Plan

S N/A (patient intubated)

O No spontaneous ventilations; vital signs: BP 132/86, HR 90, RR 10 (controlled), T 38° C (100.5° F); normal breath sounds over right lung; diminished-to-absent breath sounds over left lung; ABGs (on Fio2 = 0.50): pH 7.28, Paco2 64, image 26, Pao2 52; Spo2 80%

A

P Notify physician stat. Check CXR. Pull endotracheal tube back until breath sounds can be auscultated over both lungs. Confirm initial placement of the endotracheal tube when x-ray image is available. Mechanical Ventilation Protocol (increase tidal volume and increase Fio2). Monitor and reevaluate immediately.

3 Days after Admission

No changes in the patient’s ventilator settings were necessary over the previous 48 hours. No improvement was seen in her muscular paralysis. The woman appeared pale, and her vital signs were as follows: blood pressure 146/88, heart rate 92 bpm, and temperature 37.9° C (100.2° F). Large amounts of thick, yellowish sputum were being suctioned from her endotracheal tube approximately every 30 minutes.

Rhonchi were auscultated over both lung fields. A sputum sample was obtained and sent to the laboratory to be cultured. A portable chest x-ray examination revealed a new infiltrate in the right lower lobe consistent with pneumonia or atelectasis. The ABGs were as follows: pH 7.28, Paco2 36, image 17, and Pao2 41. Her Spo2 was 69%. On the basis of these clinical data, the following SOAP was recorded.

Respiratory Assessment and Plan

S N/A

O No improvement seen in muscular paralysis; skin: pale; vital signs: BP 146/88, HR 92, T 37.9° C (100.2° F); large amounts of thick, yellowish sputum; rhonchi over both lung fields; CXR: pneumonia and atelectasis in right lower lobe; ABGs: pH 7.28, Paco2 36, image 17, Pao2 41; Spo2 69%

A

P Up-regulate Bronchopulmonary Hygiene Therapy Protocol (med. neb. with 0.5 mL albuterol in 2 mL 10% acetylcysteine q4h; therapist to suction patient frequently; sputum culture check in 24 and 48 hours). Initiate Lung Expansion Therapy Protocol (add 10 cm H2O PEEP to ventilator settings). Up-regulate Oxygen Therapy Protocol (increase Fio2 to 0.6). Monitor closely and reevaluate (check ABGs in 30 minutes).

Discussion

As with the patient with Guillain-Barré syndrome, this case of myasthenia gravis provides another chance to discuss ventilatory failure secondary to neuromuscular disease. The presentation of this patient with double vision (diplopia), difficulty in swallowing (dysphagia), and progressive muscle weakness is classic for this condition. The positive endrophronium test noted in the history was necessary for a final diagnosis. Also important to note is that aspiration of gastric contents is not uncommon in such cases.

In the first assessment the therapist should have recognized that this case was more than simple respiratory failure. The reader sees that the patient was intubated and that breath sounds no longer were present in the entire left lung (inadvertent right main stem bronchus intubation). The therapist appropriately responded quickly and pulled the endotracheal tube back until breath sounds could be auscultated over both lung fields. The inappropriate positioning of the tube was confirmed 45 minutes later in the patient’s chest x-ray film. The patient’s respiratory status could have been seriously compromised if the therapist had waited a full 45 minutes before pulling the tube above the carina. This event further demonstrates the importance of good bedside assessment skills. In addition, because lactic acidosis was likely present at this time, oxygenating the patient was of primary importance. Increasing the Fio2 to 0.80 to 1.0 would be appropriate in such a case. Any attempt to wean the patient at this early junction should not have proceeded.

The second assessment reflected that the patient was improving and was now hyperventilated and hyperoxygenated on the current ventilator settings. The therapist adjusted the ventilator therapy accordingly and began the process of longitudinal evaluation of forced vital capacity, forced expiratory volume in 1 second, and negative inspiratory force that is appropriate for this condition if weaning is to be accomplished successfully.

The final assessment suggested that the patient had taken another turn for the worse. The sputum was now purulent, rhonchi were heard over both lung fields, and a right lower lobe pneumonia or atelectasis had developed. The patient had an uncompensated metabolic acidemia that required evaluation. The fact that the patient’s Pao2 was only 41 provided a significant clinical indicator that the cause of the metabolic acidosis was “lactic acid” generated from a low tissue oxygen level. It was clearly appropriate for the respiratory care practitioner to focus on the patient’s oxygenation status. This was done by up-regulating the Oxygen Therapy Protocol (Protocol 9-1) (increasing the Fio2 to 0.6) and starting the Lung Expansion Therapy Protocol (Protocol 9-3) (the addition of 10 cm H2O PEEP to ventilator settings).

The therapist should have anticipated this development, obtained appropriate cultures, and, if not done before, prophylactically started the Bronchopulmonary Hygiene Therapy Protocol (Protocol 9-2) and Aerosolized Medication Therapy Protocol (Protocol 9-4)—with frequent suctioning, percussion, postural drainage, and possibly mucolytics. In addition to understanding lactic acidosis, the reader may wish to review other possible causes of metabolic acidemia at this time (e.g., diabetic ketoacidosis, renal failure).

Unfortunately the patient’s pulmonary condition progressively deteriorated, and she died 3 weeks later.

Related posts:

Cardiovascular System Assessments Guillain-Barré Syndrome Bronchopulmonary Dysplasia Pulmonary Air Leak Syndromes Bronchiectasis Pulmonary Embolism