Acute Ventilatory Failure With Hypoxemia3 (Acute Respiratory Acidosis)
pH4
PaCO2
4
PaO2
SaO2 or SpO2
↓
↑
↑
↓
↓
(but normal)
3See Fig. 5.3 and Table 5.5 and related discussion for the acute pH, PaCO2, and changes associated with acute ventilatory failure.
4When tissue hypoxia is severe enough to produce lactic acid, the pH and values will be lower than expected for a particular PaCO2 level.
Oxygenation Indices5
QS/QT
DO26
VO2
O2ER
↑
↓
N
N
↑
↓
6The DO2 may be normal in patients who have compensated to the decreased oxygenation status with (1) an increased cardiac output, (2) an increased hemoglobin level, or (3) a combination of both. When the DO2 is normal, the O2ER is usually normal.
•Increased opacity (when atelectasis or consolidation are present)
If the ventilatory failure associated with GBS is properly managed (e.g., via the Airway Clearance Therapy Protocol, Protocol 10.2, Ventilator Initiation and Management Protocol, Protocol 11.1, and Ventilator Weaning Protocol, Protocol 11.2), the chest radiograph should appear normal. However, if the patient is not properly managed, mucous accumulation, alveolar consolidation, and atelectasis may develop. In these cases, the chest radiograph will show an increased density of the lung segments affected.
Autonomic Nervous System Dysfunctions
Dysautonomia (autonomic dysfunction) occurs in about 70% of cases. Symptoms include:
•Cardiac arrhythmias
•Tachycardia (the most common)
•Bradycardia, ventricular tachycardia, atrial flutter, atrial fibrillation, and asystole
•Urinary retention
•Hypertension alternating with hypotension
•Orthostatic hypotension
•Obstruction of the intestines (ileus)
•Loss of sweating
General Management of Guillain-Barré Syndrome3
GBS is a potential medical emergency, and patients must be monitored closely after the diagnosis has been made. About 30% of cases develop acute ventilatory failure and require mechanical ventilation. The primary treatment should be directed at stabilization of vital signs and supportive care for the patient. Close respiratory monitoring with frequent measurements of the patient's forced vital capacity (FVC), maximum inspiratory pressure (MIP), maximum expiratory pressure (MEP), blood pressure, oxygenation saturation, and, when indicated, arterial blood gases (ABGs) should be performed. Mechanical ventilation should be initiated when the clinical data demonstrate impending or acute ventilatory failure.
Good clinical indicators of impending acute ventilatory failure include the following:
•FVC <20 mL/kg
•MIP <–30 cm H2O: In other words, the patient is unable to generate a maximum inspiratory
pressure of –30 cm H2O or more. For example, an MIP of only –15 cm H2O would confirm severe muscle weakness and, importantly, that acute ventilatory failure is likely.
•MEP <40 cm H2O
•PaCO2 >45 mm Hg
•pH <7.35
The primary treatment modalities for GBS are (1) plasmapheresis (also called plasma exchange) and (2) intravenous immune globulin (IVIG). These two treatments have been shown to be equally effective. Plasmapheresis, or plasma
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exchange (PE), has been shown to be effective in decreasing the morbidity and shortening the clinical course of GBS. Plasmapheresis entails the removal of damaged antibodies from the patient's blood plasma, followed by the transfusion of blood. It is believed that plasmapheresis removes the antibodies from the plasma that contribute to the immune system attack on the peripheral nerves. This procedure has been shown to reduce circulating antibody titers during the early stages of the disorder. High-dose IVIG has been demonstrated to be at least as effective, and possibly more, than plasmapheresis. IVIG is a blood product that contains the pooled immunoglobulins (IgG) from thousands of donors. The effects of IVIG last between 2 weeks and 3 months. IVIG products are used to treat multiple conditions, including GBS. Glucocorticoids are not recommended.
As in any patient who is paralyzed or immobilized for prolonged periods, the risk for thromboembolism increases. Because of this danger, the patient commonly receives anticoagulants, elastic stockings, and passive range-of-motion exercises (every 3 to 4 hours) for all extremities. To prevent skin breakdown, the patient should be turned frequently.
Nonsteroidal antiinflammatory agents (NSAIDs) are helpful for pain control. A rotary bed or Stryker frame may be required. Blood pressure disturbances and cardiac arrhythmias require immediate attention. For example, episodes of bradycardia are commonly treated with atropine.
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 GBS, supplemental oxygen may be required. However, because of the alveolar consolidation and atelectasis associated with GBS, capillary shunting may be present. Hypoxemia caused by capillary shunting is refractory to oxygen therapy (see Oxygen Therapy Protocol, Protocol 10.1).
Airway Clearance Therapy Protocol
Because of the excessive mucous accumulation, airway obstruction, alveolar consolidation, and atelectasis associated with GBS, a number of airway clearance modalities may be used to enhance the mobilization of bronchial secretions (see Airway Clearance Therapy Protocol, Protocol 10.2).
Lung Expansion Therapy Protocol
Lung expansion measures are commonly administered to offset the alveolar consolidation and atelectasis associated with GBS (see Lung Expansion Therapy Protocol, Protocol 10.3).
Mechanical Ventilation Protocol
Mechanical ventilation may be necessary to provide and support alveolar gas exchange and eventually return the patient to spontaneous breathing. Because acute ventilatory failure is seen in patients with severe GBS, continuous mechanical ventilation is often required. Continuous mechanical ventilation is justified because the acute ventilatory failure is thought to be reversible. Noninvasive positive-pressure ventilation (NIPPV) may be helpful if carefully monitored (see Ventilator Initiation and Management Protocol, Protocol 11.1, and Ventilator Weaning Protocol, Protocol 11.2).
Physical Therapy and Rehabilitation
Physical therapy usually begins long before the patient recovers from the effects of GBS, often while the patient is still being mechanically ventilated. In long-term cases, for example, the arms and legs of the patient will be manually moved on a regular basis to keep the muscles flexible. After recovery, the patient frequently requires physical therapy to regain full strength and normal mobility. Hydrotherapy (whirlpool therapy) is commonly used to relieve pain and facilitate limb movement. Full recovery may occur in as little as a few weeks or as long as 3 years.
Case Study Guillain-Barré Syndrome
Admitting History and Physical Examination
A 48-year-old career US Navy physician visited the base hospital clinic because of the acute onset of severe muscle weakness. He had joined the Navy immediately after medical school. Throughout his time in the service, he had the opportunity to pursue his passion—competitive water-ski jumping. For many years he was the first-place winner at most tournaments, including the nationals held yearly. For almost 25 years, he progressed through the age divisions, always remaining the top seed, always capturing the highest title.
The man was in outstanding physical condition. He was an avid runner and weightlifter, and during the off-season he often traveled to a warm climate to practice his water-ski jumping. He had never smoked and had never been hospitalized. He had an occasional “cold.” About 2 years previously, he had begun to focus all his attention on his 19-year-old son, who was quickly following in his father's footsteps, having just captured the Men's Division I championship in collegiate ice hockey.
The man stated that he had felt good until 3 weeks before his admission, at which time he experienced a flulike syndrome for 3 days. About 10 days after returning to work, he noticed a tingling and burning sensation in his feet during his morning patient rounds. By dinner time that same day, the tingling and burning had radiated from his feet to about the level of his knees. Thinking that he was just tired from being on his feet all day, he went to bed early that evening. The next morning, however, his legs were completely numb, although he could still move them. Alarmed, he asked his son to drive him to the clinic. After examining him, his doctor (a personal friend) admitted him for a diagnostic workup and observation.
Over the next 3 days, the laboratory results showed that the patient's CSF had an elevated protein concentration with a normal cell count. The electrodiagnostic studies showed a progressive ascending paralysis of the man's legs and arms. He began to have difficulty eating and swallowing his food. The respiratory therapist, who was monitoring his forced vital capacity (FVC), maximum inspiratory pressure (MIP), pulse oximetry, and arterial blood gas values (ABGs), reported a progressive deterioration in all the values. A provisional diagnosis of Guillain-Barré syndrome was recorded in the patient's chart.
When the man's ABGs showed pH 7.29, PaCO2 53 mm Hg, 23 mEq/L, PaO2 86 mm Hg, and SaO2 96% (on a
2 L/min oxygen nasal cannula), the respiratory therapist called the attending physician and reported his assessment of acute ventilatory failure. The doctor transferred the patient to the intensive care unit (ICU), intubated him, and placed him on a mechanical ventilator. The initial ventilator settings were synchronized intermittent mandatory ventilation (SIMV) mode, 12 breaths/min, tidal volume 750 mL, and FIO2 0.50, without added PEEP.
About 15 minutes after the patient was committed to the ventilator, he appeared comfortable. No spontaneous breaths were noted between the 12 set breaths/min. His vital signs were blood pressure 126/82 mm Hg and heart rate 68 beats/min. He was afebrile. A portable chest radiograph confirmed that the endotracheal (ET) tube was in a good position and the lungs were adequately aerated. Normal vesicular breath sounds were auscultated over both lung fields.
His ABGs were pH 7.51, PaCO2 29 mm Hg, 22 mEq/L, PaO2 204 mm Hg, and SaO2 98%. On the basis of these clinical data, the following SOAP was documented.
Respiratory Assessment and Plan
S N/A (intubated on ventilator)
O Vital signs: BP 126/82, HR 68, RR 12 (SIMV); afebrile; no spontaneous breaths; CXR: normal;
normal breath sounds; ABGs (on FIO2 0.50) pH 7.51, PaCO2 29, 22, PaO2 204, SaO2 98% A
•Acute alveolar hyperventilation with excessive oxygenation on ventilator (ABGs)
•FIO2 too high (ABGs)
P Adjust mechanical ventilator settings (decrease tidal volume to 650 mL and FIO2 to 0.40)
according to Mechanical Ventilation Protocol and Oxygen Therapy Protocol. Monitor closely and reevaluate.
Three Days After Admission
The patient's cardiopulmonary status had been unremarkable. No improvement was seen in his muscular paralysis. No changes had been made in his ventilator settings over the previous 48 hours. His skin color appeared good. Palpation and percussion of the chest were unremarkable. On auscultation, however, coarse crackles could be heard over both lung fields.
Moderate amounts of thick, whitish, clear secretions were being suctioned from the patient's endotracheal tube regularly. His vital signs were blood pressure 124/83 mm Hg, heart rate 74 beats/min, and rectal temperature 37.7°C (99.8°F). A recent portable chest radiograph revealed no significant pathologic process. His ABGs on an FIO2 of 0.40, a
respiratory rate of 12 breaths/min, and tidal volume of 650 were pH 7.44, PaCO2 35 mm Hg, 24 mEq/L, PaO2 98 mm Hg, and SaO2 97%. On the basis of these clinical data, the following SOAP was documented.
Respiratory Assessment and Plan
S N/A (intubated on ventilator)
O Skin color good; coarse crackles over both lung fields; moderate amount of whitish, clear secretions being suctioned regularly; vital signs BP 124/83, HR 74, T 37.7°C (99.8°F); CXR:
•Normal acid-base and oxygenation status on present ventilator settings (ABGs)
•Excessive sputum accumulation; possible progression to mucous plugging and atelectasis (coarse crackles, whitish and clear secretions)
P Begin Airway Clearance Therapy Protocol (PRN tracheal suctioning and obtain sputum stain and culture). Begin Lung Expansion Therapy Protocol (+10 cm H2O positive end-expiratory
pressure [PEEP] to offset any early development of atelectasis). Monitor and reevaluate (4 × per shift). Continue Mechanical Ventilation Protocol and Oxygen Therapy Protocol.
Five Days After Admission
The patient remained alert and comfortable, except for the presence of the ET tube. His muscular paralysis remained unchanged. His skin color appeared good, and no abnormalities were noted during palpation and percussion of the chest. Although coarse crackles could still be heard over both lung fields, they were not as intense as they had been 48 hours earlier. A small amount of clear secretions was suctioned from the patient's ET tube. His vital signs were blood pressure 118/79 mm Hg, heart rate 68 beats/min, and temperature normal. Results of a recent portable chest radiograph appeared normal. His ABGs on an FIO2 of 0.40, a respiratory rate of 12 breaths/min, a tidal volume of 650, and PEEP of +10 cm H2O
were pH 7.42, PaCO2 37 mm Hg,
24 mEq/L, PaO2 97 mm Hg, and SaO2 97%. The sputum culture was
unremarkable. On the basis of these clinical data, the following SOAP note was recorded.
Respiratory Assessment and Plan
S N/A (intubated on ventilator)
O Skin color good; coarse crackles over both lung fields improving; small amount of clear secretions suctioned; vital signs BP 118/79, HR 68, T normal; no spontaneous respirations;
•Normal acid-base and oxygenation status on present ventilator settings (ABGs)
•Respiratory insufficiency (no spontaneous respirations)
•Secretion control improving (coarse crackles, clear secretions)
P Continue Mechanical Ventilation Protocol. Continue Airway Clearance Therapy Protocol. Continue Lung Expansion Therapy Protocol. Monitor and reevaluate (SpO2, maximum
inspiratory pressure, and forced vital capacity 2 × per shift).
Discussion
Guillain-Barré syndrome is a neuromuscular paralysis that ensues after infection with a neurotropic virus. This patient had a classic history of ascending paralysis and paresthesia and the diagnostic finding of elevated protein concentration in the spinal fluid. In this setting, serial measurements of the patient's forced vital capacity (FVC), maximum inspiratory pressure (MIP), blood pressure, oxygen saturation, and arterial blood gases (ABGs) must be measured and charted.
Once respiratory failure develops, intubation and respiratory support on a ventilator became necessary. As discussed in
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this chapter, good clinical indicators of acute ventilatory failure include FVC less than 20 mL/kg, MIP below –30 cm H2O, pH less than 7.35, and PaCO2 greater than 45 mm Hg. As noted by the respiratory therapist, a progressive deterioration was
observed in all of these clinical indicators over a 3-day period.
As shown during the first assessment, when acute ventilatory failure developed, the patient was transferred to the ICU, intubated, and placed on a mechanical ventilator. Shortly after the patient was placed on the ventilator, his ABG values showed hyperoxia and acute alveolar hyperventilation, both of which were caused by the ventilator settings. The appropriate response was to immediately adjust the ventilator settings by reducing the tidal volume or frequency (or both) and the FIO2. At the time of the assessment, the patient exhibited no evidence of airway obstruction or secretions.
Therefore the Airway Clearance Therapy Protocol (Protocol 10.2) was not indicated. Indeed, all that needed to be done at that time was to ensure adequate ventilation and oxygenation on the ventilator.
However, 3 days later, at the time of the second assessment, coarse crackles were heard over all lung fields. There was no indication of fluid overload. Clearly the time had come to initiate the Airway Clearance Therapy Protocol (Protocol 10.2). Because of the risk for atelectasis, the Lung Expansion Therapy Protocol (Protocol 10.3), in the form of PEEP on the ventilator, was indicated. In such a case, the sputum should be cultured to see whether any infectious organisms were present.
At the time of the final assessment (2 days later), the clinical indicators for airway secretions had decreased—the crackles could no longer be heard over the lung fields, and the small amount of sputum suctioned appeared clear. At that point down-regulation of the Airway Clearance Therapy Protocol (Protocol 10.2) was indicated.
Serial SpO2, FVC, or MIP measurements would continue to be made until the patient was ready to be extubated and
thereafter for at least several days. Indeed, extubation occurred about 3 weeks after the initiation of mechanical ventilation. The patient recovered without incident and returned to his active lifestyle within a year.
Self-Assessment Questions
1.In Guillain-Barré syndrome, which of the following pathologic changes develop in the peripheral nerves?
1.Inflammation
2.Increased ability to transmit nerve impulses
3.Demyelination
4.Edema
a.2 and 3 only
b.3 and 4 only
c.2, 3, and 4 only
d.1, 3, and 4 only
2.Which of the following is(are) associated with Guillain-Barré syndrome? 1. Alveolar consolidation
2. Mucus accumulation
3.Alveolar hyperinflation
4.Atelectasis
a.1 and 2 only
b.3 and 4 only
c.1, 2, and 4 only
d.2, 3, and 4 only
3.Guillain-Barré syndrome is more common in: 1. People older than 50 years of age
2.Blacks
3.Males than in females
4.Early childhood
a.1 only
b.4 only
c.1 and 3 only
d.3 and 4 only
4.Which of the following are possible precursors to Guillain-Barré syndrome? 1. Mumps
2.Swine influenza vaccine
3.Infectious mononucleosis
4.Measles
a.2 and 4 only
b.3 and 4 only
c.2, 3, and 4 only
d.1, 2, 3, and 4
5.Full recovery from Guillain-Barré syndrome is expected in approximately what percentage of cases?
a.30%
b.40%
c.50%
d.90%
6.Which of the following are indicators for intubation and mechanical ventilation in patients with Guillain-Barré syndrome?
1.pH >7.40
2.PaCO2 >45
3.FVC <20 mL/kg
4. MIP <–30 cm H2O
a.1 and 2 only
b.3 and 4 only
c.2, 3, and 4 only
d.1, 2, and 3 only
1The Guillain-Barré syndrome is named after the French physicians Georges Guillain and Jean Alexandre Barré, who described it in 1916.
2These diagnostic criteria have been used for years in research studies and are applicable to about 80% or 90% of patients with GBS in North America and Europe, particularly those with the AIDP form of GBS.
1It should be noted that the clinical manifestations associated with Guillain-Barré may occur over hours or days, depending on how quickly the paralysis progresses.
3About 80% of GBS cases have a complete recovery within a few months. Even with treatment, about 5% to 10% of cases have a prolonged course with very prolonged and incomplete recovery. About 2% to 3% die despite intensive care. In addition, relapse occurs in up to 10% of patients.
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C H A P T E R 3 0
Myasthenia Gravis
CHAPTER OUTLINE
Anatomic Alterations of the Lungs Associated With Myasthenia Gravis
Etiology and Epidemiology
Screening and Diagnosis
Clinical Presentation and History
Bedside Diagnostic Tests
Evaluation of Conditions Associated With Myasthenia Gravis
Overview of the Cardiopulmonary Clinical Manifestations Associated With Myasthenia Gravis
General Management of Myasthenia Gravis
Symptomatic Treatment
Chronic Immunotherapies
Rapid Immunotherapies
Thymectomy
Respiratory Care Treatment Protocols
Case Study: Myasthenia Gravis
Self-Assessment Questions
CHAPTER OBJECTIVES
After reading this chapter, you will be able to:
•List the anatomic alterations of the lungs associated with myasthenia gravis.
•Describe the etiology and epidemiology of myasthenia gravis.
•Discuss the screening for and diagnosis 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-assessment questions at the end of the chapter and on Evolve.
KEY TERMS
Acetylcholine (ACh)
Acetylcholinesterase inhibitors
Anticholinesterase Inhibitors
Binding AChR Antibodies Test
Chronic Immunotherapies
CMAP Amplitude
Decremental Response
Diplopia
Edrophonium (Tensilon) Test
Electromyography
Generalized Myasthenia Gravis
Ice Pack Test
Immunoglobulin G (IgG) Antibodies
Inadvertent Right Mainstem Bronchial Intubation
Intravenous Immune Globulin (IVIG)
Muscle-specific receptor tyrosine kinase (MuSK)
Myasthenic Crisis
Mycophenolate Mofetil
Neuromuscular Junction
Ocular Myasthenia Gravis
Ophthalmoparesis
Ophthalmoplegia
Plasmapheresis
Ptosis
Pyridostigmine (Mestinon)
Rapid Immunotherapies
Repetitive Nerve Stimulation (RNS)
Seronegative Myasthenia Gravis
Seropositive Myasthenia Gravis
Single-Fiber Electromyography (SFEMG)
Symptomatic Treatment
Thymectomy
Thymoma
Anatomic Alterations of the Lungs Associated With Myasthenia Gravis
Myasthenia gravis is the most common chronic disorder of the neuromuscular junction. The disorder interferes with the chemical transmission of acetylcholine (ACh) between the axonal terminal and the receptor sites of voluntary muscles (Fig. 30.1). The hallmark clinical feature of myasthenia gravis is fluctuating skeletal muscle weakness, often with true muscle fatigue. The fatigue and weakness usually improve after rest. There are two clinical types of myasthenia gravis: ocular and generalized. In ocular myasthenia gravis, the muscle weakness is limited to the eyelids and extraocular muscles. In generalized myasthenia gravis, the muscle weakness involves a variable combination of (1) muscles of the mouth and throat responsible for speech and swallowing (called bulbar muscles), (2) limbs, and (3) respiratory muscles. Neck extensor and flexor muscles are commonly affected, producing a “dropped head syndrome.” The facial muscles are often involved, causing the patient to appear expressionless. Generalized myasthenia gravis may, or may not, involve the ocular muscles. Because the disorder affects only the myoneural (motor) junction, sensory function is not lost.
FIGURE 30.1 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, producing myasthenic crisis. In these cases, mechanical ventilation is required. If the patient is not properly managed (e.g., via the Airway Clearance Therapy Protocol, Protocol 10.2, Ventilator Initiation and Management Protocol, Protocol 11.1, and Ventilator Weaning Protocol, Protocol 11.2), mucus accumulation with airway obstruction, alveolar consolidation, and atelectasis may develop.
The major pathologic or structural changes of the lungs associated with a poorly managed myasthenic crisis are as
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follows:
•Mucus accumulation
•Airway obstruction
•Alveolar consolidation
•Atelectasis
Etiology and Epidemiology
The cause of myasthenia gravis appears to be related to ACh receptor (AChR) antibodies (immunoglobulin G [IgG] antibodies) that block the nerve impulse transmissions at the neuromuscular junction. Patients who have detectable antibodies to the AChR, or to muscle-specific receptor tyrosine kinase (MuSK), are said to have seropositive myasthenia gravis, whereas those lacking both AChR and MuSK antibodies on standard assays are said to have seronegative myasthenia gravis. About 50% of patients with only ocular myasthenia gravis are seropositive. About 90% of cases of generalized myasthenia gravis are seropositive.
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 Fig. 30.1). 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 often abnormal; it is generally presumed that the antibodies arise within the thymus or in related tissue.
According to the Myasthenia Gravis Foundation of America, there are between 36,000–60,000 cases of myasthenia gravis in the United States (20 per 100,000 population). 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.
Screening and Diagnosis
Screening methods and tests used to diagnose myasthenia gravis include (1) clinical presentation and history, (2) bedside tests, (3) immunologic studies, (4) electrodiagnostic studies, and (5) evaluation of conditions associated with myasthenia gravis.
Clinical Presentation and History
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); ophthalmoplegia (paralysis or weakness of one or more of the muscles that control eye movement); 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 predisposes the patient to retain 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.
Bedside Diagnostic Tests
Ice Pack Test
The ice pack test 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 (Fig. 30.2). 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).
FIGURE 30.2 Ice pack test. (A) Myasthenia gravis in a patient who has ptosis (droopy left eye). (B) Same patient after 5-minute application of an ice pack. Note that 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) also may 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.
Edrophonium (Tensilon) Test
The edrophonium (Tensilon) test is used in patients with obvious ptosis or ophthalmoparesis. Edrophonium, a shortacting drug, blocks cholinesterase from breaking down ACh after it has been released from the terminal axon. This action increases the myoneural concentration of ACh, which in turn offsets the influx of antibodies at the neuromuscular junction.
When muscular weakness is caused by myasthenia gravis, a dramatic transitory improvement in muscle function (lasting about 10 minutes) is seen after the administration of edrophonium. A disadvantage of the edrophonium test is that it can be complicated by cholinergic side effects that include cardiac arrhythmias and cardiopulmonary arrest. Cardiac monitoring, or avoiding this test altogether, is suggested in the elderly or those with a history of arrhythmia or heart disease. Although the sensitivity of the Tensilon test for the diagnosis of myasthenia gravis is in the 80% to 90% range, it is associated with many false-negative and false-positive results.
Immunologic Studies
Serologic tests to detect the presence of circulating acetylcholine receptor antibodies (AChR-Abs) is the first step in the laboratory confirmation of myasthenia. There are three AChR-Ab assays: binding, blocking, and modulating. The binding AChR antibodies test is highly specific for myasthenia gravis (80% to 90%). Most experts use the term AChR-Absas synonymous with the binding antibodies. In some patients, assays for blocking and modulating antibodies also may be helpful. Blocking AChR-Abs are found in about 50% of patients with generalized myasthenia gravis. Assays for modulating AChR-Abs increase the diagnostic sensitivity by about 5% when combined with the binding studies. When AChR-Abs are negative, an assay for the antibodies to MuSK proteins should be performed.
Electrodiagnostic Studies
The repetitive nerve stimulation (RNS) and single-fiber electromyography (SFEMG) tests are important diagnostic supplements to the immunologic studies. The RNS study is the most frequently used electrodiagnostic test for myasthenia gravis. The RNS study is performed by electrically stimulating the motor nerve of selected muscles 6 to 10 times at low rates (2 or 3 Hz). In the normal muscle, there is no change in the compound muscle action potential (CMAP) amplitude. In patients with myasthenia gravis, there may be a progressive decline in the CMAP amplitude within the first four to five stimuli—called a decremental response. The RNS is considered positive when the decrement is greater than 10%.
The SFEMG is the most sensitive diagnostic test for myasthenia gravis, although it is technically more difficult. A specialized needle electrode allows simultaneous recording of the action potential of two muscle fibers innervated by the same motor axon. The variability in time between the two action potentials is called jitter. In patients with myasthenia gravis, the jitter is increased. The SFEMG is positive in more than 95% of patients with generalized myasthenia gravis. The sensitivity of the SFEMG ranges between 85% and 95% in ocular myasthenia gravis.
Evaluation of Conditions Associated With Myasthenia Gravis
Thymic Tumors and Other Malignancies
Thymic abnormalities are often seen in patients with myasthenia gravis. Computed tomography (CT) or magnetic resonance imaging (MRI) scans 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 of 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.
Differential Diagnosis
Studies to rule out other disease in the differential diagnosis of myasthenia gravis are indicated in some patients. For example, in cases with ocular or bulbar symptoms, an MRI of the brain is indicated. CT scanning or ultrasound of the orbits is helpful in the differential diagnosis of ocular myasthenia and thyroid ophthalmopathy. A lumbar puncture may be helpful in ruling out lymphomatous or carcinomatous meningitis in some cases. Blood tests should include thyroid function tests. In patients who have symptoms associated with a rheumatologic disorder, assays for antinuclear antibodies and rheumatoid factor should be performed.
Pulmonary Function Testing
Pulmonary function testing may be performed to help evaluate the patient's ventilatory status and the possibility of ventilatory failure—that is, a myasthenic crisis. Serial testing is advised.
Table 30.1 provides a widely accepted clinical classification system of myasthenia gravis, which was developed by the Myasthenia Gravis Foundation of America.
TABLE 30.1
Clinical Classifications of Myasthenia Gravis
Class I
Any ocular muscle weakness; may have weakness of eye closure; all other muscle strength is normal
Class II
Mild weakness affecting other ocular muscles; may also have ocular muscle weakness of any severity
Class
Predominantly affecting limb, axial muscles, or both; may also have lesser involvement of oropharyngeal
IIa
muscles
Class
Predominantly affecting oropharyngeal, respiratory muscles, or both; may also have lesser or equal
IIb
involvement of limb, axial muscles, or both
Class III
Moderate weakness affecting other ocular muscles; may also have ocular muscle weakness of any severity
Class
Predominantly affecting limb, axial muscles, or both; may also have lesser involvement of oropharyngeal
IIIa
muscles
Class
Predominantly affecting oropharyngeal, respiratory muscles, or both; may also have lesser or equal
IIIb
involvement of limb, axial muscles, or both
Class IV
Severe weakness affecting other ocular muscles; may also have ocular muscle weakness of any severity
Class
Predominantly affecting limb, axial muscles, or both; may also have lesser involvement of oropharyngeal
IVa
muscles
Class
Predominantly affecting oropharyngeal, respiratory muscles, or both; may also have lesser or equal
IVb
involvement of limb, axial muscles, or both; use of a feeding tube without intubation
Class V
Defined by the need for intubation, with or without mechanical ventilation, except when used during routine
postoperative management
From Jaretzki A, Barohn RJ, Ernstoff RM, et al. Myasthenia gravis: recommendations for clinical research standards. Task Force of the Medical Scientific Advisory Board of the Myasthenia Gravis Foundation of America. Neurology. 2000; 55(1):16-23.
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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 Fig. 10.7), alveolar consolidation (see Fig. 10.8), and excessive bronchial secretions (see Fig. 10.11)—the major anatomic alterations of the lungs associated with myasthenia gravis, which may occur when the patient is not properly managed via the Airway Clearance Therapy Protocol, Protocol 10.2, Ventilator Initiation and Management Protocol, Protocol 11.1, and Ventilator Weaning Protocol, Protocol 11.2 (see Fig. 30.1).1
Clinical Data Obtained at the Patient's Bedside
The Physical Examination
Respiratory Rate
•Varies with the degree of respiratory muscle paralysis
•Apnea (in severe cases)
Cyanosis (in Severe Cases)
Chest Assessment Findings
•Diminished breath sounds
•Crackles
Clinical Data Obtained From Laboratory Tests and Special Procedures
Pulmonary Function Test Findings2
(Restrictive Lung Pathology)
Forced Expiratory Volume and 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 ↓
Lung Volume and Capacity Findings
VT
IRV
ERV
RV
↓
↓
↓
↓
VC
IC
FRC
TLC
RV/TLC ratio
↓
↓
↓
↓
N
MAXIMUM INSPIRATORY PRESSURE (MIP) ↓
2Progressive worsening of these values is key to anticipating the onset of ventilatory failure.
Arterial Blood Gases
Moderate to Severe Myasthenia Gravis
Acute Ventilatory Failure With Hypoxemia3 (Acute Respiratory Acidosis)
pH4
PaCO2
4
PaO2
SaO2 or SpO2
↓
↑
↑
↓
↓
(but normal)
3See Fig. 5.3 and Table 5.5 and related discussion for the acute pH, PaCO2, and changes associated with acute ventilatory failure.
4When tissue hypoxia is severe enough to produce lactic acid, the pH and values will be lower than expected for a particular PaCO2 level.
6The DO2 may be normal in patients who have compensated to the decreased oxygenation status with (1) an increased cardiac output, (2) an increased hemoglobin level, or (3) a combination of both. When the DO2 is normal, the O2ER is usually normal.
changes associated with acute ventilatory failure.
values will be lower than expected for a particular PaCO
23 mEq/L, PaO2 86 mm Hg, and SaO2 96% (on a
22 mEq/L, PaO2 204 mm Hg, and SaO2 98%. On the basis of these clinical data, the following SOAP was documented.
22, PaO
24 mEq/L, PaO
24, PaO
24, PaO
changes associated with acute ventilatory failure.
values will be lower than expected for a particular PaCO2 level.