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Perioperative Management of Patients With Muscular Dystrophy
Case Synopsis
An 8-month-old boy undergoes myringotomy tube removal. Previous general anesthesia for tube placement was uneventful. Anesthesia is induced with intravenous thiopental and is maintained with nitrous oxide, halothane, and oxygen via a facemask. After removal of the myringotomy tube, the surgeon decides to perform an adenoidectomy. Airway obstruction at this time necessitates emergency intubation. After succinylcholine 2 mg/kg intravenously, an increase in masseter muscle tone is noted. The electrocardiogram (ECG) monitor shows a wide complex tachycardia progressing to bradycardia. End-tidal carbon dioxide (CO 2 ) of 40 to 50 mm Hg gradually decreases to 25 mm Hg. Arterial saturation decreases from 100% to 80%, then cannot be detected. Halothane is discontinued. Calcium chloride, epinephrine, and sodium bicarbonate are given intravenously. The ECG becomes increasingly dysmorphic, and pulses cannot be palpated. Chest compressions start. Venous blood analysis shows a pH of 7.13, CO 2 tension (P co 2 ) is 73 mm Hg, and serum potassium level is over 10 mmol/L. Calcium, epinephrine, and bicarbonate are repeated. After 13 minutes of cardiopulmonary resuscitation, the ECG shows the return of a narrow complex tachycardia, and systolic blood pressure increases to 100 mm Hg. Twenty minutes after succinylcholine administration, a venous blood sample shows a pH of 7.30, P co 2 of 49 mm Hg, and potassium of 7.1 mmol/L. A urinary catheter reveals red urine. The patient is transported to a pediatric intensive care unit. The creatine kinase (CK) level is 285,760 U/L. The patient is treated with vigorous intravenous hydration. He is discharged home in good condition. DNA studies show a deletion of the dystrophin gene, consistent with a diagnosis of Duchenne muscular dystrophy.
Problem Analysis
Definition
Muscular dystrophies are a clinically and genetically diverse group of hereditary disorders of the structure of striated muscle, characterized by progressive muscle weakness and wasting. The diagnosis of a muscular dystrophy is based on elevated serum CK, myopathic electromyogram features, and muscle biopsy. The morphologic changes common to all forms of muscular dystrophy present a random pattern of normal or hypertrophic muscle fibers, necrotic and necrotizing fibers, and interstitial accumulation of fatty and fibrous tissue. The latter changes result in the characteristic pseudohypertrophy of the calf muscles seen in Duchenne muscular dystrophy.
The previous classification of muscular dystrophies was based on patterns of inheritance and clinical features. A more recently proposed classification takes into account the type, localization, and function of defective proteins involved in the pathogenesis of different muscular dystrophies.
Plasma Membrane–Associated Proteins
Defective plasma membrane–associated proteins or the lack of such proteins causes the most common muscular dystrophies, including Duchenne muscular dystrophy (DMD), Becker muscular dystrophy (BMD), the sarcoglycanopathies, and other forms of limb-girdle muscular dystrophy (LGMD).
Dystrophinopathies
The most common muscular dystrophies are X-linked recessive disorders caused by mutations of the dystrophin gene. Dystrophin is a large sarcolemmal protein essential for maintaining the integrity of the sarcolemma. The severe DMD form results from deficiency of dystrophin. The milder allelic form (BMD) is associated with a reduced amount of the truncated protein. The incidence of DMD is approximately 1 in 3500 live male births. A DMD or BMD phenotype may be expressed by a female patient with the dystrophin gene mutation and an X0 karyotype (Turner syndrome). Affected patients have delayed motor development, and when they start walking, they present with gait abnormalities. By the age of 5 years, muscle weakness is evident and calf pseudohypertrophy develops. Lumbar hyperlordosis and toe-walking result from progressive loss of muscle strength and tendon contractures. By age 12, most patients are confined to a wheelchair. Scoliosis, chest deformity, and diaphragmatic weakness lead to restrictive pulmonary disease by age 16 to 18. Respiratory failure, the most common cause of death, occurs in the third decade of life. Almost all patients have cardiomyopathy, but this rarely causes death. Intellectual impairment is common.
Compared with DMD, BMD has a later onset and a milder clinical course. Symptoms of proximal muscle weakness commonly start between ages 5 and 15, although the onset may be delayed until the third or fourth decade of life. Patients generally ambulate beyond age 15. Calf enlargement occurs early and is prominent. Patients have a short life expectancy, but many live to their thirties or forties. Mental retardation is milder than in DMD. In patients with mild or subclinical BMD, dilated cardiomyopathy may be the presenting feature of the disease. Most BMD patients die of complications of cardiomyopathy.
Sarcoglycanopathies
These disorders are caused by mutations of genes encoding four transmembrane glycoproteins of the sarcoglycan complex. Mutations of any of the four sarcoglycan genes (alpha, beta, gamma, and delta) result in LGMD 2D, 2E, 2C, and 2F. Males and females are similarly affected. Proximal leg muscle weakness generally appears in the second or third decade but may be delayed. Upper limb involvement with scapular winging develops. Diaphragmatic weakness with respiratory insufficiency, cardiomyopathy, congestive heart failure (CHF), and arrhythmias may develop. Intellectual function is normal.
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