I. Definition. Hyaline membrane disease (HMD) is another name for respiratory distress syndrome (RDS). This clinical diagnosis is warranted in a preterm newborn with respiratory difficulty, including tachypnea (>60 breaths/min), chest retractions, and cyanosis in room air that persists or progresses over the first 48–96 h of life, and a characteristic chest radiographic appearance (uniform reticulogranular pattern and peripheral air bronchograms). The clinical course of the disease varies with the size of the infant, severity of disease, use of surfactant replacement therapy, presence of infection, degree of shunting of blood through the patent ductus arteriosus (PDA), and whether or not assisted ventilation was initiated.
II. Incidence. HMD occurs in 44% of infants with birthweight between 501 and 1500 g. The incidence is inversely proportional to the gestational age and birthweight. The incidence and severity of HMD are expected to decrease after the increase in use of antenatal steroids in recent years. Survival has improved significantly, especially after the introduction of exogenous surfactant, and is now at >90%. During the surfactant era, HMD accounts for <6% of all neonatal deaths, but declines in HMD mortality have been greater for white than for black infants.
III. Pathophysiology. Surfactant deficiency is the primary cause of HMD, often complicated by an overly compliant chest wall. Both factors lead to progressive atelectasis and failure to develop an effective functional residual capacity (FRC). Surfactant is a surface-active material produced by airway epithelial cells called type II pneumocytes. This cell line differentiates, and surfactant synthesis begins at 24–28 weeks' gestation. Type II cells are sensitive to and decreased by asphyxial insults in the perinatal period. The maturation of this cell line is delayed in the presence of fetal hyperinsulinemia. The maturity of type II cells is enhanced by the administration of antenatal corticosteroids and by chronic intrauterine stress such as pregnancy-induced hypertension, intrauterine growth restriction, and twin gestation. Surfactant, composed chiefly of phospholipid (75%) and protein (10%), is produced and stored in the characteristic lamellar bodies of type II pneumocytes. This lipoprotein is released into the airways, where it functions to decrease surface tension and maintain alveolar expansion at physiologic pressures.
A. Lack of surfactant. In the absence of surfactant, the small airspaces collapse; each expiration results in progressive atelectasis. Exudative proteinaceous material and epithelial debris, resulting from progressive cellular damage, collect in the airway and directly decrease total lung capacity. In pathologic specimens, this material stains typically as eosinophilic hyaline membranes lining the alveolar spaces and extending into small airways.
B. Presence of an overly compliant chest wall. In the presence of a chest wall with weak structural support secondary to prematurity, the large negative pressures generated to open the collapsed airways cause retraction and deformation of the chest wall instead of inflation of the poorly compliant lungs.
C. Decreased intrathoracic pressure. The infant with HMD who is <30 weeks' gestational age often has immediate respiratory failure because of an inability to generate the intrathoracic pressure necessary to inflate the lungs without surfactant.
D. Shunting. The presence or absence of a cardiovascular shunt through a PDA or foramen ovale, or both, may change the presentation or course of the disease process. Shortly after birth, the predominant shunting is right to left across the foramen ovale into the ...
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