Infant respiratory distress syndrome ("RDS")(previously called hyaline membrane disease) is a syndrome caused by developmental lack of surfactant and structural immaturity in the lungs of premature infants. RDS affects about 1% of newborn infants. The incidence decreases with advancing gestational age (length of pregnancy), from about 50% in babies born at 26-28 weeks, to about 25% at 30-31 weeks. The syndrome is more frequent in infants of diabetic mothers and in the second born of premature twins.

Clinical course: Respiratory distress begins shortly after birth, and is manifest by a whining noise, flaring of the nostrils and "sucking in" of the chest wall during breathing efforts. The baby may become cyanotic ("blue") from lack of oxygen in the blood. As the disease progresses, the baby may have prolonged cessations of breathing ("apnea"). Untreated, the baby's condition may worsen over the first 24 hours, and death may ensue. Complications include patent ductus arteriosus, high serum potassium levels, low serum calcium, low blood sugar, low blood pressure, edema, and intracranial hemorrhage. Pathology: The characteristic pathology seen in babies who die from RDS was the source of the name "hyaline membrane disease." These waxy-appearing layers line the collapsed tiny air sacs ("alveoli") of the lung. In addition, the lungs show bleeding, over-distention of airways and damage to the lining cells.

Pathophysiology: The lungs are developmentally deficient in a material called surfactant, which allows the alveoli to remain open thoughout the normal cycle of inhalation and exhalation. Surfactant is a complex system of lipids, proteins and glycoproteins which are produced in specialized lung cells called Type II cells. The surfactant is packaged by the cell in structures called lamellar bodies, and extruded into the alveoli. The lamellar bodies then unfold into a complex lining of the alveoli. This layer serves the purpose of reducing the surface tension which would tend to cause the alveoli to collapse in the presence of gas. Without adequate amounts of surfactant, the alveoli collapse and are very difficult to expand. Microscopically, it is characterized by collapsed alveoli alternating with hyperaerated alveoli, vascular congestion and hyaline membranes (resulted from fibrin, cellular debris, red blood cells, rare neutrophils and macrophages). Hyaline membranes appear like an eosinophilic (pink), amorphous material, lining or filling the alveolar spaces and blocking the gases exchange 1. The blood (which normally receives oxygen from the alveolar gas and unloads carbon dioxide into the alveol) passes through the lungs without this vital exchange. Blood oxygen levels fall, and carbon dioxide rises, resulting in rising blood acid levels. Structural immaturity, as manifest by low numbers of alveoli, also contributes to the disease process. It is also clear that the oxygen and breathing treatments used, while life-saving, can also damage the lung. The diagnosis is made by the clinical picture and the chest xray, which has a "ground-glass" appearance.

Prevention: Most cases of hyaline membrane disease can be prevented if mothers who are about to deliver prematurely can be given a hormone-like substance called glucocorticoid. This speeds the maturation of the lungs and surfactant system. For very premature deliveries, glucocorticoid is given without testing the fetal lung maturity. In pregancies of greater than 30 weeks, the fetal lung maturity may be tested by sampling the amount of lipid in the amniotic fluid, obtained by inserting a needle though the mother's abdomen and uterus. The maturity level is expressed a the lecithin-sphingomyelin (or "L/S") ratio. If this ratio is less than 2, the fetal lungs are probably immature, and glucocorticoid is given.

Treatment: Oxygen is given with a small amount of continuous positive airway pressure ("CPAP"), and intravenous fluids are administered to stabilize the blood sugar, blood salts, and blood pressure. If the baby's condition worsens, a breathing tube is inserted into the trachea and intermittent breaths are given by a mechanical device. Medication which simulates surfactant is given through the breathing tube into the lungs. With these treatments, many babies will be breathing normally within 48-72 hours. Smaller premature infants may remain bound to the breathing machine for months, developing a chronic lung disorder called bronchopulmonary dysplasia. The mortality rate for babies greater than 27 weeks gestation is less than 10%.

A similar syndrome in adults is the adult respiratory distress syndrome.

See: Wyman ML. "Neonatal Respiratory Distress" in Essentials of Pediatric Intensive Care (2 volume set) by Levin and Morriss, 1997.

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Images of hyaline membranes