Hyaline Membrane Lung Disease
Hyaline membrane disease (HMD) is an old name for a severe condition of newborn infants that today is called respiratory distress syndrome (RDS). In the past, while RDS was used for newborns born younger than 38 weeks of gestation but more than 32 weeks, HMD referred to those with respiratory distress born younger than 32 weeks gestation and weighing less than 1,200 grams.
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Symptoms
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Clinical signs of RDS/HMD include grunting during expiration, rapid breathing (greater than 60 breaths per minute), retractions of subcostal and intercostals tissues, nasal flaring and cyanosis (bluish coloring). These signs develop within the first few hours after birth and peak at three days. If untreated, this can lead to lung collapse, acidosis (acidity of blood) and death.
Diagnosis
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RDS/HMD can be diagnosed based on symptoms and clinical signs, plus a chest X-ray and measurement of arterial blood gases (ABG).
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Incidence
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Nearly 40,000 infants are born with RDS/HMD each year and the incidence and severity are related to gestational age and weight at birth. For those born after 34 weeks of gestation, the incidence is 5 percent. At 30 to 34 weeks, it is 25 percent. For those born earlier than 30 weeks of gestation, the rate is 60 percent. The condition is somewhat more common in white than black infants, and twice as common in males than females of any given gestational age. As might be expected, the risk of giving birth to a newborn with RDS/HMS increases with the presence of any factors that increase the risk of premature birth, the major ones being smoking and young maternal age.
Pathophysiology
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RDS/HMD develops as a result of immaturity of the lungs and deficiency of surfactant production. Surfactant is a mixture of "lipo-proteins" (special proteins with attached fat-like entities). With detergent-like properties, surfactant decreases surface tension in alveoli, or air sacks, in healthy lungs. Without surfactant, air sacks can collapse during exhalation.
Production of Surfactant
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Production of surfactant begins at 24 to 28 weeks of gestation, but is not up to peak until roughly the 38th gestational week. Initially, HMD was named for hyaline membranes, which are seen at the severe range of RDS, meaning very low gestational age. Hyaline membranes, however, develop as a result of several other serious respiratory conditions of the newborn. Moreover, since hyaline membranes develop only after a few hours of breathing, even in RDS, hyaline membranes often are not present. Thus, HMD is not an accurate name and today RDS is the accepted label for the condition.
Treatment
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RDS/HMD is treatable with corticosteroids given prior to birth to speed development of the lungs, plus surfactant therapy, which can be sprayed into the airway, and gentle, mechanical ventilation.
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