Infant respiratory distress syndrome (IRDS), also known as surfactant deficiency disorder (SDD), and previously called hyaline membrane disease (HMD), is a syndrome in premature infants caused by developmental insufficiency of pulmonary surfactant production and structural immaturity in the lungs. It can also be a consequence of neonatal infection and can result from a genetic problem with the production of surfactant-associated proteins. IRDS affects about 1% of newborns and is the leading cause of morbidity and mortality in preterm infants. Data have shown the choice of elective caesarean sections to strikingly increase the incidence of respiratory distress in term infants; dating back to 1995, the UK first documented 2,000 annual caesarean section births requiring neonatal admission for respiratory distress. The incidence decreases with advancing gestational age, from about 50% in babies born at 26–28 weeks to about 25% at 30–31 weeks. The syndrome is more frequent in males, Caucasians, infants of diabetic mothers and the second-born of premature twins. IRDS is distinct from pulmonary hypoplasia, another leading cause of neonatal death that involves respiratory distress. The European Consensus Guidelines on the Management of Respiratory Distress Syndrome highlight new possibilities for early detection, and therefore treatment of IRDS. The guidelines mention an easy-to-use rapid point-of-care predictive test that is now available and how lung ultrasound, with appropriate training, expertise and equipment, may offer an alternative way of diagnosing IRDS early.
Signs and symptoms IRDS begins shortly after birth and is manifested by fast breathing (more than 60 breaths per minute), a fast heart rate, chest wall retractions (recession), expiratory grunting, nasal flaring, and blue discoloration of the skin during breathing efforts. As the disease progresses, the baby may develop ventilatory failure (rising carbon dioxide concentrations in the blood) and prolonged cessations of breathing (apnea). Whether treated or not, the clinical course for the acute disease lasts about two to three days. During the first day, the child worsens and requires more support. During the second day, the baby may be remarkably stable on adequate support and resolution is noted during the third day, heralded by a prompt diuresis. Despite huge advances in care, IRDS remains the most common single cause of death in the first month of life in the developed world. Complications include metabolic disorders (acidosis, low blood sugar), patent ductus arteriosus, low blood pressure, chronic lung changes and bleeding in the brain. The syndrome is frequently complicated by prematurity and its additional effect on other organ functions.
Histopathology The characteristic histopathology seen in babies who die from RDS was the source of the name "hyaline membrane disease", first described in 1903 by Hochheim. Waxlike layers of hyaline membrane line the collapsed alveoli of the lung. In addition, the lungs show bleeding, overdistention of airways, and damage to the lining cells.
Pathophysiology The lungs of infants with respiratory distress syndrome are developmentally deficient in a material called surfactant, which helps prevent the collapse of the terminal air spaces (the future site of alveolar development) throughout the normal cycle of inhalation and exhalation. This deficiency of surfactant is related to inhibition from the insulin that is produced in the newborn, especially those of diabetic mothers. Pulmonary surfactant is a complex system of lipids, proteins and glycoproteins that is produced in specialized lung cells called Type II cells or Type II pneumocytes. The surfactant is packaged by the cell in structures called lamellar bodies, and extruded into the air spaces. The lamellar bodies then unfold into a complex lining of the air space. This layer reduces the surface tension of the fluid that lines the alveolar air space. Surface tension is responsible for approximately 2/3 of the inward elastic recoil forces. In the same way that a bubble will contract to give the smallest surface area for a given volume, so the air/water interface means that the liquid surface will tend toward being as small as possible, thereby causing the air space to contract. By reducing surface tension, surfactant prevents the air spaces from completely collapsing on exhalation. In addition, the decreased surface tension allows reopening of the air space with a lower amount of force. Therefore, without adequate amounts of surfactant, the air spaces collapse and are very difficult to expand. Microscopically, a pulmonary surfactant-deficient lung is characterized by collapsed air spaces alternating with hyperexpanded areas, vascular congestion, and, in time, hyaline membranes. Hyaline membranes are composed of fibrin, cellular debris, red blood cells, rare neutrophils and macrophages. They appear as an eosinophilic, amorphous material, lining or filling the air spaces and blocking gas exchange. As a result, blood passing through the lungs is unable to pick up oxygen and unload carbon dioxide. Blood oxygen levels fall and carbon dioxide rises, resulting in rising blood acid levels and hypoxia. Structural immaturity, as manifested by a decreased number of gas exchange units and thicker walls, also contributes to the disease process. Therapeutic oxygen and positive-pressure ventilation, while potentially life-saving, can damage the lung.
Diagnosis The diagnosis is made by the clinical picture and the chest X-ray, which demonstrates decreased lung volumes (bell-shaped chest), absence of the thymus (after about six hours), a small (0.5–1 mm), discrete, uniform infiltrate (sometimes described as a "ground glass" appearance or "diffuse airspace and interstitial opacities") that involves all lobes of the lung and air-bronchograms (i.e. the infiltrate will outline the larger airways passages, which remain air-filled). In severe cases, this becomes exaggerated until the cardiac borders become indiscernible (a 'white-out' appearance).
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