Disclosure
Background: Respiratory distress syndrome (RDS), also known as hyaline membrane disease (HMD), is an acute lung disease of the newborn caused by surfactant deficiency. It is seen primarily in neonates younger than 36-38 weeks' gestational age and weighing less than 2500 g. In comparison, HMD tends to occur in neonates younger than 32 weeks' gestational age and weighing less than 1200 g. The incidence and severity of RDS is inversely related to gestational age. RDS is the most common cause of respiratory failure during the first days after birth. In addition to prematurity, other factors contributing to the development of RDS are maternal diabetes, cesarean delivery without preceding labor, fetal asphyxia, and being the second born of twins. In recent years, the outcome of patients with RDS has improved with the increased use of antenatal steroids to improve pulmonary maturity, early postnatal surfactant therapy to replace surfactant deficiency, and gentle techniques of ventilation to reduce barotrauma to the immature lungs. Pathophysiology: RDS is the result of anatomic pulmonary immaturity and a deficiency of surfactant. Pulmonary surfactant synthesis, in type II pneumocytes, begins at 24-28 weeks of gestation, and gradually increases until full gestation. Pulmonary surfactant decreases surface tension in the alveolus during expiration, allowing the alveolus to remain partly expanded, thereby maintaining a functional residual capacity. In premature infants, an absence of surfactant results in poor pulmonary compliance, atelectasis, decreased gas exchange, and severe hypoxia and acidosis. Premature infants must expend a great deal of effort to expand their lungs with each breath, and respiratory failure ensues. The lack of surfactant and the resultant poor compliance of the lungs causes debris consisting of damaged or desquamated cells, exudative necrosis, and leaked protein, which lines the alveolar sacs. On hematoxylin-eosin staining, this lining stains like hyaline cartilage. HMD was originally named for these hyaline membranes, but their presence is not specific for the disease. These hyaline membranes can be seen with other conditions, such as meconium aspiration, bronchopulmonary dysplasia (BPD), and neonatal pneumonia. In addition, hyaline membranes are frequently absent in infants with RDS who die at less than 4 hours of age. (Approximately 4 hours of breathing are usually required for pathologically identifiable hyaline membranes to form.) The disparity between the clinical and pathologic findings has led to use of the term respiratory distress syndrome, or RDS, instead of hyaline membrane disease, or HMD. Hyaline membranes may form in response to pulmonary hemorrhage; pulmonary edema; and various irritants to the terminal airways, alveolar sacs, and alveoli. Intrauterine stress may cause an outpouring of endogenous steroids, which cause the type 2 alveolar-lining cells to mature and produce surfactant. This steroid output also causes the thymus to shrink, as thymic atrophy can be induced by corticosteroids or corticotropin. Therefore, a premature neonate with no thymus is less likely than other neonates to have RDS. Frequency:
Mortality/Morbidity: RDS is a leading cause of mortality in infants and accounts for 20% of all neonatal deaths.
Race: White infants are more commonly affected than black infants Sex: RDS is twice as common in boys as in girls at every gestational age. Age: RDS predominantly occurs in infants younger than 32 weeks' gestational age and in those weighing less than 1200 g.
Clinical Details: Infants with RDS have all of the clinical signs of respiratory distress. The clinical presentation of expiratory grunting (due to partial closure of glottis), tachypnea, subcostal and intercostals retractions, nasal flaring, and cyanosis usually manifests in the first few hours and almost always before 8 hours of age. If symptoms do not develop until after 8 hours of normal breathing, RDS is excluded. On auscultation, air movement is diminished despite vigorous respiratory effort. Cyanosis and hypoxia frequently become severe. Tachypnea, with respiratory rate greater than 60 breaths per minute, develops early. Functional residual capacity and pulmonary compliance are greatly reduced. Cyanosis, apnea, and circulatory collapse are grave clinical prognostic indicators. A mixed respiratory and metabolic acidosis usually develops. Arterial blood gas studies show hypoxemia, hypercapnia, and respiratory acidosis. Hypoglycemia, hyperkalemia, and hypocalcemia are also common. Increased pulmonary vascular resistance develops because of a noncompliant lung, hypoxia, and acidosis. This effect increases the right-to-left shunt through a patent ductus arteriosus (PDA). Perfusion of atelectatic air spaces and uneven distribution of inspired air result in a ventilation-perfusion mismatch that initiates a chain of physiologic events that accounts for most of the findings in RDS. Death often directly results from pulmonary disease. However, it may also result from complications related to hypoxemia (eg, intracranial hemorrhage, disseminated intravascular coagulation [DIC], pulmonary hemorrhage, congestive heart failure [CHF] due to left-to-right shunting through PDA), or air blockage complications of assisted ventilation (eg, pulmonary interstitial emphysema [PIE], pneumothorax, pneumomediastinum, gas embolism). The symptoms of RDS usually peak by the third day, and they may resolve quickly when diuresis starts and when infants begin to need less oxygen and mechanical ventilation. Clinical improvement is accompanied by a rapid fall in pulmonary vascular resistance and a rise in systemic arterial pressure, which sometimes leads to the development of a large left-to-right shunt through a PDA. Therefore, the patient's recovery may be interrupted by the development of CHF and pulmonary edema. In RDS, symptoms appear shortly after birth and always within first 8 hours of life. Respiratory symptoms starting after 8 hours of age are unlikely to be the result of RDS. Findings include dyspnea, inspiratory retractions, tachypnea, nasal flaring, expiratory grunting, and progressive cyanosis. Subxiphoid retraction reflects decrease lung volume. In fetal aspiration syndrome, TTN, and neonatal pneumonia, clinical symptoms appear in 12-24 hours, within 6 hours, and in less than 6 hours, respectively. In addition, lung volumes are usually increased in these conditions. The absence of cyanosis excludes congenital pulmonary lymphangiectasis as a diagnosis. In infants not receiving assisted ventilation, clinical improvement is associated with the slow clearing of the lungs and a patchy return of normal alveolar aeration. No residual changes are observed, and postrecovery pulmonary function is normal. In contrast, in infants of RDS who receive assisted ventilation, residual pulmonary changes are common and referred to as BPD. Preferred Examination: RDS is usually diagnosed with a combination of clinical signs and/or symptoms, chest radiographic findings, and arterial blood gas results.
Meconium Aspiration
Congenital pulmonary alveolar proteinosis
Findings: Classic findings In RDS, the classic chest radiographic findings consist of pronounced hypoaeration, bilateral diffuse reticulogranular opacities in the pulmonary parenchyma, and peripherally extending air bronchograms. The reticulogranularity is due to superimposition of multiple acinar nodules caused by atelectatic alveoli. The development of air bronchograms depends on the coalescence of areas of acinar atelectasis around aerated bronchi and bronchioles. In nonintubated infants, cephalic doming of the diaphragms and hypoexpansion are observed. Radiologic spectrum The radiologic spectrum of RDS ranges from mild to severe and is generally correlated with the severity of the clinical findings. In the early stages of the disease, notable air bronchograms are lacking because the major bronchi lie in the more anterior portions of the lungs and because alveolar atelectasis tends to involve the dependent areas of the lungs, which are posterior in recumbent infants. However, a bubble appearance, which represents overdistended bronchioles and alveolar ducts, can be observed. As RDS progresses, the reticulogranular pattern becomes prominent due to coalescence of the small atelectatic areas. This coalescence leads to larger areas of increased lung opacity. As the anterior portions of the lung become involved with microatelectasis, the granularity becomes uniformly distributed, and air bronchograms can be seen. With increasing severity of disease, progressive opacification of the anterior portions of the lungs cause obscuration of cardiac silhouette and the formation of prominent air bronchograms. With severe disease, the lungs appear opaque and display prominent air bronchograms, with total obscuration of cardiomediastinal silhouette. In infants with mild-to-moderate RDS, hypoaeration and reticulogranular opacities persist for 3-5 days. Clearing from the peripheral to the central areas and from the upper lobe to the lower lobe begins at the end of the first week. Infants with severe RDS have progressive hypoaeration and diffuse bilateral opacities. Superimposed parenchymal hemorrhage may be noted. This type of severe and progressive RDS often leads to death, usually within 72 hours. The radiographic findings of RDS also depend on the timing of the administration of surfactant. Early on, despite prevention with surfactant, the lungs are hypoaerated and have a reticulogranular pattern due to interstitial fluid and atelectatic alveoli. The administration of surfactant usually produces some clearing, which may be symmetrical or asymmetrical; the asymmetry usually disappears in 2-5 days. Because the surfactant is not evenly distributed throughout the lungs, areas of improving lung alternating with areas of unchanged RDS is common. This uneven distribution leads to a radiographic appearance similar to that of other entities, such as neonatal pneumonia and meconium aspiration syndrome. The clearing is sometimes irregular, creating a cystic appearance. Relapse may occur after initial improvement. Infants who are being ventilated with intermittent positive pressure with positive end-expiratory pressure may have well-aerated lungs without air bronchograms. Infants with severe disease may be unable to expand their lungs; they have total opaque radiographs. Late in the course of the disease, pulmonary edema, air leaks, or pulmonary hemorrhage can affect the radiographic appearance. With positive-pressure ventilation, the lungs opacity decreases, and they appear radiographically improved. However, the positive pressure required to aerate the lungs can disrupt the epithelium, producing interstitial and alveolar edema. It can also cause the dissection of air into the interlobar septae and their lymphatics, producing PIE, which has the appearance of tortuous, 1- to 4-mm linear lucencies that are relatively uniform in size. These radiate outward from the hilar regions. The lucencies do not empty on expiration and extend to the periphery of the lungs. PIE can be symmetrical, asymmetrical, or localized to 1 portion of a lung. Peripheral PIE can produce subpleural blebs and ultimately rupture into pleural space to produce pneumothorax (usually tension pneumothorax), or they can extend centrally to produce pneumomediastinum or pneumopericardium. Because infants are supine and because air rises to the highest point of the thorax, the pneumothorax is located paramediastinally, resulting in the sharp mediastinum sign, whereby the mediastinum/heart is sharply outlined by adjacent free air rather than aerated lung tissue. A continuous diaphragm sign, which is caused by air in the mediastinum beneath the heart, may be seen with pneumomediastinum. When alveoli rupture, air may become localized and may coalesce in the parenchyma to produce pulmonary pseudocyst. In addition to parenchymal pseudocysts and PIE, alveolar rupture may allow air to enter the pulmonary venous system, leading to systemic air embolism with intravascular air. After days of ventilatory support, interstitial fibrosis results from the cumulative effect of therapeutic insult to the pulmonary parenchyma. This fibrosis is often accompanied by exudative necrosis and a honeycomb appearance of the lungs on chest radiography. This condition is referred to as BPD. The honeycomb appearance represents focally distended alveolar groups in a scarred, and immature lungs. Because infants with RDS are usually hypoxic, the ductus arteriosus may remain patent. Early in the disease, shunting is from right to left. By the end of first week, shunting becomes left to right as pulmonary artery pressure decreases because of the increased compliance of the healing lungs. Interstitial pulmonary edema may develop. Therefore, when the granular pattern of HMD changes to a homogenously opaque appearance, pulmonary edema due to PDA or early chronic pulmonary changes should be suspected. Radiographic findings in conditions mimicking RDS Meconium aspiration syndrome usually occurs in postterm infants, especially in those with meconium staining. Clinical symptoms usually appear 12-24 hours after birth. (In contrast, clinical symptoms of RDS always appear within the first few hours of life.) The most common radiographic features are hyperaeration and bilateral, diffuse, and grossly patchy areas of increased radiopacity. Pneumothorax in fetal aspiration syndrome is usually not tension pneumothorax; therefore, it often requires no specific therapy. In RDS, the lungs are hypoaerated, and the abnormal lung radiopacities due to alveolar resorption atelectasis are finely granular. In addition, pneumothorax related to RDS is often under tension, and surgical intubation is required. Transient tachypnea of the newborn (TTN) usually occurs in term infants, usually after cesarean delivery. Clinical symptoms usually manifest within 6 hours of birth. Radiographic findings include increased or normal lung volume, with interstitial edema and pleural effusions. In RDS, bilateral reticular or granular parenchymal opacities are present for at least 3-4 days, whereas with transient tachypnea these opacities are fleeting. Hypoaeration is typical of RDS, in contrast to the hyperaeration of transient tachypnea. Neonatal pneumonia is usually associated with premature rupture of membranes. Clinical symptoms appear less than 6 hours after birth. Radiographic findings include perihilar streaking. Neonatal pneumonia often produces hyperaeration of the lungs, but, in general, the areas of pneumonia are focal rather than diffuse. Pleural effusions may be the only distinguishing feature; they are not a feature of uncomplicated RDS but are present in as many as two thirds of patients with pneumonia. Group B beta-hemolytic streptococcal pneumonia often occurs with RDS, or it can mimic the appearance of RDS. Hence, many neonatal units give antibiotics to all neonates in this condition until blood cultures are negative. Differentiating RDS from diffuse pulmonary hemorrhage may be difficult. One feature that aids in the differential diagnosis is the identification of a pleural effusion. Pleural effusions are rare in RDS but are common in pulmonary hemorrhage. Degree of Confidence: If chest images in a premature infant show reticulogranular opacities, RDS can be diagnosed with 90% confidence. False Positives/Negatives: Other entities that may produce opacities similar to those of RDS include immature lung, wet lung disease, neonatal pneumonia, idiopathic hypoglycemia, CHF, maternal diabetes, and early pulmonary hemorrhage. |
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Findings: Homogenous opacification of the lungs due to consolidation of the lower lobes may been seen on upper abdominal ultrasonography. In addition, ultrasonography can be useful to diagnose or exclude a simultaneous or complicating pleural effusion.
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