Pathology of the Lung in Surfactant-Treated Neonates

Respiratory distress syndrome (RDS) is associated with prematurity-related deficiency of surfactant. Surfactant replacement therapy has been used in premature infants to prevent RDS or reduce its severity. In this study we describe the pathology of the lungs after surfactant replacement therapy. All the neonatal autopsies during the years 1989 and 1990 (n = 235) were examined. Infants ≥ 31 weeks gestation, with congenital anomalies or who lived more than 2 weeks were excluded from the study. Infants who had received intratracheal Survanta, a modified surfactant extracted from cow lung (n = 14), were compared with infants who did not receive exogenous surfactant (n = 20). The two groups were statistically comparable in terms of weight, gestational and postnatal age, gender, and clinical management. H&E-stained lung sections were examined independently by two pathologists without knowledge of surfactant treatment status; any discrepancies in histological evaluation were resolved by joint review. Nine histological features were evaluated including hyaline membranes, necrosis of the epithelium, hemorrhage, edema, inflammation, metaplasia, arteriolar muscular hyperplasia, interstitial fibrosis, and pulmonary interstitial emphysema (PIE). Histological changes were graded from 0 to 3+. When it was present, cerebral periventricular-intraventricular hemorrhage (PVH-IVH) was graded 1-4. The presence or absence of sepsis and necrotizing enterocolitis (NEC) were also determined. Comparisons between patient groups were performed using the Mann-Whitney U, Student's t and χ² tests. The severity of hyaline membrane disease, PIE, and epithelial necrosis was less severe in the surfactant-treated group than in the untreated group. There were no differences between the two groups in the degree of pulmonary hemorrhage or in the incidence of PVH-IVH, sepsis, or NEC.     

Surfactant phospholipids and surface activity among preterm infants with respiratory distress syndrome who develop bronchopulmonary dysplasia

Our objective was to determine if preterm infants with respiratory distress syndrome who develop bronchopulmonary dysplasia have abnormalities in surfactant phospholipids and/or function. Tracheal aspirate samples obtained from preterm infants with respiratory distress syndrome on days 1, 3-5, 7-10, 14-17, 21-24 and 27-30 were analyzed for total phospholipids and phospholipids fractions by determination of total phospholipid phosphorus and thin layer chromatography, respectively. Surfactant properties were assessed with captive bubble surfactometer. Sixteen out of 56 (29%) infants died during the first 30 d of life. Infants who died were more immature, required more ventilatory support and had a surfactant with lower surface-tension-reducing properties than infants who survived (p < 0.05). Surviving infants were divided into group I (no bronchopulmonary dysplasia at 27-30 d, n = 25) and group II (with bronchopulmonary dysplasia at 27-30 d, n = 15). No significant differences in concentrations of surfactant phospholipids nor measurements of surface tension were noted among groups of infants. Surfactant therapy after birth was associated with a significant increase in concentrations of total phospholipids, lecithin, phosphatidylinositol and lower surface-tension measurements at 3-5 d of age among surviving infants (p < 0.01). Abnormalities in concentrations of surfactant phospholipids or surfactant function could not be demonstrated during the first month of life among preterm infants with respiratory distress syndrome who develop bronchopulmonary dysplasia.     

New developments in neonatal respiratory management

Respiratory distress syndrome (RDS) is the major cause of respiratory failure in preterm infants due to immature lung development and surfactant deficiency. Although the concepts and methods of managing respiratory problems in neonates have changed continuously, determining appropriate respiratory treatment with minimal ventilation-induced lung injury and complications is crucially important. This review summarizes neonatal respiratory therapy's advances and available strategies (i.e., exogenous surfactant therapy, noninvasive ventilation, and different ventilation modes), focusing on RDS management.     

Pathophysiology of respiratory distress syndrome

Respiratory distress syndrome (RDS) is a major cause of neonatal mortality and morbidity, especially in preterm infants. Its aetiology includes developmental immaturity of the lungs, particularly of the surfactant synthesizing system. Surfactant is produced, stored and recycled by type II pneumocytes and is detectable from about 24 weeks’ gestation. It is a mixture of phospholipids, neutral lipids and proteins and is spread as a film over the alveolar surface to lower surface tension and to prevent alveolar collapse. The resulting clinical correlates of RDS can be predicted from the immature lung structure and atelectasis which occur due to surfactant deficiency. Various clinical factors are known to dysregulate surfactant production and function, leading to the development of RDS. Apart from preventing the incidence of prematurity, antenatal steroids and prophylactic surfactant are of proven benefit in reducing the incidence of RDS.     

Hyaline Membrane Disease: I: Cellular Changes

Cellular changes were studied in 1 μ thick sections of lungs from 84 perinatal deaths, including 44 with hyaline membrane disease (HMD). The presence or absence of osmiophilic granules was related to surface tension measurements in 69 cases. The presence of numerous granules usually indicated normal surfactant and their absence a lack. It is concluded that the granules represent surfactant material.Osmiophilic granules were found first at 20 weeks'' gestation (in 2 out of 6 fetuses). After 24 weeks'' gestation almost all infants had many granules, except those with HMD.The earliest stages in hyaline membrane formation consisted of interstitial oedema accompanied by localized areas of necrosis and desquamation of alveolar epithelial cells. Osmiophilic granules were virtually absent.Infants dying at a later stage of the disease showed more extensive hyaline membranes, but from 36 hours almost all cases displayed some signs of repair of the denuded alveolar surfaces. In 5 out of 10 cases with evidence of repair, normal values for surface tension were obtained.In the late stages of HMD some of the cells lining the alveoli were highly abnormal. They consisted of large thick squames with very few capillaries in apposition to them; the appearances were thus consistent with a severe degree of alveolo-capillary block.     

Surfactant replacement therapy for respiratory distress syndrome. American Academy of Pediatrics. Committee on Fetus and Newborn

Respiratory failure secondary to surfactant deficiency is a major cause of morbidity and mortality in low birth weight immature infants. Surfactant therapy substantially reduces mortality and respiratory morbidity for this population. The statement summarizes the indications for surfactant replacement therapy. Because respiratory insufficiency may be a component of multiorgan dysfunction in sick infants, surfactant should be administered only at institutions with qualified personnel and facilities for the comprehensive care of sick infants.     

Pulmonary surfactant for neonatal respiratory disorders

Surfactant therapy has revolutionized neonatal care and is used routinely for preterm infants with respiratory distress syndrome. Recent investigation has further elucidated the function of surfactant-associated proteins and their contribution toward surfactant and lung immune defense functions. As the field of neonatology moves away from intubation and mechanical ventilation of preterm infants at birth toward more aggressive use of nasal continuous positive airway pressure, the optimal timing of exogenous surfactant therapy remains unclear. Evidence suggests that preterm neonates with bronchopulmonary dysplasia and prolonged mechanical ventilation also experience surfactant dysfunction; however, exogenous surfactant therapy beyond the first week of life has not been well studied. Surfactant replacement therapy has been studied for use in other respiratory disorders, including meconium aspiration syndrome and pneumonia. Commercial surfactant preparations currently available are not optimal, given the variability of surfactant protein content and their susceptibility to inhibition. Further progress in the treatment of neonatal respiratory disorders may include the development of "designer" surfactant preparations.     

Plasminogen did not affect lung function in surfactant-treated preterm lambs

Preterm infants with respiratory distress syndrome develop fibrin-rich hyaline membranes within the alveoli and have depressed fibrinolytic activity, which is thought to be due to a relative deficiency of plasminogen. Local fibrin deposition inhibits surfactant function and amplifies inflammation. We hypothesized that plasminogen administration to surfactant-treated preterm lambs would prevent fibrin-rich hyaline membrane formation, resulting in the amelioration of lung pathology and improved lung function. We randomly treated preterm lambs (gestational age 127-129 days) with either 16 mg of lysine-plasminogen (n = 10) or saline (n = 10), and ventilated them for 5 h. There were no significant differences in physiologic measurements of lung function (ventilation efficiency index, oxygenation index, dynamic compliance, quasi-static pressure volume curve), measures of lung injury (alveolar wash protein content and (125)I-albumin recovery) or surfactant pool size. The degree and extent of bronchiolar erosion and hyaline membrane formation were similar in the two groups. Plasminogen administration did not improve lung function or prevent hyaline membrane formation in surfactant-treated lambs.     

Survival of very preterm infants: Epipage, a population based cohort study

OBJECTIVE: To evaluate the outcome for all infants born before 33 weeks gestation until discharge from hospital. DESIGN: A prospective observational population based study. SETTING: Nine regions of France in 1997. PATIENTS: All births or late terminations of pregnancy for fetal or maternal reasons between 22 and 32 weeks gestation. MAIN OUTCOME MEASURE: Life status: stillbirth, live birth, death in delivery room, death in intensive care, decision to limit intensive care, survival to discharge. RESULTS: A total of 722 late terminations, 772 stillbirths, and 2901 live births were recorded. The incidence of very preterm births was 1.3 per 100 live births and stillbirths. The survival rate for births between 22 and 32 weeks was 67% of all births (including stillbirths), 85% of live births, and 89% of infants admitted to neonatal intensive care units. Survival increased with gestational age: 31% of all infants born alive at 24 weeks survived to discharge, 78% at 28 weeks, and 97% at 32 weeks. Survival among live births was lower for small for gestational age infants, multiple births, and boys. Overall, 50% of deaths after birth followed decisions to withhold or withdraw intensive care: 66% of deaths in the delivery room, decreasing with increasing gestational age; 44% of deaths in the neonatal intensive care unit, with little variation with gestational age. CONCLUSION: Among very preterm babies, chances of survival varies greatly according to the length of gestation. At all gestational ages, a large proportion of deaths are associated with a decision to limit intensive care.     

Surfactant Replacement Therapy in Preterm Neonates: A Comparison of Postmortem Pulmonary Histology in Treated and Untreated Infants

New histological lesions have been reported in the lungs of preterm neonates treated with surfactant for respiratory distress syndrome (RDS). Globular deposits of hyaline material in parenchymal air spaces, absence of hyaline membranes, and increased interstitial cellularity and edema without associated fibrosis have been described. Fifteen histological findings were assessed in the lung pathology of 76 infants with RDS from three study groups. Group I (24 infants) died in the presurfactant era (before 1982), group II (26 infants) died despite having surfactant treatment, and group III (26 infants) were either untreated controls or did not receive surfactant for other reasons. The three groups were comparable in respect of sex and survival time. All infants were 34 weeks of gestation or less. Infants with a significant congenital abnormality or pulmonary hypoplasia were excluded. The 76 cases were assessed independently and “blindly” by two pathologists. The histological findings assessed were alveolar collapse; epithelial necrosis, proliferation, and metaplasia; hyaline membranes; dilated lymphatics; pulmonary interstitial emphysema; interstitial edema, inflammation, and fibrosis; arteriolar muscular hyperplasia; interstitial and intra-alveolar hemorrhage; massive pulmonary hemorrhage; and pneumonia. No significant differences were found in any of the histological findings between the three groups. The hyaline membranes seen in the surfactant-treated infants were identical to those in the untreated lungs and were of the characteristic linear type. Interstitial fibrosis, inflammation, and edema were present in all three groups. It has also been suggested that surfactant therapy protects preterm infants from interstitial hemorrhage but predisposes them to intra-alveolar hemorrhage. No significant difference in the incidence of intra-alveolar and interstitial hemorrhage in the three groups was identified.     

Phosphatidylglycerol in tracheal aspirates for diagnosis of hyaline membrane disease.

Lecithin:sphingomyelin ratio and phosphatidylglycerol were determined by a rapid, simple method in tracheal aspirates obtained from 132 newborn infants with respiratory diseases, sixty five of whom developed hyaline membrane disease. Phosphatidylglycerol determination was more sensitive (97%) than lecithin:sphingomyelin ratio, but their specificities were similar (76%).     

Surfactant abnormalities in infants with severe viral bronchiolitis

To determine whether abnormalities of pulmonary surfactant occur in infants with acute viral bronchiolitis, surfactant indices were measured in lung lavage fluid from 12 infants with severe bronchiolitis and eight infants without lung disease. Compared with controls, the bronchiolitis group showed deficiency of surfactant protein A (1.02 v 14.4 micrograms/ml) and disaturated phosphatidylcholine (35 v 1060 micrograms/ml) which resolved as the disease improved. Surfactant functional activity was also impaired (minimum surface tension 22 v 17 mN/m). These findings indicate that surfactant abnormalities occur in bronchiolitis, and may represent one of the pathophysiological mechanisms causing airway obstruction.     

History of surfactant up to 1980

Remarkable insight into disturbed lung mechanics of preterm infants was gained in the 18th and 19th century by the founders of obstetrics and neonatology who not only observed respiratory failure but also designed devices to treat it. Surfactant research followed a splendid and largely logical growth curve. Pathological changes in the immature lung were characterized in Germany by Virchow in 1854 and by Hochheim in 1903. The Swiss physiologist von Neergard fully understood surfactant function in 1929, but his paper was ignored for 25 years. The physical properties of surfactant were recognized in the early 1950s from research on warfare chemicals by Pattle in Britain and by Radford and Clements in the United States. The causal relationship of respiratory distress syndrome (RDS) and surfactant deficiency was established in the USA by Avery and Mead in 1959. The Australian obstetrician Liggins induced lung maturity with glucocorticoids in 1972, but his discovery was not fully believed for another 20 years. A century of basic research was rewarded when Fujiwara introduced surfactant substitution in Japan in 1980 for treatment and prevention of RDS.     

Fetal lung hypoplasia: biochemical and structural variations and their possible significance.

Quantitative biochemical criteria for lung growth and maturation were compared with the histological appearances in hypoplastic lungs from 20 fetuses and newborn infants. Cases associated with oligohydramnios showed a characteristic series of changes with narrow airways, retardation of epithelial and interstitial growth, delay in development of blood-air barriers, and low concentrations of phospholipid phosphorus, lecithin phosphorus, total palmitate, and lecithin palmitate. The growth and maturation arrest appeared to affect the peripheral part of the acinus. Examples of other types of lung hypoplasia showed different features. Hypoplastic lungs from infants with normal or increased amniotic fluid were of mature structure with phospholipid concentrations similar to those of infants with normally developed lungs at term. The hypoplastic left lung in 2 cases of congenital diaphragmatic hernia had an immature structure with low phospholipid concentrations, whereas the right lung has structurally and biochemically more mature. It is suggested that fetal lung growth may be impaired by any influence which reduces thoracic volume but that maturation arrest is due specifically to loss of the ability to retain lung liquid.     

Surfactant protein B deficiency: radiographic manifestations

Surfactant is a complex structure primarily composed of phospholipids, but containing essential proteins as well. Congenital deficiency of Surfactant Protein-B (SPB) has recently been documented for the first time in two siblings. The pathologic findings in these infants was that of congenital pulmonary alveolar proteinosis and the radiographic manifestations were strikingly similar to hyaline membrane disease.     

FIBRINOLYTIC ACTIVITY IN LUNG TISSUE FROM NEONATES WITH HYALINE MEMBRANE DISEASE

The fibrinolytic activity of lung tissue was studied with Todd's histochemical method in a material of 29 newborn infants containing normal lungs, hyaline membrane disease, atelectasis and massive pulmonary haemorrhage. No significant difference was found between a group of hyaline membrane lungs and a group of atelectatic lungs without hyaline membranes. In one case with massive pulmonary haemorrhage the activity was high. Our findings make it questionable whether a lack of plasminogen activator in the lung of infants with hyaline membrane disease is a consistent finding and argue against an alteration of the intrapulmonary fibrinolytic system being of any major importance in the formation of the membranes.     

Inherited disorders of neonatal lung diseases

Although genetic factors are assumed to have a role in the etiology of respiratory distress syndrome (RDS), specific genes underlying this susceptibility are incompletely known. The most promising candidates are the genes coding for the lung-specific protein components of the surfactant. In congenital absence of surfactant protein A in mice, lung mechanics or surfactant homeostasis is normal. However, there is an increased susceptibility to infections. The major surfactant protein A alleles, 6A(2) and 1A(0), are the general high-risk RDS alleles, while the allele 6A(3) carries a decreased risk of RDS. The allele 6A(6) is also over-represented in infants with bronchopulmonary dysplasia. To date, no human infants who lack surfactant protein A have been identified, and the human respiratory phenotype associated with the 1A(0) allele has been demonstrated to be variable, therefore, surfactant protein A polymorphisms are not currently useful for estimation of individual risk of having an affected infant. Surfactant protein B (SP-B) plays an essential role in the structure of tubular myelin. Mutations resulting in an absence of surfactant protein B have been identified. They cause a recessively inherited, progressive respiratory disease. More than 27 loss of function mutations have been identified in the surfactant protein B gene that result in lethal neonatal respiratory failure. Of the several known common variants of the surfactant protein B gene, the most common mutation is 121ins22 that accounts for 60-70% of the mutant cases. Although the frequency of the 121ins2 mutation is rare, the consistent phenotype is exhibited by infants with a homozygous genotype. The clinical presentation in infants homozygous for the 121ins2 mutation is full-term infants who develop respiratory distress within the first 12-24 hours of life. Surfactant replacement therapy fails to reverse this outcome, and without lung transplantation, they expire within the first 1-6 months of life. Surfactant protein B gene mutations may also result in milder phenotypes. These mutations resulting in reduced synthesis of SP-B appear to be family-specific and result in respiratory distress, but sometimes with more gradually progressive or chronic respiratory failure. Surfactant protein C plays a role in the stabilization of surfactant and may also have a role in the intracellular processing of the surfactant complex. Surfactant protein B is important in the intracellular processing and production of surfactant protein C. Although surfactant protein C-deficient mice are viable and survive to adulthood without obvious pulmonary abnormalities, their lung have reduced viscoelasticty. Human respiratory disease in the neonatal period caused by loss-of-function mutations in the surfactant protein C gene has not been identified. However, an autosomal dominant inherited mutation at the surfactant protein C gene causes chronic interstitial lung disease. Surfactant protein D is a member of the collectin family like surfactant protein A, therefore it opsonizes pathogens and enhances their phagocytosis by alveolar macrophages and neutrophils. Unlike surfactant protein A, it does not contribute to lowering surface tension. Surfactant protein D-deficient mice have no respiratory abnormalities at birth, but it causes development of emphysema and predisposition to specific infections. No human infant or child with respiratory distress and mutation in the surfactant protein D gene has been identified.     

Mechanisms to explain surfactant responses

Surfactant is now standard of care for infants with respiratory distress syndrome. Surfactant treatments are effective because of complex metabolic interactions between surfactant and the preterm lung. The large treatment dose functions as substrate; it is taken up by the preterm lung and is reprocessed and secreted with improved function. The components of the treatment surfactant remain in the preterm lung for days. If lung injury is avoided, then surfactant inhibition is minimized. Prenatal corticosteroids complement surfactant to further enhance lung function. The magic of surfactant therapy results from the multiple interactions between surfactant and the preterm lung.     

Respiratory distress syndrome

RDS, due to surfactant deficiency or abnormality, remains a major cause of neonatal mortality and morbidity. It is now a condition primarily affecting very immature infants. Extensive research has been directed at optimising respiratory support of premature infants with RDS. Effective prophylaxis is now available both maternally administered and as surfactant replacement therapy.     

Disturbed surface properties in preterm infants with pneumonia

Congenital pneumonia in preterm infants is often associated with respiratory insufficiency requiring mechanical ventilation. This study was performed to show whether pneumonia in these infants is associated with an inhibition or deficiency of surfactant. The ratio of lecithin and sphingomyelin (L/S ratio) and minimal surface tension were determined in pharyngeal aspirates from 90 term born infants (healthy) and in tracheal aspirates from preterm infants with wet lung (n = 13), congenital pneumonia (n = 21) and respiratory distress syndrome (RDS) (n = 90). The L/S ratio was lower (p < 0.0001) in the RDS group (8.6) when compared with healthy (48.6), wet lung (42.9) and pneumonia (28.9). Surface tension was higher (p < 0.001) in RDS (37 mN/m) and pneumonia (33.7) when compared with healthy (22.9) or wet lung (21.2). For infants with RDS, L/S ratio <16.5 detects surfactant deficiency with 96% specificity and 70% sensitivity, surface tension >29 mN/m represents surfactant inhibition (specificity 97%, sensitivity 92%). Using these cut-off values in infants with pneumonia, 81% had a sufficient amount of surfactant but only 21% of infants with pneumonia had appropriate surface tension. Our study shows that lung effluent of respiratory insufficient infants with pneumonia, who need mechanical ventilation, has disturbed surface properties despite a sufficient amount of surfactant. In these infants, surfactant substitution could be beneficial.