Chronic intrauterine meconium aspiration causes fetal lung infarcts, lung rupture, and meconium embolism

Colloidal iron staining used for amniotic fluid in lung sections of stillborn infants in histopathology

In forensic pathology, intrauterine amniotic fluid aspiration must be examined in lung sections of a stillborn fetus in late pregnancy. Alcian blue staining and immunohistochemistry are routinely performed using anti-cytokeratin antibody to detect the components. Colloidal iron staining is another method for acidic mucus polysaccharides. Retrospectively, we examined 15 forensic cases of stillborn and neonatal deaths with autopsies conducted at our department. Colloidal iron staining caused positive features of amniotic mucin in 14 cases, but Alcian blue staining could not be confirmed clearly in 8 cases because of weak color development. For detection of amniotic mucin, colloidal iron staining showed a richer color with contrast than other methods. Moreover, mucin masses were observed in two live-born infants, and in all stillborn infants, one of which was clinically diagnosed as having meconium aspiration syndrome. Non-specific reaction to iron ions such as hemosiderin-phagocytic macrophages might occur in the lung sections. However, colloidal iron staining should be the alternative routine for lung tissue from stillborn infants.

Meconium aspiration syndrome: a role for fetal systemic inflammation

BACKGROUND

Meconium aspiration syndrome (MAS) is a leading cause of morbidity and mortality in term infants. Meconium-stained amniotic fluid (MSAF) occurs in approximately 1 of every 7 pregnancies, but only 5% of neonates exposed to MSAF develop MAS. Why some infants exposed to meconium develop MAS while others do not is a fundamental question. Patients with MSAF have a higher frequency of intraamniotic inflammation/infection than those with clear fluid. We propose that fetal systemic inflammation is a risk factor for the development of MAS in patients with MSAF.

OBJECTIVE

We sought to investigate whether intraamniotic inflammation and funisitis, the histopathologic landmark of a fetal inflammatory response, predispose to MAS.

STUDY DESIGN

A prospective cohort study was conducted from 1995 through 2009. Amniotic fluid (AF) samples (n = 1281) were collected at the time of cesarean delivery from women who delivered singleton newborns at term (gestational age ≥38 weeks). Intraamniotic inflammation was diagnosed if the AF concentration of matrix metalloproteinase-8 was >23 ng/mL. Funisitis was diagnosed by histologic examination if inflammation was present in the umbilical cord.

RESULTS

The prevalence of MSAF was 9.2% (118/1281), and 10.2% (12/118) of neonates exposed to MSAF developed MAS. There were no significant differences in the median gestational age or umbilical cord arterial pH at birth between neonates who developed MAS and those who did not (each P > .1). Mothers whose newborns developed MAS had a higher median of AF matrix metalloproteinase-8 (456.8 vs 157.2 ng/mL, P < .05). Newborns exposed to intraamniotic inflammation had a higher rate of MAS than those who were not exposed to intraamniotic inflammation [13.0% (10/77) vs 0% (0/32), P = .03], as did those exposed to funisitis [31.3% (5/16) vs 7.3% (6/82); relative risk, 4.3; 95% confidence interval, 1.5-12.3]. Among the 89 newborns for whom both AF and placental histology were available, MAS was more common in patients with both intraamniotic inflammation and funisitis than in those without intraamniotic inflammation and funisitis [28.6% (4/14) vs 0% (0/28), P = .009], while the rate of MAS did not show a significant difference between patients with intraamniotic inflammation alone (without funisitis) and those without intraamniotic inflammation and funisitis [10.9% (5/46) vs 0% (0/28)].

CONCLUSION

The combination of intraamniotic inflammation with fetal systemic inflammation is an important antecedent of MAS. This concept has implications for the understanding of the mechanisms of disease responsible for MAS and for the development of prognostic models and therapeutic interventions for this disorder.

Bacteria and endotoxin in meconium-stained amniotic fluid at term: could intra-amniotic infection cause meconium passage?

BACKGROUND

Meconium-stained amniotic fluid (MSAF) is a common occurrence among women in spontaneous labor at term, and has been associated with adverse outcomes in both mother and neonate. MSAF is a risk factor for microbial invasion of the amniotic cavity (MIAC) and preterm birth among women with preterm labor and intact membranes. We now report the frequency of MIAC and the presence of bacterial endotoxin in the amniotic fluid of patients with MSAF at term.

MATERIALS AND METHODS

We conducted a cross-sectional study including women in presumed preterm labor because of uncertain dates who underwent amniocentesis, and were later determined to be at term (n = 108). Patients were allocated into two groups: (1) MSAF (n = 66) and (2) clear amniotic fluid (n = 42). The presence of bacteria was determined by microbiologic techniques, and endotoxin was detected using the Limulus amebocyte lysate (LAL) gel clot assay. Statistical analyses were performed to test for normality and bivariate comparisons.

RESULTS

Bacteria were more frequently present in patients with MSAF compared to those with clear amniotic fluid [19.6% (13/66) versus 4.7% (2/42); p < 0.05]. The microorganisms were Gram-negative rods (n = 7), Ureaplasma urealyticum (n = 4), Gram-positive rods (n = 2) and Mycoplasma hominis (n = 1). The LAL gel clot assay was positive in 46.9% (31/66) of patients with MSAF, and in 4.7% (2/42) of those with clear amniotic fluid (p < 0.001). After heat treatment, the frequency of a positive LAL gel clot assay remained higher in the MSAF group [18.1% (12/66) versus 2.3% (1/42), p < 0.05]. Median amniotic fluid IL-6 concentration (ng/mL) was higher [1.3 (0.7-1.9) versus 0.6 (0.3-1.2), p = 0.04], and median amniotic fluid glucose concentration (mg/dL) was lower [6 (0-8.9) versus 9 (7.4-12.6), p < 0.001] in the MSAF group, than in those with clear amniotic fluid.

CONCLUSION

MSAF at term was associated with an increased incidence of MIAC. The index of suspicion for an infection-related process in postpartum women and their neonates should be increased in the presence of MSAF.

Meconium aspiration in the midtrimester fetus: an autopsy study

We studied the incidence, severity, and significance of intrauterine meconium aspiration (IUMA) in the midtrimester fetus. Lung sections from 219 midtrimester fetuses were examined for meconium, identified by finding meconium corpuscles. There were 21 cases (9.6%) with IUMA. Gestational age ranged from 13 to 23 weeks (mean 18.7 weeks); 11 were female, and 10 were male. Eighteen were spontaneous abortions, 11 of which were macerated, and 1 of these was associated with an intrauterine contraceptive device. There were 3 terminations of pregnancy, 2 for fetal abnormality and 1 for severe preeclampsia. One fetus was conceived by in vitro fertilization. Fifteen cases (71.4%) showed slight IUMA, 5 cases (23.8%) showed moderate IUMA, and in 1 case (4.8%) IUMA was massive. The incidence of IUMA in an autopsy series of midtrimester fetuses was 9.6%. The amount of aspirated meconium was usually small and possibly physiologic. When IUMA was moderate or massive, it was associated with severe abnormality of the umbilical cord or placenta, particularly subchorionic hematoma.

The Respiratory System

One of the most critical events of birth is the conversion of the fluid-filled lung, unimportant to fetal intrauterine existence, into a hollow organ distended with air and capable of gaseous exchange sufficient to support life. Indeed, it has been argued that the major determinant of perinatal survival is respiratory function (Wigglesworth and Desai 1982).

The failure to make this conversion adequately may lead, directly or indirectly, to infant death, and the pathologist often needs to assess the contribution made by respiratory inadequacy to the sequence of events leading to death. In the preterm infant, problems are mainly related to pulmonary immaturity and associated therapy. In the mature infant, birth asphyxia primarily results in cerebral damage but can engender significant respiratory complications when associated with aspiration of meconium. Even in stillbirths, where primary pulmonary pathology is rarely a cause of death, lung pathology may provide clues to antecedent events. Poor lung growth and maturation may point to the presence of pathology elsewhere. Consequently, adequate pathological investigation of the fetal or infant respiratory system is critical in any perinatal autopsy.

Meconium induced IL-8 production and intratracheal albumin alleviated lung injury in newborn pigs

We have recently shown that albumin added to meconium before intratracheal instillation in newborn pigs limits detrimental effect on the lungs and reduces increase of IL-8. The aim of this study was to test the effect of albumin instillation as rescue treatment in meconium aspiration syndrome (MAS). MAS was induced in hypoxic piglets by lung instillation of meconium (MAS I = 675 mg/kg, n=12; MAS II=540 mg/kg, n=14). Morbidity and mortality differed (MAS I, dead=7/12; MAS II, dead=5/14). MAS groups were randomized to postmeconium instillation of either bovine albumin (30%, 1.4 mL/kg; MAS I, n=6; MAS II, n=7) or isotonic saline (9 mg/mL, 1.4 mL/kg; MAS I, n=6; MAS II, n=7). The controls (n=4) were tested by sequential instillation of saline (9 mg/mL, 5 mL/kg) and albumin (30%, 1.4 mL/kg). Lung function and gas exchange deteriorated significantly after instillation of meconium [oxygenation index (OI): MAS I, +814%; MAS II, +386%; ventilation index (VI): MAS I, +256%; MAS II, +162%; compliance: MAS I, -53%; MAS II, -44%]. Increases of tracheal IL-8 correlated to deterioration of lung function were 10- (MAS I) and 5-fold (MAS II) (p <0.001). Lung compliance was higher in albumin instillation versus saline instillation (MAS I, p=0.008; MAS II, p=0.002). Albumin did not influence intergroup differences in IL-8, hemodynamics, OI, or VI. MAS-induced IL-8 increases correlated with deterioration of lung function (OI, VI, and compliance). Rescue treatment with albumin in meconium aspiration improved lung compliance in piglets and may represent a new therapeutic approach to MAS.

Severe fetal placental vascular lesions in term infants with neurologic impairment

OBJECTIVE

This study tests the hypothesis that placental disease can identify antepartum processes that either progress into the intrapartum period or predispose to intrapartum brain injury.

STUDY DESIGN

Lesions that affect large fetal vessels were compared in the placentas of 125 neurologically impaired term infants who were the focus of clinical negligence litigation and 250 consecutive singleton deliveries of >/=36 weeks of gestation.

RESULTS

One or more of 4 severe placental fetal vascular lesions (fetal thrombotic vasculopathy, chronic villitis with obliterative fetal vasculopathy, chorioamnionitis with severe fetal vasculitis, and meconium-associated fetal vascular necrosis) were found in 51% of index cases versus 10% of the comparison group ( P <.0001). Prevalence of these lesions in the 64 infants with cerebral palsy was 52% ( P <.0001).

CONCLUSION

Severe fetal placental vascular lesions are correlated highly with neurologic impairment and cerebral palsy. Their nature, duration, and anatomic location make them strong candidates for the antepartum processes that place fetuses at risk for brain injury during the intrapartum period.

Complement activation reflects severity of meconium aspiration syndrome in newborn pigs

Meconium aspiration syndrome (MAS) is a serious condition in newborns, associated with a poorly characterized inflammatory reaction. The aim of this study was to investigate a possible role for complement in pulmonary pathophysiology and systemic inflammation in experimental MAS. MAS was induced by instillation of meconium into the lungs of 12 hypoxic piglets. Six controls received saline under otherwise identical conditions. Hemo- and lung dynamics were recorded for 5 h. Plasma complement activation, revealed by the terminal sC5b-9 complex (TCC), and cytokines were measured by enzyme immunoassays. TCC increased substantially in MAS animals compared with controls (p <0.0005). The increase in TCC correlated with lung dysfunction: closely with oxygenation index (r=0.51, p <0.0001) and ventilation index (r=0.64, p < 0.0001) and inversely with lung compliance (r=-0.22, p=0.05). IL-1beta and tumor necrosis factor-alpha increased significantly in MAS animals compared with the controls (p=0.004 and 0.008, respectively). The cytokine increase occurred later than TCC and showed correlations with lung dysfunction similar to TCC. IL-10 did not discriminate between MAS animals and controls (p=0.32). Finally, the subgroup of MAS animals that died (n=5) had substantially higher TCC concentration compared with the surviving MAS animals (n=7; p <0.0005). TCC increased substantially in MAS and was closely correlated to lung dysfunction. Complement activation preceded cytokine release, which may suggest a primary role for complement in the pathophysiology of MAS.

Postmortem findings in term neonates

Neonatal deaths in infants born at term are relatively rare in the USA, occurring in 0.9/1000 live births. Congenital malformations, perinatal asphyxia, infections and inborn errors of metabolism are the leading causes. Chromosomal malformation syndromes, congenital heart disease, pulmonary hypoplasia and severe neural tube defects comprise the majority of lethal malformations. Several skeletal dysplasias are lethal in the newborn infant. Group B Streptococcus still plays a major role in neonatal mortality while deaths due to other infectious agents have decreased. Hypoxic ischaemic encephalopathy is a significant cause of neonatal death. Inborn errors of metabolism have variable presentations but some, such as the fatty acid oxidation disorders, may present in neonates and cause sudden death.

Meconium is a potent activator of complement in human serum and in piglets

Meconium aspiration syndrome (MAS) is a clinical condition in the newborn infant with a significant morbidity and mortality. The complex pathophysiology of MAS, leading to both pulmonary and systemic complications, is characterized by an incompletely understood inflammatory reaction. Treatment is symptomatic, mainly limited to airway cleaning and ventilatory support. In this study, we show for the first time that meconium is a potent activator of complement, a key mediator of inflammation. In vitro, meconium activated the alternative complement pathway in human umbilical cord serum as judged by a substantial increase in the alternative pathway convertase C3bBbP. The activation proceeded through C3 (C3bc) and the terminal C5-9 pathway (terminal SC5b-9 complement complex), whereas the classical and lectin pathways were not activated (C1rs-C1-inhibitor complexes and C4bc). The lipid fraction, containing, e.g. free fatty acids, and the water fraction, containing, e.g. bile acids, contributed equally to the complement activation. A blocking antibody to factor D (alternative pathway) completely inhibited the meconium-induced complement activation, whereas blocking antibodies to mannose-binding lectin (lectin pathway) and C2 (classical and lectin pathway) had no effect. In vivo, meconium induced systemic complement activation in a piglet model of MAS, paralleling the increase in lung dysfunction. In conclusion, meconium is a potent activator of the complement system both in vitro and in vivo. Complement may be important in the pathogenesis of MAS, and specific complement inhibition might be a possible treatment approach in MAS.

Meconium aspiration syndrome in term neonates with normal acid-base status at delivery: is it different?

OBJECTIVE

Our aim was to compare the clinical characteristics of meconium aspiration syndrome in cases with pH > or =7.20 and in those with pH <7.20.

STUDY DESIGN

Medical records of diagnostic codes from the International Classification of Diseases, Ninth Revision, were used to identify neonates with severe meconium aspiration syndrome who had been delivered at our institution from 1994 through 1998. Severe meconium aspiration syndrome was defined as a mechanical ventilator requirement of >48 hours. Clinical data including neonatal outcomes of cases of meconium aspiration syndrome associated with umbilical pH > or =7.20 at delivery were compared with data on outcomes of cases with pH <7.20.

RESULTS

During this 4-year study period, 4985 singleton term neonates were delivered through meconium-stained amniotic fluid. Forty-eight cases met all study criteria, and pH values at delivery were as follows: pH > or =7.20, n = 29, and pH <7.20, n = 19. There were no differences between groups in the incidence of clinical chorioamnionitis, in the presence of meconium below the vocal cords, or in birth weight. Neonates with meconium aspiration syndrome and umbilical pH > or =7.20 at delivery developed seizures as often as those with pH <7.20 (20.1% vs 21.1%; P = 1.0).

CONCLUSION

Normal acid-base status at delivery is present in many cases of severe meconium aspiration syndrome, which suggests that either a preexisting injury or a nonhypoxic mechanism is often involved.