Impact of preterm premature rupture of membranes and oligohydramnios on in-hospital outcomes of very-low-birthweight infants

Abstract Objective To analyze neonatal outcomes in very-low-birthweight (VLBW) infants depending on the presence of preterm premature rupture of membranes (PPROM), oligohydramnios, or both. Methods The electronic medical records of VLBW infants admitted during the study period, January 2013 to September 2018, were reviewed. Neonatal outcomes (primary outcome: neonatal death; secondary outcome: neonatal morbidity) were compared depending on whether the infant was affected by PPROM or oligohydramnios. Logistic regression analysis was performed to assess the association of PPROM and oligohydramnios with neonatal outcomes. Results Three hundred and nineteen VLBW infants were included: (1) 141 infants in the PPROM group vs. 178 infants in the non-PPROM group, and (2) 54 infants in the oligohydramnios group vs. 265 infants in the non-oligohydramnios group. The infants affected by PPROM were at significantly younger gestational ages at birth with lower 5-min Apgar scores than those not affected by PPROM. Histologic chorioamnionitis was significantly more frequent in the PPROM group than in the non-PPROM group. The proportions of small-for-gestational-age infants and infants affected by multiple births were significantly higher in the non-PPROM group. The median (interquartile range) PPROM latency and onset were 50.5 (9.0 − 103.0) h and 26.6 (24.1 − 28.5) weeks, respectively. Based on the logistic regression analysis assessing the association of PPROM and oligohydramnios with the significant neonatal outcome, oligohydramnios was significantly associated with neonatal death (odds ratio [OR] = 2.831, 95% confidence interval [CI] 1.447 − 5.539), air leak syndrome (OR = 2.692, 95% CI 1.224 − 5.921), and persistent pulmonary hypertension (PPH) (OR = 2.380, 95% CI 1.244 − 4.555). PPROM per se was not associated with any neonatal outcome. However, early onset PPROM and prolonged PPROM latency were associated with neonatal morbidity and mortality. When PPROM was accompanied by oligohydramnios, it was associated with increased odds for PPH (OR = 2.840, 95% CI 1.335 − 6.044), retinopathy of prematurity (OR = 3.308, 95% CI 1.325 − 8.259), and neonatal death (OR = 2.282, 95% CI 1.021 − 5.103). Conclusion PPROM and oligohydramnios affect neonatal outcomes differently. Oligohydramnios, but not PPROM, is a significant risk factor for adverse neonatal outcomes, which is presumably related to pulmonary hypoplasia. Prenatal inflammation appears to complicate neonatal outcomes in infants affected by early PPROM and prolonged PPROM latency.


Introduction
Mid-trimester preterm premature rupture of membranes (PPROM) occurs in 0.4-0.7% of pregnancies [1]. PPROM is associated with adverse fetal and neonatal outcomes due to premature delivery [2]. Studies in the 1980s reported that the neonatal survival rate following PPROM before 25 gestational weeks was 24% [3]; however, recent studies reported increased survival rates of $70% [4]. Nevertheless, in neonates affected by PPROM, morbidities are common, with 40% of survivors developing adverse outcomes (death, severe retinopathy of prematurity, bronchopulmonary dysplasia (BPD), and severe neurological injury) compared to age-adjusted controls [5].
In comparison, isolated oligohydramnios, which occurs in 1-5% of pregnancies [6,7], is also associated with an increased risk of preterm birth [8,9]. One of the important factors that determines neonatal survival in pregnancies affected by oligohydramnios is pulmonary hypoplasia, which develops at rates ranging from 8 to 26% and even higher depending on the study [10,11]. Meanwhile, oligohydramnios can sometimes occur as a consequence of PPROM, particularly in cases with prolonged latency. In a study by Williams et al. [4], over half of the infants affected by PPROM with onset before 25 weeks' gestation and latency !14 d presented with clinical signs of pulmonary hypoplasia at birth. The majority of these infants survived to discharge, but required intensive care, such as high-frequency ventilation, and were subsequently affected by persistent pulmonary hypertension (PPH), severe cardiac insufficiency, and BPD at variable rates. Therefore, PPROM and oligohydramnios remain challenges for obstetricians and neonatologists.
Although PPROM is the most common cause of oligohydramnios and may lead to a decrease in amniotic fluid [9], not all cases of PPROM result in oligohydramnios. Similarly, oligohydramnios can be caused by etiologies other than PPROM, such as placental insufficiency or congenital urogenital anomalies [12]. Furthermore, these two factors are believed to involve different pathophysiologies. PPROM is a major risk factor for intrauterine infections and inflammation [13]. By comparison, one of the main hazards of oligohydramnios is the hindrance of fetal pulmonary development [14]. Therefore, while the two risk factors share the common feature of deviation from normal amniotic fluid status, postnatal consequences may differ depending on the risk factor(s) to which the fetus has been exposed. Previous studies usually focused on one of the two factors when evaluating the association with neonatal outcomes. This study aimed to analyze neonatal outcomes of very-low-birthweight (VLBW) infants according to the presence of PPROM and/or oligohydramnios.

Study design and enrollment
This was a retrospective cohort study performed at a level III neonatal intensive care unit (NICU). Electronic medical records of VLBW infants admitted to our NICU during the study period, January 2013 to September 2018, were reviewed. VLBW infants, defined as infants born at <37 weeks' gestation and weighing <1500 g at birth, were included in the study. The exclusion criteria were major congenital malformations, gestational age <23 weeks, critical intrauterine/perinatal illness (e.g. hydrops fetalis, severe hypoxic-ischemic encephalopathy), and outborn infants who had been transferred to our hospital lacking sufficient information on amniotic fluid or PPROM duration. Infants whose mothers had hypertension were also excluded from the study because maternal hypertension is primarily associated with poor neonatal outcomes due to decreased placental perfusion in gestational hypertensive disorders [14]. In the case of multiple pregnancy, only the infant directly exposed to PPROM or oligohydramnios was categorized as such. The institutional review board (IRB) of our institution approved this study (approval number: KC21RISI0957), and the need for informed consent was waived due to the retrospective nature of the study.

Definitions and data collection
The interval between membrane rupture and delivery was determined. PPROM was defined as premature rupture of the membrane at the time of delivery. Therefore, "non-PPROM" referred to infants whose membranes were intact at the time of delivery. PPROM was diagnosed by the attending obstetrician based on visualization of amniotic fluid leakage from the cervical os or amniotic fluid pooling in the posterior vagina via sterile speculum examination and the positivity of diagnostic test(s) such as nitrazine and/or AmniSure tests [15,16]. The amniotic fluid index (AFI) was defined by summing the vertical diameter of the largest amniotic fluid pocket in each of the four quadrants. An AFI <5 cm was considered oligohydramnios [17].
In addition to information concerning PPROM and oligohydramnios, the baseline neonatal and maternal characteristics were collected as follows: gestational age (weeks) at birth, birth weight (g), small-for-gestational-age (SGA, defined when the birthweight was less than the 10th percentile for gestational age, based on the Fenton growth chart [18]), sex, delivery mode (cesarean section or vaginal delivery), 1-and 5min Apgar scores, maternal age (years), maternal diabetes, histologic chorioamnionitis, placental abruption, singleton or multiple births, method of conception (natural pregnancy or via assisted reproduction therapy), and antenatal corticosteroid administration. Placental abruption was diagnosed by the attending obstetrician, based on clinical signs of abrupt vaginal bleeding accompanied by acute abdominal or back pain, uterine contraction or tenderness, and/or nonreassuring fetal status, and antepartum sonographic or gross visualization at delivery of abruption or retroplacental blood clots [19].
Data on neonatal outcomes were collected and compared, depending on whether the infant was affected by PPROM, oligohydramnios, or both. The primary outcome was neonatal mortality, and the secondary outcome was major neonatal morbidity. Major neonatal morbidity was defined as the presence of any of the following conditions: Respiratory distress syndrome (RDS), air leak syndrome, massive pulmonary hemorrhage, PPH, moderate to severe BPD, hemodynamically significant patent ductus arteriosus (hsPDA), intraventricular hemorrhage (IVH), cystic periventricular leukomalacia (PVL), neonatal sepsis, necrotizing enterocolitis (NEC), and retinopathy of prematurity(ROP) requiring treatment. RDS was defined as clinically evident respiratory distress (tachypnea, labored breathing, apnea, nasal flaring, or chest retraction) necessitating invasive/noninvasive respiratory support and a fraction of inspired oxygen !40% to achieve an oxygen saturation (SpO 2 ) !92-95%, accompanied by radiologic findings of ground-glass opacities and/or air bronchograms. Air leak syndrome was defined as one of the following conditions: pneumothorax, pneumomediastinum, or pulmonary interstitial emphysema requiring chest tube insertion or needle aspiration. Massive pulmonary hemorrhage was defined as fresh blood in the endotracheal tube or oral cavity accompanied by cardiovascular collapse/acute respiratory failure. PPH was defined as the presence of pulmonary vasodilators used to treat clinical symptoms of labile hypoxemia and/or differential desaturation (preductal SpO 2 , postductal SpO 2 >5-10%) with or without echocardiographic findings of the right-to-left or bidirectional shunt at the atrial level or through the patent ductus arteriosus (PDA), tricuspid regurgitation, or interventricular septum flattening [20]. Moderate to severe BPD was defined by one of the following conditions according to the 2001 National Institute of Child Health and Human Development definition: (1) infants born at <32 weeks requiring supplemental oxygen or positive pressure ventilation at 36 weeks' corrected age or at the time of discharge, whichever is first, or (2) infants born at !32 weeks requiring supplemental oxygen or positive pressure ventilation on the 56th day after birth or at the time of discharge, whichever is first [21]. HsPDA was defined as a left-to-right shunt through the PDA observed in the color Doppler view on echocardiograms, which required treatment in infants manifesting at least two of the following five conditions: (1) systolic murmur or continuous murmur, (2) bounding pulse or hyperactive precordial pulsation, (3) fluctuation of blood pressure, (4) aggravation of respiratory condition not attributed to lung disease, and (5) pulmonary edema and/or cardiomegaly (cardiothoracic ratio >60%) on chest radiographs. IVH referred to grades 3 and 4 IVH according to Papile's criteria [22]. IVH and cystic PVL were defined based on the findings obtained from brain ultrasound or brain magnetic resonance imaging during the NICU stay. Neonatal sepsis was defined as positive blood culture and administration of systemic antibiotics for at least five days. NEC was defined as stage 2 or greater according to the modified Bell's staging criteria [23]. ROP requiring treatment was defined when infants received photocoagulation surgery or antivascular endothelial growth factor therapy.

Statistical analysis
Data was analyzed using SPSS, version 20.0 (IBM Corp., Armonk, N.Y., USA). The included infants were sequentially categorized according to the presence of (1) PPROM and (2) oligohydramnios. Baseline characteristics and neonatal outcomes were evaluated. Continuous variables were analyzed using the Wilcoxon rank sum test because of the non-parametric distribution of the data, and categorical variables were analyzed using the chi-square and Fisher's exact tests, as appropriate. Logistic regression analyses were performed to assess the association of PPROM and oligohydramnios with neonatal mortality/major neonatal morbidity. Statistical significance was set at p < .05.

Results
A total of 504 VLBW infants were admitted during the study period. Of these, 185 infants were excluded for the following reasons: major congenital malformations or critical illness (n ¼ 29), <23 gestational weeks (n ¼ 8), delivery outside our hospital (n ¼ 61), polyhydramnios/unknown AFI (n ¼ 27), and maternal hypertensive disorder (n ¼ 60). Three hundred and nineteen VLBW infants met the inclusion criteria for the final analysis: 141 and 178 infants were included in the PPROM and non-PPROM groups, respectively; and 54 and 265 infants in the oligohydramnios and non-oligohydramnios groups, respectively ( Figure 1). Table 1 describes the baseline characteristics of the enrolled infants, grouped according to the presence of PPROM. The infants affected by PPROM were significantly younger in terms of gestational age at birth (median: 26.8 vs. 28.1 weeks, p < .001) with lower 5min Apgar scores than those not affected by PPROM. Histologic chorioamnionitis was significantly more frequent in the PPROM group than in the non-PPROM group. In contrast, the proportion of SGA infants and infants affected by multiple births was significantly higher in the non-PPROM group. The prevalence of oligohydramnios was 24.1 and 11.2% in the PPROM and non-PPROM groups, respectively (p ¼ <.001). The median (interquartile range) PPROM latency and onset were 50.5 (9.0 À 103.0) h and 26.6 (24.1 À 28.5) weeks, respectively. PPROM occurred at periviable gestational age <25 weeks in 34.0% of the infants affected by PPROM. Table 2 presents the characteristics concerning PPROM according to the presence of oligohydramnios.
The "PPROM and oligohydramnios" group exhibited significantly longer PPROM latency compared to the "only PPROM" group (median: 67 vs. 48 h; p ¼ .013, respectively), and also PPROM onset was earlier (median: 25.6 vs. 26.7 weeks; p ¼ .029), respectively. The proportion of infants affected by PPROM at <25 weeks' gestation was greater in the "PPROM and oligohydramnios" group compared to the "only PPROM" group (47.1 vs. 29.9%; p ¼ .066, respectively). Figure 2 shows the distribution of different PPROM  In Tables 3 and 4, neonatal mortality and major neonatal outcomes were analyzed in the infant groups depending on the presence of PPROM and oligohydramnios. Neonatal outcomes were not significantly different based on the presence of PPROM. However, neonatal death before discharge (31.5 vs. 14.0%, p ¼ .002), air leak syndrome (20.4 vs. 8.7%, p ¼ .011), and PPH (33.3 vs. 17.4%, p ¼ .008) occurred significantly more frequently in the oligohydramnios group than in the non-oligohydramnios group. Logistic regression analyses were performed to assess the association of PPROM and oligohydramnios with significant neonatal outcomes from the above analysis. Specifically, neonatal death, air leak syndrome, ROP, and PPH were evaluated (

Discussion
Based on our results, oligohydramnios was associated with increased odds of neonatal death, air leak syndrome, and PPH. Although PPROM per se was not associated with these adverse neonatal outcomes, it was associated with increased odds of PPH, ROP, and neonatal mortality when accompanied by oligohydramnios. In addition, PPROM onset at <25 weeks' gestation was associated with increased odds for neonatal death, air leak syndrome, PPH, and ROP requiring treatment; PPROM latency !168 h significantly increased the odds for PPH. The different impacts of PPROM and oligohydramnios on neonatal outcomes demonstrated in our study may be attributed to the predominant pathogenesis. PPROM is a major risk factor for intrauterine infection and inflammation. There is a high incidence of acute infection, including chorioamnionitis, after prolonged membrane rupture [24]. Reciprocally, infection/inflammation is implicated in the pathogenesis of PPROM [25]. Inflammation adversely affects pulmonary vascular development, such as smooth muscle hypertrophy of the pulmonary arterioles and decreased production of endothelial nitric oxide synthase, which are likely to  be mediated by cytokines such as interleukin-6 and tumor necrosis factor-a [26]. On the other hand, oligohydramnios has a negative impact on fetal lung development. Following oligohydramnios, the fetal radial alveolar count is decreased, and the pulmonary vascular bed reduces in size with decreased vessel count and increased pulmonary vascular muscular development [27][28][29]. According to our findings, the prevalence of oligohydramnios was significantly higher (more than doubled) in the PPROM group than that in the non-PPROM group. PPROM latency also differed depending on the presence of oligohydramnios. In the oligohydramnios group, neonates with PPROM latency >168 h (7 d) and those with PPROM latency <24 h accounted for the same proportion (29.4%). The such contrasting phenomenon is also observed in previous studies. In a nationwide study including VLBW infants in Korea, infants affected by PPROM latency >7 d exhibited a significantly higher proportion of oligohydramnios compared to those with shorter PPROM latency [30]. Conversely, Ekin et al. [31] demonstrated that PPROM accompanied by oligohydramnios is associated with a shorter latency to delivery, which is probably attributable to aggravation of subclinical or existing intraamniotic infection and subsequent increased uterine contractility leading to amniotic fluid loss through the ruptured membrane.
Meanwhile, despite the lower prevalence, oligohydramnios was still identified in the infants who were not affected by PPROM. Since we excluded infants with congenital anomaly including urogenital anomaly and those affected by maternal hypertensive disorders, some possible etiologies contributing to oligohydramnios in these  infants could be placental malperfusion caused by conditions other than maternal hypertensive disorders and multiple births. The two variables of interest that are associated with deviation from normal amniotic fluid status in the current study exerted different impacts on neonatal outcomes. In this study, PPROM was not associated with neonatal mortality. The risk of increased mortality in patients with PPROM has been controversial, with some studies reporting an association of PPROM with increased neonatal mortality [24,32] and others finding no association [5,33]. Different inclusion criteria (such as gestational age at the time of rupture) and cutoff durations for defining PPROM may potentially contribute to inconsistent results in previous literature. However, as demonstrated in our research as well as in previous studies, although PPROM itself was not associated with neonatal outcomes, the outcomes may vary depending on how early membrane rupture occurs [34,35]. For instance, Pendse et al. [34] demonstrated that PPROM onset at a previable gestational age, particularly before 20 weeks, significantly increased neonatal morbidity and postnatal death. In comparison, consistent with our results, the association between oligohydramnios and neonatal mortality in preterm infants has been described in several studies [36,37]. Meanwhile, PPROM latency was not significantly associated with neonatal mortality but PPROM accompanied by oligohydramnios yielded statistically significant results. Since our study criteria excluded urogenital anomalies and maternal hypertension, one of the main etiological factors contributing to oligohydramnios is probably prolonged PPROM latency and early gestational age at PPROM [32]. Therefore, it is reasonable to infer that prolonged PPROM complicated with oligohydramnios, rather than mere PPROM, would influence neonatal mortality.
Concerning neonatal morbidities, our study demonstrated that PPROM itself was not significantly associated with any morbidities in the logistic regression analysis, but PPROM that occurred before 25 gestational weeks, prolonged PPROM latency, PPROM accompanied by oligohydramnios, and oligohydramnios (with or without PPROM) were associated with increased odds for various types of adverse neonatal outcomes. Consistent with our findings, Furman et al. [38] reported that PPROM is not an independent risk factor for major neonatal morbidities. In contrast, Park et al. [30] found that PPROM was associated with increased odds of early PPH and BPD. The duration criterion for PPROM set in their report was a latent period exceeding 7 d. In our study, despite the lack of impact of PPROM per se, PPROM latency of ! 168 h, which conforms to the criteria of their work, significantly increased the odds for PPH. Of note, in their report, oligohydramnios was also a significant risk factor for early PPH, with greater odds compared to those of prolonged PPROM. In another study [39], oligohydramnios, but not prolonged PPROM (occurring before 25 weeks of gestation and persisting for !7 d), was associated with early PPH. However, when only those exposed to prolonged PPROM were sub-analyzed, oligohydramnios increased the odds for early PPH to an even greater extent. This implies that rather than PPROM itself, the involvement of oligohydramnios may play an important role in the development of PPH. Our study results also imply that oligohydramnios is an important factor that independently impacts neonatal outcomes, and also when present with PPROM.
An increased risk of air leak syndrome and PPH in infants with oligohydramnios may involve the pathophysiology of pulmonary hypoplasia [20]. The development of pulmonary hypoplasia associated with oligohydramnios has been previously described [40]. In addition to gestational age at PPROM and latency period, amniotic fluid volume was an important predictive factor for pulmonary hypoplasia, and the presence of oligohydramnios at any time during the latency period was deemed unfavorable. A lack of amniotic fluid below the critical amount may hamper appropriate lung development by mechanical compression of the fetal thorax, restricting normal episodic fetal breathing movements, and resulting in failure to generate distended pressure formed by fluid [41]. Reduced lung collagen levels and the absence of elastic tissues observed in animal models [42] exposed to maternal oligohydramnios may additionally explain the pathogenesis of oligohydramnios with regard to pulmonary hypoplasia.
Our study suggests that PPROM occurring at <25 weeks' gestation and PPROM accompanied by oligohydramnios contribute to increased odds for ROP requiring treatment. There is limited literature describing the direct relationship between ROP, PPROM, and oligohydramnios. However, the association between inflammatory mediators derived from amniotic fluid and the development of ROP has been described recently [43]. The association with ROP requiring treatment in our study may be attributed to prenatal inflammation. More research is required to elucidate the role of prenatal inflammation and postnatal factors such as sepsis and/or hyperoxia that may differently add to the risk of ROP occurrence and severity [44].
It is noteworthy that in our study two factors concerning abnormal amniotic fluid status (PPROM and oligohydramnios) were selected as variables of interest. Furthermore, not only the effect of each factor on neonatal outcomes, but also various settings of the two factors were used to analyze the association with neonatal outcomes, which offers a different viewpoint from previous work evaluating the role of the two factors. Considering the dissimilar impact on neonatal outcomes, the presence of either PPROM itself, the onset and latency of PPROM, the presence of concomitant oligohydramnios, or isolated oligohydramnios, should be considered by clinicians and treatment strategies be guided accordingly.
Our study had some limitations. First, this was a single-center study; therefore, the sample size was restricted. Future multicenter studies with larger cohorts suitable for further risk stratification and multivariable analysis are required to validate our findings. Second, we used AFI rather than deepest vertical pocket (DVP) to define oligohydramnios. Although DVP is regarded as a stricter definition of oligohydramnios that could reduce the risk of overestimation [17], AFI is frequently used in clinical settings. Further, the final determination regarding the superiority of one criterion over the other is still unresolved [45]. Moreover, according to a recent survey in Korea, the majority (90.1%) of obstetricians used AFI to define and record oligohydramnios in the medical charts [46]. Therefore, AFI was selected to define oligohydramnios in the current study. Acknowledging the possible difference in the prevalence of oligohydramnios if the DVP definition were incorporated, further studies using DVP to define oligohydramnios are warranted. Finally, although the theoretical link between oligohydramnios and pulmonary hypoplasia has been previously described [9,10,12,37], we were unable to concretely assess the presence of pulmonary hypoplasia in our cohort. Despite some suggestions for diagnosing pulmonary hypoplasia [47,48], a clear definition of pulmonary hypoplasia is not well established in clinical practice. In addition, because the exact onset of oligohydramnios was not readily available from the medical records, particularly when the mothers were transferred from local clinics, the accurate duration of oligohydramnios could not always be obtained. This limitation unfortunately restricted us from directly investigating the link between the duration of oligohydramnios and pulmonary hypoplasia. Future studies to correctly diagnose and investigate appropriate antenatal and postnatal management of high-risk pregnancies affected by PPROM and/or oligohydramnios, depending on the development and extent of pulmonary hypoplasia are necessary.

Conclusion
PPROM and oligohydramnios affect neonatal outcomes differently. Early PPROM before 25 weeks' gestation and prolonged latency of PPROM, which results in oligohydramnios rather than isolated PPROM, is potentially hazardous for preterm infants. The impact of oligohydramnios on adverse neonatal outcomes is likely linked to maldeveloped pulmonary vascular beds and presumed pulmonary hypoplasia. Future research is warranted to refine antenatal and postnatal diagnosis and management strategies for pregnancies affected by oligohydramnios.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
None.