The multivariate analysis performed in this study excluded the impact of confounding factors. The study then suggested that oligohydramnios may be a risk factor for BPD. Additionally, analysis in which an ROC curve was used indicated that oligohydramnios that lasts for four days or longer may be a risk factor for BPD.
No consensus has been reached on how to deal with pPROM cases. At present, each case is individually treated, taking into consideration factors such as: estimated body weight, number of gestational weeks, the capacity of the facility for neonatal resuscitation and management (13). Past studies have found that prolonged oligohydramnios is associated with contracture and abnormalities of the musculoskeletal system (14) as well as pulmonary hypoplasia (15).
It is still extremely difficult to determine whether the termination of pregnancy is appropriate or not for pPROM cases. The following conditions may be clear criteria for the termination of pregnancy in pPROM cases: NRFS, clinical CAM, and clearly recognisable premature separation of the normally implanted placenta (16). On the other hand, if these conditions are not observed, the termination of pregnancy must be carefully determined on an individual basis by taking into account the number of gestational weeks (16).
BPD was first coined by Northway et al. in 1967 (17). Since then, subsequent studies gradually shed light on its pathophysiology. The pathology of BPD in a preterm neonate is as follows: pulmonary development in the neonate is inhibited after birth as the neonate with the disease starts pulmonary respiration while their lungs are underdeveloped. Since the structure of their lungs is not fully developed, the neonate suffers from hypoxemia, which may require mechanical ventilation management and the administration of oxygen. These interventions damage pulmonary tissue. Moreover, while cytokine-induced, intrauterine inflammatory changes frequently cause premature birth, a study has pointed out that inflammatory changes are also associated with pulmonary tissue injuries (18).
This study took into account the above-mentioned pathophysiological findings in analysing factors associated with the onset of BPD. Existing studies have reported that oligohydramnios is a risk factor for BPD (19). However, as mentioned above, it is considered that various factors are intricately interrelated when a neonate develops BPD, such as: premature birth, intrauterine infection and infection after birth, the sex of the neonate and SGA. After excluding the impact of confounding factors such as the number of gestational weeks and intrauterine infection, multivariate analysis in this study found that oligohydramnios is a risk factor for BPD. Prolonged oligohydramnios may be associated with neonatal pulmonary hypoplasia (15, 20). It has been pointed out that various factors, including premature lungs, oxygen toxicity and inflammatory mediators, are also involved (21). Diagnostic criteria for and the definition of BPD vary, and consensus is yet to be reached (22, 23). Meanwhile, risk factors for neonatal BPD during pregnancy include intrauterine growth restriction and chorioamnionitis (21). One study reported that the incidence rate of BPD was higher in cases who experienced pPROM before 31 weeks of gestation than those who did not experience the condition (24), Another study reported that from among 36 neonates who were born to mothers diagnosed with pPROM before 24 weeks of gestation and who were discharged following delivery, 17 (47%) developed BPD (25). Oligohydramnios was found to be a risk factor for BPD. It can be considered that neonates with BPD required the administration of oxygen after birth because the extensibility of their pulmonary tissue was reduced, from prolonged oligohydramnios that prevented the foetal development of the lungs.
It has been found that a neonate with BPD has increased mid- to long-term risks of pulmonary hypertension syndrome and chronic obstructive pulmonary disease (26). Since this study did not examine the mid- to long-term prognosis of neonates, the studied neonates should be closely monitored for the onset of prolonged pulmonary hypertension going forward.
The ROC curve indicated that oligohydramnios that lasts four days or longer increases the risk of BPD. If a mother has persistent oligohydramnios for a period of four days or longer, an onset of BPD should be taken into account in managing the case. There has been some research that has reported that a neonate develops respiratory complications if prolonged oligohydramnios was observed in their mother (27). However, to the best of our knowledge, no research has been identified that explores the relationship between the duration of preterm oligohydramnios and the onset of neonatal BPD. The authors agree that the appropriateness of early pregnancy termination for a pPROM case should be determined on an individual basis by taking into account the number of gestational weeks and other individual background factors (13). However, it is still necessary for health professionals to consider the risks of neonatal BPD when attempting to prolong pregnancy in a mother with persistent oligohydramnios in whom NRFS or clinical chorioamnionitis is not observed.
This study has several limitations. In this study, the endpoint for analysis was the onset of neonatal BPD. Future studies may need to examine the long-term prognosis of neonates as well. Also, although the onset of BPD was examined by using an ROC curve, pregnancy termination was determined on an individual basis by taking into account the number of gestational weeks and patient background factors. Hence, it should be noted that interventions were performed at the discretion of the attending health professionals.
The following aspects should also be noted: firstly, this study is a retrospective epidemiological study which was performed at a single centre and which did not involve large-scale data; and secondly, there was a bias in selecting patients because the study site was a general perinatal care hospital which primarily accepts severe cases in its area.