Prenatal diagnosis and postnatal outcome of type III vasa previa: systematic review of literature.

OBJECTIVE: Vasa previa (VP) is a condition characterized by fetal blood vessels located on the amniotic membranes and outside the placenta, within 2 cm from internal os. The most common forms of VP are associated to velamentous cord insertion (type I) or to bilobed-succenturiate placenta (type II), located across the cervix. A rare form of VP, not necessarily associated to other placental or vascular anomalies, involves aberrant vessels running from the placenta to the amniotic membranes, nearby the internal os, and then turning back to the placenta (type III). Although early diagnosis of type III VP is important, it often technically challenging and difficult. The objective of our study is to gather the current available evidence on type III VP related to prenatal diagnosis and perinatal outcome.

METHODS: A systematic review of the literature was carried out according to PRISMA guidelines from inception up to March 2023 using common internet-based databases (PubMed, Scopus and EMBASE) looking for perinatal diagnosis of atypical type III vasa previa. Data extraction and tabulation was carried out by two different operators and double-checked by a third senior author. Quality assessment of the included studies was realized using the National Institute of Health (NIH) tool for the quality assessment of Case Series Studies. Recent unpublished cases were searched in our local ultrasound database. Frequencies and proportions of prenatally vs. postnatally diagnosed type III VP were summarized, including clinical expression and perinatal outcomes.

RESULTS: Eighteen cases of type III VP were detected: 16 with prenatal diagnosis (14 from 10 publications and 2 unpublished from our center) and 2 with postnatal diagnosis from two publications. All cases with prenatal diagnosis were assessed by transvaginal ultrasound at a mean gestational age of 29 weeks (median 31 weeks; range 19-38 weeks). Conception was achieved with IVF in 4/16 (25%), there were no prenatal symptoms in 15/18 cases (83.3%), in two cases (11.1 %) there was vaginal bleeding and in one (5.6%) preterm labour. Only 3/18 (16.6%) cases had no placental abnormality and 15/18 (83.3 %) showed at least one placental abnormality including: low-lying insertion (9/17), succenturiate or accessory lobe (1/17), velamentous cord insertion (3/18), marginal insertion (9/18). All cases with prenatal diagnosis were liveborn with cesarean section before rupture of membranes at a mean gestational age of 35.4 weeks (range 32-38) without neonatal complications. Emergent cesarean section was observed in 2/16 cases with prenatal diagnosis and 1/2 cases with postnatal diagnosis (rates: 12.5% vs 50%; p-value: 0.179), Apgar score≤7 at 1 or 5 minutes in prenatally vs postnatally diagnosed cases was observed in 5/16 vs 1/2 cases (p=0.595) and 3/16 vs 1/2 cases (p=0.316), respectively.

CONCLUSIONS: The prenatal diagnosis of VP type III is challenging with few cases reported in the literature; however, it minimizes the risks of an adverse outcome with intact membranes early-term elective cesarean delivery. Since clinical manifestations and risk factors are not specific, and type III VP cannot be excluded when there is a normal cord insertion or just one placental mass, a generalized screening by transvaginal ultrasound in the general population is recommended, particularly in presence of low-lying or morphologically abnormal placentas and medically assisted reproduction. This article is protected by copyright. All rights reserved.