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INTRODUCTION AND OBJECTIVES: Although ultrasound (US) has traditionally be used to evaluate amniotic fluid (AF) levels and guide prenatal intervention, fetal magnetic resonance imaging (MRI) has been gaining acceptance as a potential alternative to US in the antenatal evaluation of complex genitourinary anomalies. However, the role of fetal MRI in guiding prenatal intervention has not been established. The objective of this study is to examine the association of fetal MRI and US findings of abnormal amniotic fluid (AF) levels with likelihood of prenatal intervention in fetuses with suspected bladder outlet obstruction (BOO).

METHODS: We conducted a retrospective cohort study of patients seen by our multidisciplinary maternal and fetal health program with a prenatal diagnosis of BOO from 9/1/2012 to 10/1/2014. Inclusion criteria included prenatal evaluation with both fetal MRI and US on the same day. All images were reviewed by a single pediatric radiologist with expertise in fetal imaging who was blinded to treatment. AF levels were classified as normal or abnormal (oligohydramnios or anhydramnios) based on deepest vertical pocket (DVP) by ultrasound and based on DVP as well as subjective axial T2- weighted HASTE/SSH images by MRI. Treatment was classified as observation, fetal intervention (valve ablation, vesicoamniotic shunt, or amnioinfusion), and termination of pregnancy. MRI and US findings were compared using Wilcoxon signed rank sum test. Associations between MRI and US findings and treatment were analyzed using Fisher's exact test and compared using McNemar's test.

RESULTS: A total of 26 patients meeting study criteria were identified. There was 73.1% agreement between US and MRI findings, with abnormal AF levels more often seen by MRI (p=0.016). Overall, 30.8% patients were observed, 34.6% elected termination, and 34.6% underwent fetal intervention. Likelihood of fetal intervention or termination were significantly associated with findings of abnormal AF levels by ultrasound (p=0.001) and by MRI (p<0.001). All patients with abnormal AF levels by MRI underwent fetal intervention or termination compared to 78% of patients with these findings by US (p=0.04).

CONCLUSIONS: In this series, fetal MRI findings of abnormal AF levels were more strongly associated with decisions to intervene or terminate the pregnancy than US findings, suggesting that fetal MRI may be a better predictor of fetal intervention. Further studies are needed to more clearly assess the role of fetal MRI in identifying fetuses with suspected bladder outlet obstruction at risk for adverse postnatal pulmonary and renal outcomes.

Source of Funding: None