Ventriculomegaly (VM) is the most common brain anomaly diagnosed during fetal life and encompasses a large spectrum of conditions characterized by a dilatation of the lateral ventricles of the brain, typically defined as greater than 10 mm at the level of the atria, with or without dilatation of the third or fourth ventricles
Cause, severity and presence of associated anomalies are the major determinants in predicting the outcome of fetuses affected by VM; thus, the main issue when approaching a fetus with VM is to rule out CNS and extra-CNS anomalies.
The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) recommends that every fetus presenting with isolated VM on ultrasound should undergo a thorough multiplanar examination of the fetal brain.
Ultrasound (US) is the primary imaging tool for the assessment of fetal brain while fetal magnetic resonance imaging (MRI) is usually performed to confirm diagnosis and rule out associated anomalies.
Fetal magnetic resonance imaging (MRI) has been shown to add additional information compared to ultrasound in fetuses affected by isolated VM on ultrasound. However, the large majority of these studies does not specify which type of ultrasound imaging protocol was adopted to assess fetal brain. This is fundamental as most of CNS anomalies cannot be detected on the standard axial plane of fetal brain.
MRI is commonly performed in the third trimester of pregnancy, but recent evidences suggested that early (second trimester) MRI is as affective as late assessment in ruling out such associated anomalies.
The primary aim of this research will be to evaluate the role of fetal MRI in detecting associated anomalies in fetuses presenting with isolated VM undergoing a detailed ultrasound evaluation; the secondary aim will be to determine the optimal gestational age at MRI in fetuses affected by such anomaly.
VM is the most common fetal CNS anomaly diagnosed on ultrasound. Isolated VM represents a considerable diagnostic dilemma as it can be an apparently benign finding, but can also be associated with chromosomal abnormalities, congenital infections, cerebral vascular accidents or hemorrhage, and other fetal cerebral and extracerebral abnormalities. This highlights the need for an accurate imaging assessment, in order to confirm that the anomaly is isolated. Furthermore, invasive testing and maternal serological screening should be offered to parents in order to rule out aneuploidy and congenital infection.
Even when isolated, VM has been reported to increase the risk of neurodevelopmental delay in childhood and adolescence. The prevalence of neurodevelopmental disorders in fetuses with both unilateral and bilateral mild VM has been reported to be only slightly higher than that of the general population. Conversely, neurodevelopmental delay is higher in case of VM associated with additional CNS anomalies, and in case of severe ventricular dilatation.
The ISUOG guidelines on sonographic examination of the fetal CNS suggest that a multiplanar assessment of fetal head should be always performed when a suspicion of brain anomaly is raised at the basic US examination. Furthermore, in a recent consult series on mild VM, the Society for Maternal-Fetal Medicine (SMFM) has stated that MRI may be considered in cases of mild or moderate fetal VM, although it may be of less value if the women undergone detailed US assessment by an individual with speci¿c experience and expertise in sonographic imaging of the fetal brain.
MRI is the imaging technique of choice in analyzing brain anomalies postnatally. Compared to US, MRI has a better regional resolution, thus being theoretically superior to US in detecting abnormalities of the cortical development.
In a systematic review by Rossi and Prefumo, MRI was reported to add additional information compared to US in case of isolated brain anomalies.
Moreover, in 2017, a large prospective, multicenter study involving 16 centers across the United Kingdom (the MERIDIAN Study) was designed to evaluate the diagnostic and clinical role of fetal MRI in fetuses with a previous US with a suspicion or diagnosis of CNS anomaly. This study showed a much greater diagnostic accuracy of MRI compared to US (93% versus 68%), with additional findings detected exclusively on MRI in 49% of brain abnormalities and in up to 19.4% when focusing on the subgroup analysis of fetuses with mild and moderate VM.23 Of note, the rate of associated callosal anomalies detected exclusively on MRI was lower in our cohort compared to the MERIDIAN study (10% versus 55% of cases of failed commissuration).
Based on these findings, the current practice today suggests a prenatal MRI examination in the late second, early third trimester of pregnancy, although there is no complete agreement among different authors regarding the need and time of MRI examination.
However, the large majority of studies on MRI does not specify which type of ultrasound imaging protocol was adopted to assess fetal brain, and this is fundamental as most of CNS anomalies cannot be easily detected on the standard axial plane of fetal brain.
In this scenario, the project proponent has recently demonstrated in a systematic review that MRI can detect 10.0% of fetal anomalies missed on ultrasound. However, when stratifying the analysis according to the type of ultrasound assessment, the rate of associated anomalies detected only on MRI was 5.0% when dedicated neurosonography was performed, compared with 16.8% in cases diagnosed using basic assessment of the fetal brain in the axial plane. (doi: https://doi.org/10.1002/uog.20197).
Anyway, retrospective design, small sample size, different gestational
ages at examination and lack of stratification of the results according to laterality and degree of ventricular dilatation in most of the included studies represent the main limitations of this systematic review. Furthermore, not all cases presenting an additional anomaly on MRI were screened for aneuploidy or infection.
This highlights the need of larger studies sharing objective protocols for ultrasound imagining of the fetal brain to elucidate better the actual role of fetal MRI when fetal neurosonography has been performed.
The next most important Congresses on Maternal Fetal Medicine and Prenatal Diagnosis - SMFM World Congress and ISUOG World Congress - will have entire sessions focused on MRI and neurosonography and the project proponent (Dr Daniele Di Mascio) has been invited to share his experience and his data on this topic.