Decoding Fetal Septum Pellucidum/ Verum complex using Multimodality Imaging: Implications in prenatal diagnosis of Septopreoptic holoprosencephaly

BackgroundFrom the radiographic perspective, the septum pellucidum (SP) and septum verum (SV) Complex (SPVC) has been tacitly understood. Microdissection and diffusion tensor imaging (DTI) have now well established that they are not mere membranes but contain septal nuclei and nerve fibers; the Superior (SF) and Inferior fascicles (IF) forming the SP, and precommissural fibers of the fornix (PrCFx) in SV. ObjectiveWe aimed to delineate the topography of normal and abnormal SPVC using ultrasound (US), T2-weighted magnetic resonance imaging (MRI), and DTI in fetuses and provide an algorithm for prenatal diagnosis and evaluation of septopreoptic holoprosencephaly (SPrH). MethodsTwenty-nine fetuses included in the study were divided based on US into Group 1 (five of 29): normal Cavum Septum Pellucidum (CSP) on axial transthalamic (aTTP) and transventricular (aTVP) planes; Group 2 (eleven of 29): non-visualization of the SP in aTVP, coronal transcaudate plane (cTCP) and beyond; Group 3 (three of 29): single septum in aTVP; Group 4 (ten of 29): small /echogenic CSP in aTTP and aTVP. ResultsAll three fascicles forming the SPVC were demonstrated in all cases prenatally and/or postnatally on US, MRI and DTI. All fetuses in Groups 2 to 4 showed an abnormal hypointense band bridging the region of septal and/or preoptic nuclei on T2-weighted fetal and postnatal MR, suggestive of SPrH. ConclusionThis study contributes to understanding the topography of normal and abnormal SPVC by prenatal US, MRI, and DTI. Based on this understanding, we outline an algorithm for prenatal diagnosis and evaluation of SPrH. HighlightsO_LIFetal Septum pellucidum/verum complex (SPVC) contain septal nuclei and 3 nerve fiber groups: Superior fascicle, Inferior fascicle and Precommissural fornix C_LIO_LIThese are seen on ultrasound, T2-weighted MRI and Diffusion tensor imaging in cases with both normal and abnormal cavum septum pellucidum (CSP). C_LIO_LIHypointense band in the septopreoptic region on T2-weighted MRI in fetuses with abnormal CSP are potential markers of septopreoptic holoprosencephaly C_LIO_LIRecognition of this entity may help in prenatal counselling and prognosis C_LI