EP06.10: The forgotten space: what fetal MRI teaches us about the subdural and other external cerebrospinal fluid spaces

The concept of a naturally occurring space between the arachnoid and dura was advocated in the studies of Key and Retzius, and by subsequent developmental and experimental investigations conducted in the mid late 1800s (1). Collectively, these data suggested that a space appeared at the dura-arachnoid junction early in development by a partial separation of the arachnoid from the dura, that the subdural space was lined by a flattened “mesothelium”, and that a small amount of fluid was found therein. This study aims to assess the visibility of the subdural space in fetuses without brain abnormalities and prove that the external CSF spaces of human fetuses are subdivided into the subarachnoid and the subdural space by a thin MR depictable membrane/dural leaflet. This is a retrospective study of 102 fetal MRI examinations at a single tertiary centre. All fetal cases had a normal brain at the time of examination, aged between 18 – 39 GW. The width of the subdural space and external CSF were obtained via 2D measurements on T2W images, across the convexities. While the total brain volume and intracranial volume were obtained on super-resolution T2 MRI that were segmented via ITK-snap (an open-source software that allows image segmentation). Our results showed an increase in the total external CSF volume and total brain volume in relation to increasing GA of the fetuses while the mean subdural space decreased. The total subarachnoid space also at increased in relation to increasing GA of the fetuses while interestingly the mean subdural space showed at first an increase from 18 until around 32-33 weeks, followed by a shot plateau followed then by a drop around 34 to 39 GW. During fetal life, the outer CSF spaces are subdivided into a subdural and a subarachnoid space by a thin fetal MRI-visible membrane. This presumable increases in size and thereafter decreases until birth.