Fluid accumulation in the fetal extracerebral spaces: External hydrocephalus or subdural hygromas?

External hydrocephalus is defined as enlargement of the subarachnoidal space with accumulation of cerebrospinal fluid (CSF). In adults, there are several reasons for external hydrocephalus, such as disruption of CSF resorption or meningeal inflammations. During fetal development the occurrence of external hydrocephalus is a very rare event and pathogenetic mechanism as well as neuropathological classification considerably differs from that of adults. Here we report on a 25^th gestational week fetus with the ultrasonic diagnosis of an external hydrocephalus with macrocephalus and microencephaly. Neuropathological examination revealed extensive subdural hygroma, associated with chronic subdural haemorrhages. The central nervous system (CNS) showed a distinct microencephaly with pronounced neocortical migration defects and various stages of neuronal maturation. Due to our findings we were able to exclude the diagnosis of an external hydrocephalus. Instead, we suggest, that the atrophic hemispheres lead to a chronic stretch of the bridging veins with drainage of fluid into the subdural space.