Arachnoid cysts are lined by arachnoid membranes and filled with cerebrospinal fluid. Etiologically, they are thought to be due to maldevelopment of the arachnoid or secondary to
trauma or
infection. Postnatally, many are asymptomatic and remain quiescent for years, although others expand and cause symptoms by compressing adjacent brain and/or expanding the overlying skull. Being congenital, it should be possible to identify them in the fetus, and this has proved to be so. But are they anatomically similar and do they behave the same as those detected postnatally? Fifteen fetuses with fluid-filled
cysts were identified from the database of the Fetal Management Unit at St. Mary's Hospital in Manchester. Five were diagnosed at or before 20 weeks of gestation, 4 between 21 and 30 weeks, and 6 at 31 weeks or more. Thirteen
cysts were in the supratentorial compartment, and 2 in the posterior fossa. Eleven fetuses were delivered, and 4 pregnancies were terminated. One of the born children had
Pallister-Hall syndrome and died on day 19, and another had
Aicardi's syndrome, is retarded and has fits, 1 with a posterior fossa
cyst developed
hydrocephalus in utero and was delivered early for the insertion of a
ventriculoperitoneal shunt, he is moderately mentally delayed. Of the remaining 8 children, 1 has been lost to follow-up, and 7 appear to be reaching their early milestones on time. The 4 terminated fetuses had a postmortem examination, 2 did not have
arachnoid cysts; 1 had an expanding glioependymal
cyst which had destroyed most of the cerebral hemispheres, and the other had a
sagittal sinus thrombosis with extensive cavitation of one cerebral hemisphere. The diagnosis of an arachnoid
cyst in the fetus can be difficult and may be confused with other fluid-filled
cysts.