Fetal arachnoid cysts: Their site, progress, prognosis and differential diagnosis

Arachnoid cysts are lined by arachnoid membranes and filled with cerebrospi nal fluid. Etiologically, they are thought to be due to maldevelopment of t he arachnoid or secondary to trauma or infection. Postnatally, many are asy mptomatic 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, an d this has proved to be so. But are they anatomically similar and do they b ehave the same as those detected postnatally? Fifteen fetuses with fluid-fi lled cysts were identified from the database of the Fetal Management Unit a t St. Mary's Hospital in Manchester. Five were diagnosed at or before 20 we eks of gestation, 4 between 21 and 30 weeks, and 6 at 31 weeks or more. Thi rteen cysts were in the supratentorial compartment, and 2 in the posterior fossa. Eleven fetuses were delivered, and 4 pregnancies were terminated. On e 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 posterio r fossa cyst developed hydrocephalus in utero and was delivered early for t he insertion of a ventriculoperitoneal shunt, he is moderately mentally del ayed. Of the remaining 8 children, 1 has been lost to follow-up, and 7 appe ar 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 expa nding glioependymal cyst which had destroyed most of the cerebral hemispher es, 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.