The brain and spine of a developing baby are normally bathed in fluid called cerebrospinal fluid (CSF). Sometimes, an arachnoid cyst can compress the channels or openings that allow this fluid to flow through the brain and spine, similar to a kink in a garden hose.
Excess fluid then builds up in the baby's brain, resulting in conditions like ventriculomegaly (enlarged ventricles) or hydrocephalus (excess fluid on the brain). Associated hydrocephalus is present in more than 30% of cases of arachnoid cysts but, unless it is significant, the pregnancy likely will not be affected.
Abnormalities outside of the brain are not commonly seen, though they may be possible. Rare cases of arachnoid cysts have been associated with Tetralogy of Fallot, a heart condition, and neurofibromatosis type 1, which uniquely affects the skin, bones, eyes, brain and spine. Seizures may also occur in infants with an arachnoid cyst.
If the cyst was caused by bleeding, trauma or infection in the brain, that brings its own set of complications. That's why diagnosis of an arachnoid cyst and its underlying causes are critical for timely and effective treatment.