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.
How are arachnoid cysts diagnosed?
Most cases of arachnoid cysts are detected after 20 weeks of pregnancy during ultrasound. On ultrasound, an arachnoid cyst may look like a clear, round, oval or crescent moon-shaped mass in the back of the brain. When an arachnoid cyst is detected within the fetal brain, the patient will be given a complete anatomic sonographic fetal diagnosis to rule out other abnormalities.
Sometimes, a prenatal MRI may be necessary, as well. If other abnormalities are present, amniocentesis may be recommended to look for a chromosomal or genetic abnormality. If the arachnoid cyst is the result of a chromosomal abnormality, genetic counseling may also become part of the mother and baby's treatment plan.
Based on the measurement of cerebrospinal fluid in the brain, as well as the size of a baby's ventricles, an arachnoid cyst may be suspected. If the volume of fluid is high, a baby with a cyst may also be diagnosed with hydrocephalus.