Children's Hospital Colorado
Colorado Fetal Care Center

Arachnoid Cysts

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What is an arachnoid cyst?

There are two types of arachnoid cysts: congenital and acquired. Congenital arachnoid cysts are the result of abnormal development of a brain structure called leptomeninges. Acquired congenital arachnoid cysts are the result of bleeding, trauma or infection.

An arachnoid cyst is usually a single or simple cyst rather than a collection of multiple cysts. It can be round, oval or crescent moon-shaped. These cysts usually do not grow quickly, remaining stable during pregnancy. Arachnoid cysts tend to account for only 1% of neonatal intracranial masses and mostly affect the left side of the brain. The precise of arachnoid cysts is unknown.

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.

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