An epithelial lined cyst filled with gelatinous material. The gelatinous material commonly contains mucin, old blood, cholesterol, and ions. Most colloid cysts identified are currently asymptomatic and identified incidentally on imaging. When a colloid cyst does cause issues, it most commonly causes obstructive hydrocephalus.
Comment:
The colloid cyst is most commonly found in the rostral third ventricle at or near the foramen of Monro. The foramen of Monro is the conduit of cerebrospinal fluid (CSF) outflow from the lateral ventricles to the third ventricle. A colloid cyst can act as a ball valve, stopping CSF flow out of the lateral ventricles. If this occurs, CSF backs up into the lateral ventricles and causes ventriculomegaly and hydrocephalus.