What Is the Average Size of an Arachnoid Cyst?

The size of an arachnoid cyst varies from person to person. Some cysts are small enough that they do not affect the surrounding tissue and don't cause symptoms. In fact, they do not, in some cases, even require treatment, according to the National Institute of Neurological Disorders and Stroke.

There are two types of arachnoid cysts. Primary arachnoid cysts are present at birth and form during the early weeks of gestation. Secondary arachnoid cysts develop as a result of an injury, brain tumors, meningitis or as a complication of brain surgery, states the National Institute of Neurological Disorders and Stroke.