An arachnoid cyst is a cyst that occurs in the space between your brain, or spinal column, and your arachnoid membrane. Oftentimes, this cyst develop in your head and it will grow between your brain and skull or in pockets around your brain called ventricles.
Arachnoid cysts are usually filled with cerebrospinal fluid (CSF). CSF is a naturally occurring protective fluid that surrounds your brain and spinal column. The walls of an arachnoid cyst don’t allow this fluid to drain into your CSF system, causing it to accumulate inside.
Arachnoid cyst in children are usually congenital, or present at birth. These are called primary arachnoid cyst and the most common type. On the other hand, secondary arachnoid cyst develops later in life.