What Is an Arachnoid Cyst? An arachnoid (pronounced ah-RACK-
noid) cyst is a fluid-filled sac located inside the head or
spine. Arachnoid cysts develop between the brain or spinal cord
and the arachnoid membrane. This is one of three membranes that
cover the brain and spinal cord. The fluid in the cyst often is
cerebrospinal fluid (CSF), a clear liquid that protects and
nourishes the brain and spinal cord. Arachnoid cysts that are
located around the spinal cord are less common than those that
appear inside the head. In a child's head, an arachnoid cyst can
be located between the brain and the skull. A cyst can also be
located inside the brain. Arachnoid cysts in the brain form in
open spaces called ventricles that normally contain and
circulate cerebrospinal fluid. About two-thirds of the arachnoid
cysts we find in the head are located in the upper part of the
brain. The other third are in the cerebellum, located in the
lower part of the brain. Arachnoid Cyst in Children Arachnoid
cysts are often present when