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Arachnoid Cyst

What is an arachnoid cyst?

An arachnoid (pronounced ah-RACK-noid) cyst is a fluid-filled sac found inside the head or spine. The fluid is often cerebrospinal fluid.

An arachnoid cyst can appear outside the brain, between the brain and the skull. It can also appear inside the brain, in the open spaces called ventricles that normally contain and circulate CSF.

Two-thirds of arachnoid cysts appear in the upper part of the brain. We see the other third in the cerebellum located in the lower part of the brain.

A large cystic mass pushing on the third ventricle in the brain.
A large cystic mass pushing on the third ventricle in the brain.

We often discover these cysts accidentally when a child has a CT scan or an MRI of the brain for some other reason, like a head injury.

Who gets an arachnoid cyst?

This kind of cyst is usually present at birth (congenital).

An arachnoid cyst may be related to head trauma in a small number of young children.

What is your experience with arachnoid cyst?

Our medical team has a great deal of experience caring for and operating on children with arachnoid cyst. Our diagnostic skills and state-of-the-art procedures and equipment make us the logical choice for both evaluation and treatment.

When a doctor at Children’s finds an arachnoid cyst during a CT, she sends the child to the Neurosurgery Clinic for evaluation. Our first step is usually to do an MRI to rule out other problems, like tumors.

What surgery or procedure do you use to treat an arachnoid cyst?

If the cyst is causing symptoms in your child, like headaches, nausea, vomiting, changes in activity, or trouble with vision and balance, he may need treatment.

A cyst can cause several problems, such as increased intracranial pressure, that produce symptoms.

If a cyst enlarges, it can put direct pressure on the surrounding brain structures. It can block circulation of cerebrospinal fluid, which leads to hydrocephalus and an increase in total intracranial pressure.

If your child has no symptoms, we usually watch the cyst, and if it does not change size, we do not operate.

Because of the nature of a fluid-filled sac, we cannot simply cut it out. Depending on a child’s unique needs, the neurosurgeon will use one of two types of procedures to remove the cyst.

1. Open the cyst

We use an instrument called an endoscope to see and open the cyst. An endoscope has a light and a camera attached to it. The neurosurgeon makes a small cut called an incision. He then puts the endoscope through the incision to see and open the cyst.

He lets the fluid drain into other areas of the brain that contain CSF. This is called fenestration. The body reabsorbs the fluid from the cyst along with the CSF it normally reabsorbs.

2. Put a shunt into the cyst

A shunt is a tiny tube called a catheter placed in the cyst to drain the fluid into another part of the child’s body, like the abdomen. Learn more about shunts.