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Arachnoid Cyst Causes, Symptoms, and Treatment Options

Arachnoid cysts are sacs filled with cerebrospinal fluid (CSF). They are often present at birth.

The UPMC Pituitary Center of Excellence treats a range of conditions, including Arachnoid cysts.

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What Is an Arachnoid Cyst?

Arachnoid cysts are the most common type of brain cyst.

An arachnoid cyst can form:

  • At the base of the skull.
  • On the surface of the brain.
  • On the arachnoid membrane.
  • On the spinal cord (this is most rare).

Three different membranes make up the protective covering that envelops the central nervous system (the meninges). These layers, beginning with the most external, are the:

  • Dura mater.
  • Arachnoid mater.
  • Pia mater.

Cerebrospinal fluid is mainly contained between the arachnoid layer and pia mater. An arachnoid cyst forms when the layers of the arachnoid membrane split apart and become filled with cerebrospinal fluid. It is normal to have CSF in this space and arachnoid cysts are usually simply larger than normal collections of this fluid, often present since birth and with no consequence.

Not all arachnoid cysts need treatment, and some people never know they have one. Clear neurological symptoms related to mass effect from an arachnoid cyst may rarely be an indication for surgery.

But arachnoid cysts can grow to be large if they continue to collect CSF. In extreme cases, the size of a cyst may cause the head to change shape or displace nearby lobes of the brain.

What Are the Common Types of Brain Cysts?

The four most common types of brain cysts are:

  • Arachnoid cysts.
  • Colloid cysts.
  • Dermoid cysts.
  • Epidermoid cysts.

How Serious Are Brain Cysts?

How severe a brain cyst is depends on its type, size, and where it is. Some cysts cause headaches, vision issues, and nausea because of the pressure they create from pressing against brain tissue.

Your doctor may choose to watch a cyst over time, or advise surgery to drain and remove a cyst.

Is an arachnoid cyst more serious than other brain cysts?

Most arachnoid cysts are present at birth and don't cause symptoms throughout a person's life. How serious an arachnoid cyst is depends on its size, where it is, and if it grows over time.

If an arachnoid cyst creates pressure, it may cause more serious issues, such as vision problems, seizures, and delays in development. But if there aren't clear neurological signs or symptoms, they're usually left alone.

Symptoms of Arachnoid Cysts

The symptoms of an arachnoid cyst depend on its size and where it is. A small cyst may not cause any symptoms, and your doctor may only find it during an exam for something else.

But large arachnoid cysts can cause neurologic symptoms, including:

  • Headaches.
  • Nausea and vomiting.
  • Seizures.
  • Hearing and vision problems.
  • Balance problems.
  • Hydrocephalus (enlargement of the head due to a buildup of CSF).
  • Stroke-like symptoms such as weakness, coordination problems or issues with speech.

Diagnosing an Arachnoid Cyst

To diagnose an arachnoid cyst, your doctor may ask you about symptoms you're having, do a physical exam, and order imaging tests.

Imaging tests for diagnosing arachnoid cysts

Doctors often find larger cysts causing symptoms through imaging tests.

As part of a neurologic exam, your doctor may order a:

  • MRI, which can tell fluid-filled cysts apart from other types of cysts.
  • CT cisternogram, where injected dye can find trapped fluid in CSF spaces.

Arachnoid Cyst Treatment

Only arachnoid cysts that cause symptoms need treatment. Surgery is only done if symptoms have no other causes or don't ease from other treatments.

Surgery for arachnoid cysts

During surgery, doctors drain the cyst and prevent it from refilling.

To prevent the cyst from refilling, doctors remove the outer membrane of the cyst or open a small window on the cyst (fenestrating the capsule) to create wide communications with the normal surrounding subarachnoid space. If no other spaces are next to the cyst, shunting of the cyst may be necessary, where the cyst fluid is shunted into the belly to relieve any pressure in the cyst.

Doctors can access certain arachnoid cysts of the skull base, such as sellar and suprasellar cysts, with state-of-the-art, minimally invasive treatments:

Surgeons use these methods to access the cyst through the nose or through the brain with an endoscope. Surgeons then drain the cyst or shunt it if necessary.

Benefits of EEA include:

  • No large incisions to heal.
  • No disfigurement.
  • Faster recovery time.