Arachnoid Cyst (Leptomeningeal Cyst)

An arachnoid cyst is a fluid-filled sac of the brain or spinal cord. It forms between the brain or spinal cord and the arachnoid membrane. This membrane is a thin layer of tissue around the brain. It’s one of the protective coverings around the brain. An arachnoid cyst contains cerebrospinal fluid (CSF). This is a normal fluid that’s found around the brain and spinal cord. These cysts appear most often in children, but they may also occur in adults.

What causes an arachnoid cyst?

The cyst may form during the first few weeks when a baby is growing in the womb. This is called a primary arachnoid cyst. In some cases, the cyst may be caused later in life. It can be caused by a head injury, meningitis, or tumors, or from brain surgery. This type is called a secondary arachnoid cyst. These are less common than primary arachnoid cysts.

Symptoms of an arachnoid cyst

The symptoms vary depending on where the cyst is. In some cases, a small cyst may not cause any symptoms. Some cysts cause no symptoms until they grow large.

Symptoms can occur a bit differently in each person, but can include:

  • Headache

  • Nausea and vomiting

  • Vertigo or dizziness

  • Hearing or vision problems

  • Trouble with balance and walking

  • Seizures

  • Behavioral changes

An arachnoid cyst on the spinal cord can compress the cord or nerve roots and cause:

  • Back and leg pain

  • Numbness and tingling in arms or legs

  • Weakness in arms or legs

  • Problems with bowel and bladder control

Diagnosing an arachnoid cyst

In some cases, your healthcare provider may discover a cyst when it shows up on an imaging scan done for another reason. In other cases, you may have symptoms from the cyst. Your healthcare provider may refer you to a neurologist. This is a doctor who specializes in diagnosing and treating diseases of the central nervous system (brain and spinal cord). Or you may be referred to a neurosurgeon. This is a surgeon who does brain or spinal cord surgery.

The process to diagnose a cyst starts with a medical history and a physical exam. Your healthcare provider will ask about your symptoms and past medical conditions. He or she may also ask about your family’s medical history. The physical exam may include a neurologic exam. Imaging tests may be done to look at the brain. Contrast dye may be used to help show more detail in the images. The tests may include:

  • CT scan. This is a test that uses a series of X-rays and a computer to create images of the inside of the body. Scans may be done of your brain and spinal cord.

  • MRI. This test uses large magnets and a computer to create images of the body. MRI scans of your brain and spinal cord may be done to get more information about the cyst and nearby tissues.

Scans may be repeated over time to learn if the cyst is growing.

Treatment for an arachnoid cyst

If a brain cyst is causing problems, your healthcare provider may advise removing it with surgery. If the cyst isn’t causing symptoms and isn’t growing, your healthcare provider may choose to watch it closely with repeated brain scans. Treatment depends mostly on the location and size of the cyst.

If you need treatment, your healthcare provider may puncture the cyst sac and drain the fluid. The fluid drains into the CSF. Surgical removal of the cyst may also be a choice.

Possible complications of an arachnoid cyst

An arachnoid cyst that is untreated may cause permanent neurological damage as the cyst expands or bleeding occurs. Discuss treatment choices with your healthcare provider.

Call 911

Call 911 or seek emergency treatment if you have: 

  • Severe sudden headache

  • Seizures


When to call your healthcare provider

Call your healthcare provider right away if you have any of these:

  • Nausea and vomiting

  • Vertigo or dizziness

  • Hearing or vision problems

  • Difficulties with balance and walking

  • Back and leg pain

  • Numbness and tingling in arms or legs