Arachnoid Cysts

What is an Arachnoid Brain Cyst?

Arachnoid cysts are sacs filled with cerebrospinal fluid that form between your brain and spinal cord, in a space called the arachnoid membrane.

There are two types of arachnoid cysts, according to the National Institutes of Health:

  • Primary arachnoid cysts, which are present when you’re born, and are typically caused by abnormal development in the womb
  • Secondary arachnoid cysts, which are less common, and may be the result of trauma, tumors or meningitis

Where Are Arachnoid Cysts Located?

Arachnoid cysts are located in the space between the brain or spinal cord and the arachnoid membrane. The arachnoid membrane is one of the three membranes that encase the brain and spinal cord. The cysts develop in the brain or spine and grow primarily in a region outside of the temporal lobe of the brain, known as the middle cranial fossa.

What is an Arachnoid?

Arachnoid cysts are the most common type of brain cysts. They are often congenital but may also develop later in life from a head injury or trauma. The cysts are not tumors, but rather fluid-filled sacs. Arachnoid cysts happen four times as often in males, and typically do not require treatment if the cyst remains stable. 

How Serious is a Cyst on the Brain?

The prognosis for a cyst on the brain is dependent on the stability of the cyst, and whether the expansion of it, or bleeding into the cyst, is causing damage to the brain or spinal cord. In an extreme case, if the cyst becomes unstable and is left untreated, it can create permanent severe neurological damage. However, most arachnoid cysts do not require treatment. If symptoms are present, it is important to reach out to your Baptist Health primary care physician for diagnosis and treatment. Symptoms usually resolve or improve with treatment.

Subarachnoid Cysts

Subarachnoid cysts are located within the intracranial compartment, and less commonly in the spinal cord. The cysts are typically found within the subarachnoid space and contain cerebrospinal fluid.


Symptoms of arachnoid cysts, and when they begin, usually depend on the size and location. Most people with primary arachnoid cysts in the brain, the more common location, experience symptoms such as these before age 20:

  • Headache
  • Nausea and vomiting
  • Seizures
  • Vertigo
  • Nerve conduction studies
  • Trouble hearing or seeing

If an arachnoid cyst is present around the spinal cord, which is less common, you may also experience other side effects such as progressive numbness, tingling and/or pain in your arms, back and legs.

Diagnosis of Arachnoid Cysts

Arachnoid cysts are usually diagnosed with imaging tests, including Magnetic Resonance Imaging (MRI) scan of brain or spine.

Your doctor may use an arachnoid cysts MRI to produce detailed images of your spine. This helps them determine the exact location and characteristics of the cyst. Your doctor may also use an arachnoid cyst CT scan to produce images of your brain. The CT stands for Computed Tomography.

If you or a loved one experience any of the symptoms of arachnoid cysts, please contact a neurologist at Baptist Health.

Treatment of Arachnoid Cysts

Whether you need treatment for arachnoid cysts depends on the severity of your symptoms and the size and location of the cyst.

The size of arachnoid cysts varies. The average arachnoid cyst size is less than 3 cm. An arachnoid cyst size of 3 cm or greater is considered dangerous.

If your cyst is in a potentially dangerous location, has enough size and is causing symptoms, minimally-invasive surgery may be an option.

Arachnoid cyst treatment without surgery is also possible. Even large cysts that do not trigger symptoms may be treated solely through scheduled monitoring and revaluation. In these cases, the treatment is focused on reducing or preventing specific symptoms.

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