Arachnoid cysts are fluid-filled sacs in the brain or spinal cord. They can be present before birth or result from a head injury. They typically do not cause symptoms or require treatment.

Many cysts are benign and do not cause symptoms. However, a cyst can expand. This can compress surrounding structures and cause symptoms.

This article explores arachnoid cysts, their symptoms, causes, treatment, and more.

A note about sex and gender

Sex and gender exist on spectrums. This article will use the terms “male,” “female,” or both to refer to sex assigned at birth. Click here to learn more.

A scan of an arachnoid cyst in a person's brain.Share on Pinterest
Design by Medical News Today; photograph James Heilman, MD, CC BY-SA 4.0, via Wikimedia Commons

Arachnoid cysts are a type of brain cyst that account for 1% of all lesions found in the brain. They are sacs filled with cerebrospinal fluid (CSF).

Three membrane layers cover and protect the brain and spinal cord. The arachnoid membrane is the middle meninges layer between the dura mater and pia mater. Arachnoid cysts occur in the arachnoid membrane.

Arachnoid cysts mainly occur in the brain. However, they can also appear in the spinal cord. They may be small or large, are typically incidental, and do not often cause symptoms.

There are two types of arachnoid cysts:

  • Primary arachnoid cysts: These are present at birth and occur during early embryonic development.
  • Secondary arachnoid cysts: These are less common. They develop due to conditions affecting the brain.

Some arachnoid cysts do not cause symptoms, especially if they are small. When symptoms do occur, they depend on the cyst’s size, location, and any structures it is compressing. Most symptoms occur in childhood.

If the cyst is in the brain, it can cause the following symptoms:

An arachnoid cyst in the spinal cord may compress the cord or nerve roots. It can cause symptoms such as:

Experts do not know the exact cause of congenital arachnoid cysts. They may happen due to the abnormal splitting or tearing of the arachnoid membrane during its development.

Secondary arachnoid cysts can occur as a result of a range of factors, including:

Doctors often discover arachnoid cysts incidentally, which means discovered while investigating another health concern.

A thorough clinical examination, medical history, and a series of specialized tests can help doctors identify the presence of an arachnoid cyst.

CT imaging is enough to make a definitive diagnosis of the condition. However, a doctor may also use MRI to rule out other conditions.

Arachnoid cysts can affect anyone. However, they are two times more common in males than in females, depending on the location of the cyst.

Hereditary factors may play a role in the development of the condition. This means that individuals who belong to families with a predisposition for this condition may be at higher risk of developing a cyst.

Some syndromes also have associations with arachnoid cysts, including mucopolysaccharidosis and Marfan syndrome.

Arachnoid cysts that do not cause symptoms typically do not need treatment. However, doctors may monitor these people regularly and reevaluate the cyst if symptoms appear.

In cases where arachnoid cysts cause symptoms, a person may require surgery. The type of surgery will depend on the location of the cyst and whether there is involvement of surrounding neurological structures.

Surgical options may include:

  • Surgical excision: This involves excising (cutting) into the membrane wall between the subarachnoid space and the lesion.
  • Fenestration: This involves opening the cyst and allowing its contents to drain. Depending on the location, the doctor may perform an endoscopy or a craniotomy.
  • Shunting: This procedure diverts fluid from the cyst. However, doctors rarely do this due to the complications associated with long-term shunt placement.

Emergency treatment may be necessary if hydrocephalus (a buildup of fluid in the brain) occurs, or the cyst grows and causes more symptoms.

The symptoms of an arachnoid cyst may be similar to that of other conditions, including:

A person should contact a doctor immediately if they show symptoms of an arachnoid cyst. Prompt treatment may help stop the symptoms and reduce the risk of the cyst causing long-term problems.

No treatment is necessary in the majority of cases. For cysts that do need treatment, surgery can be an effective treatment option.

A 2022 systematic review suggests that the recurrence rate of arachnoid cysts after surgery is very low. Of the 21 people involved in the studies, only one had cyst recurrence.

However, the review suggested that if doctors do not address any dural defects, the recurrence rate increases slightly. (A dural defect is a layer of tissue in the spine that a cyst protrudes through.)

If arachnoid cysts damage the brain or spinal cord through progressive expansion or bleeding, they may cause permanent neurological damage if left untreated.

Here are some answers to frequently asked questions about arachnoid cysts.

Do arachnoid cysts require removal?

Most arachnoid cysts do not require removal. Only those causing symptoms may require surgery.

Can an arachnoid cyst become cancerous?

No, arachnoid cysts are not cancerous. They are fluid-filled sacs containing CSF.

What happens if an arachnoid cyst ruptures?

A ruptured arachnoid cyst may cause sudden and life threatening symptoms, including sustained intracranial hypertension.

Arachnoid cysts are often not a cause of concern and require no treatment. However, a doctor should check them regularly to monitor for any changes in the cyst.

A person may need surgery to drain or remove a cyst if it is causing symptoms.

It is essential for a person to immediately contact a healthcare professional if they experience any symptoms of an arachnoid cyst. Prompt evaluation of the cyst is crucial to avoid complications.