Arachnoid Cysts

Medically Reviewed By William C. Lloyd III, MD, FACS
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What are arachnoid cysts?

Arachnoid cysts are fluid-filled sacs that develop between the central nervous system (brain and spinal cord) and the arachnoid membrane, one of three protective membranes that surround the central nervous system. Cells from the arachnoid layer produce cerebrospinal fluid, the fluid that protects and nourishes the brain and spinal cord. Consequently, arachnoid cysts are filled with cerebrospinal fluid.

Arachnoid cysts can be primary (present at birth) or secondary (arising due to disease or injury). Sometimes, cysts will develop without leading to symptoms. Most arachnoid cysts are harmless and produce no symptoms. If the cyst is large, however, it may press on the central nervous system, which can cause headaches, lethargy, and neurological damage. In these cases, the cyst may require surgical draining.

Arachnoid cysts usually develop before the age of 20 and are present in men more often than in women. Left untreated, large arachnoid cysts may lead to complications, such as hydrocephalus (fluid buildup in the skull), early onset of puberty, and visual impairment.

Seek immediate medical care (call 911) for serious symptoms of pressure on the brain, including a sudden, severe, unexplained headache or a seizure.

Seek prompt medical care if you have ongoing symptoms that may be related to the presence of an arachnoid cyst, such as unexplained headaches or a change in vision. Also seek prompt medical care if you are being treated for arachnoid cysts but mild symptoms recur or are persistent.

What are the symptoms of arachnoid cysts?

Small arachnoid cysts are usually symptomless and may not require treatment. If arachnoid cysts grow significantly, they may cause symptoms, such as headache, weakness and fatigue. In serious cases, symptoms may become life threatening. Symptoms occur far more frequently in children compared to adults. Serious symptoms of arachnoid cysts include seizures, developmental problems, sensory or motor impairment, and hydrocephalus (fluid buildup in the skull). The symptoms of arachnoid cysts depend on which part of the brain they affect.

Early symptoms of arachnoid cysts

Arachnoid cysts that are small in size may cause no symptoms or mild symptoms. If arachnoid cysts are large enough, you may experience arachnoid cyst symptoms daily or just once in a while. At times any of these symptoms can be severe:

Later symptoms of arachnoid cysts

Larger arachnoid cysts can put pressure on the brain, leading to serious symptoms including:

  • Changes in hearing
  • Developmental delays and failure to thrive
  • Difficulty hearing
  • Head bobbing
  • Impaired balance and coordination
  • Loss of vision or changes in vision
  • Muscle twitching, spasms or seizures

Serious symptoms that might indicate a life-threatening condition

In some cases, arachnoid cysts can be life threatening. Seek immediate medical care (call 911) if you, or someone you are with, have any of these life-threatening symptoms including:

  • Seizure
  • Worst headache of your life

What causes arachnoid cysts?

Arachnoid cysts develop when cerebrospinal fluid, the fluid that surrounds and nourishes the brain and spinal cord, fills sacs that develop in the arachnoid mater, one of the layers of tissue that surrounds the brain. Often, arachnoid cysts are present at birth. These are called primary arachnoid cysts and are related to developmental abnormalities. Secondary arachnoid cysts can develop because of infection or surgery of the brain or spinal cord and are uncommon.

What are the risk factors for arachnoid cysts?

A number of factors increase the risk of developing arachnoid cysts. Not all people with risk factors will get arachnoid cysts. Risk factors for arachnoid cysts include:

How are arachnoid cysts treated?

If arachnoid cysts are small, they generally do not require treatment. But if they begin to press on the brain or spinal cord, treatment is necessary to alleviate symptoms.

Surgical treatment for arachnoid cysts

Surgery to treat arachnoid cysts is aimed at draining the fluid from the cyst or puncturing a hole in the arachnoid membrane, which forms the sac. Types of surgery for arachnoid cysts include:

  • Needle aspiration or burr hole to drain the cyst (cyst more likely to recur with this treatment)

  • Neurosurgery to create multiple holes that continuously drain the cyst (fenestration)

  • Placement of a shunt to drain cerebrospinal fluid, which is a relatively simple procedure but may lead to acute and long-term complications (infection, obstruction, scarring, etc.)

What are the potential complications of arachnoid cysts?

Early detection and treatment of arachnoid cysts will help prevent symptoms from developing. If the cyst is allowed to grow, it may put pressure on the brain and spinal cord, leading to permanent neurological complications. Complications of untreated arachnoid cysts can be serious, even life threatening in some cases. You can help minimize your risk of serious complications by following the treatment plan you and your health care professional design specifically for you. Complications of arachnoid cysts include:

  • Brain damage

  • Failure to thrive in infants and children

  • Hydrocephalus (fluid buildup in the skull)

  • Permanent nerve damage including paralysis

  • Seizures and tremors

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2021 Jan 19
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THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
  1. NINDS arachnoid cysts information page. National Institute of Neurological Disorders and Stroke.
  2. Arachnoid cysts. UCLA Neurosurgery.
  3. Al-Holou WN, Terman S, Kilburg C, et al. Prevalence and natural history of arachnoid cysts in adults. J Neurosurg 2013; 118:222.