What is an arachnoid cyst?

An arachnoid cyst is most likely to develop in your head, but it can also develop around your spinal cord. It’s called an arachnoid cyst because it occurs in the space between your brain, or spinal column, and your arachnoid membrane. This is one of three membrane layers that surround your brain and spine. If an arachnoid cyst develops in your head, it will grow between your brain and skull or in pockets around your brain called ventricles.

Arachnoid cysts are usually filled with cerebrospinal fluid (CSF). CSF is a naturally occurring protective fluid that surrounds your brain and spinal column. The walls of an arachnoid cyst don’t allow this fluid to drain into your CSF system, causing it to accumulate inside.

In children, arachnoid cysts are usually congenital, or present at birth. These cysts are called primary arachnoid cysts. Arachnoid cysts that develop later in life are called secondary arachnoid cysts. Primary arachnoid cysts are more common than secondary arachnoid cysts.

Arachnoid cysts are usually asymptomatic. That means they don’t tend to produce symptoms. As a result, most people who have an arachnoid cyst don’t realize it until they’re examined for other issues, such as head injuries.

In some cases, arachnoid cysts do cause symptoms. Symptoms depend on the location and size of the cyst. For example, symptoms may develop if you have a cyst that presses on nerves or sensitive areas of your brain or spinal cord. If it’s located in your brain, the cyst may produce one or more of the following symptoms:

  • headache
  • dizziness
  • nausea
  • vomiting
  • lethargy
  • seizures
  • problems hearing, seeing, or walking
  • balance issues
  • developmental delay
  • dementia

If it’s located in your spinal column, it may cause symptoms such as:

  • back pain
  • scoliosis
  • muscle weakness or spasms
  • lack of feeling or tingling in your arms or legs
  • problems controlling your bladder or bowels

If you develop these symptoms, contact your doctor.

Primary, or congenital, arachnoid cysts are usually caused by an abnormal growth of your brain and spinal column while you’re developing in utero. The exact cause of this growth is not known. It might be genetic.

Secondary arachnoid cysts, also known as noncongenital arachnoid cysts, can be caused by several things. These include:

  • injury to your head or spinal cord
  • complications from brain or spinal surgery
  • meningitis
  • tumors

Arachnoid cysts are most common in children. They’re more likely to develop in men than women, reports the National Organization for Rare Disorders.

If your doctor suspects you have an arachnoid cyst, they will likely order imaging tests. For example, they may use CT and MRI scans to look at your brain or spine.

If you have an arachnoid cyst that isn’t causing any symptoms or other complications, your doctor may advise you to leave it untreated. They will likely monitor the cyst over time to watch for potential growth or other changes. If it begins to cause problems, they may recommend treatment.

If you have an arachnoid cyst in your brain that’s causing symptoms, your doctor will likely drain it. They may recommend one of two procedures. In the first procedure, they will make a small incision near the cyst and insert an endoscope with a small camera on the end. They will use this endoscope to gently open the cyst, allowing the fluid inside to drain into your CSF system, where it will be redistributed through your body. This procedure is called fenestration. In the second procedure, your doctor will insert a shunt into the cyst. This will allow the fluid inside to drain to another part of your body, such as your belly.

If you have a symptomatic cyst in your spinal column, your doctor may recommend surgery to remove it completely. If that’s not feasible, they may use fenestration or shunting to drain it.

If you have an asymptomatic cyst, you will likely be able to enjoy a normal life, even without treatment. Your doctor will generally encourage you to schedule regular check-ups to monitor it for changes.

If you have a symptomatic cyst, draining or removing it should resolve your symptoms.

In rare cases, an untreated expanding arachnoid cyst could cause permanent neurological damage.

Ask your doctor for more information about your specific condition, treatment options, and long-term outlook.