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What is an Arachnoid Cyst?

An arachnoid cyst is a noncancerous fluid-filled sac that occurs on the arachnoid membrane that covers the brain and the spinal cord.

Three layers of tissue surround the brain and spinal cord. The middle layer is called the arachnoid membrane, and this is where an arachnoid cyst develops. Arachnoid cysts usually occur in the brain, but can also appear in the spinal cord as well.

Arachnoid cysts are the most common type of brain cyst. These cysts are filled with cerebrospinal fluid (CSF), the clear fluid that surrounds the brain and spinal cord. Arachnoid cysts are typically congenital, meaning they are present at birth (primary arachnoid cysts), and they may develop in various locations within the brain or spinal cord. Arachnoid cysts can also develop later in life due to head injury or trauma, meningitis, or brain surgery. This is referred to as a secondary arachnoid cyst. Primary arachnoid cysts are more common than secondary arachnoid cysts.

Untreated arachnoid cysts may cause permanent, severe neurological damage due to expansion of the cyst, hydrocephalus, or hemorrhage. With treatment, most individuals with arachnoid cysts have a good prognosis.

Causes of Arachnoid Cyst

Primary, or congenital, arachnoid cysts are thought to result from developmental abnormalities during fetal growth. The exact reason for this is not known, although it might be due to factors such as genetics, inflammatory processes, or infections.

Secondary arachnoid cysts can be caused by several factors such as trauma to the head or spinal cord, tumors, meningitis, and complications from brain or spinal surgery.

Symptoms of Arachnoid Cyst

Most arachnoid cysts do not cause symptoms and may go unnoticed for many years. However, if a cyst grows larger or puts pressure on nearby structures, it can lead to symptoms. In general, symptoms depend on the location and size of the cyst. If the cyst is located in the brain, the cyst may produce one or more of the following symptoms:

  • Headaches
  • Seizures
  • Developmental delays
  • Nausea
  • Vomiting
  • Problems hearing, seeing, or walking
  • Dementia
  • Balance issues
  • Fluid build-up in the brain (hydrocephaly)

If the cyst is located in the spinal column, it may cause symptoms such as:

  • Scoliosis
  • Back pain
  • Muscle spasms or weakness
  • Trouble controlling bladder or bowels
  • Tingling or lack of feeling in your legs or arms

Diagnosis of Arachnoid Cyst

An Arachnoid cyst is typically diagnosed with imaging studies such as a CT scan or a brain MRI. These tests also show the cyst's size and location, which will assist your physician to devise a proper treatment plan.

Treatment for Arachnoid Cyst

The treatment for arachnoid cysts depends on several factors, including the size and location of the cyst, the presence and severity of symptoms, and the individual's overall health. Some common approaches for the treatment of arachnoid cysts include:

  • Observation: If the arachnoid cyst is small, asymptomatic, and not causing any issues, your physician may choose a conservative approach of regular monitoring without active intervention. In many cases, these cysts do not require treatment.
  • Symptomatic Management: Medications may be prescribed to manage specific symptoms associated with arachnoid cysts. For example, pain relievers may be given for headaches, and antiepileptic drugs may be prescribed for seizures if they are present.
  • Surgical Intervention: Surgical treatment may be considered in cases where the cyst is causing significant symptoms or if there is concern about its potential to grow and cause complications. Surgical options may include:
    • Cyst Fenestration: This involves creating an opening in the cyst wall to allow the cerebrospinal fluid to flow out and be absorbed by surrounding tissues.
    • Cyst Shunting: A shunt may be inserted to create a drainage pathway for the cerebrospinal fluid, diverting it away from the cyst and relieving pressure.
    • Endoscopic Treatment: Minimally invasive endoscopic procedures may be used to address the cyst, including creating fenestrations or removing a portion of the cyst wall.
  • Cyst Excision: In some cases, particularly when the cyst is causing severe symptoms and is accessible, surgical removal (excision) of the cyst may be considered.
  • Hydrocephalus Management: If the arachnoid cyst is associated with hydrocephalus (accumulation of fluid in the brain), the healthcare team may address both conditions through surgical interventions such as shunt placement.

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