Arachnoid Cysts

Arachnoid cysts are fluid-filled sacs that form on the arachnoid membrane, one of the three membranes covering the brain and spinal cord. These cysts contain cerebrospinal fluid, the clear fluid that surrounds and protects the brain and spinal cord. Most arachnoid cysts are present from birth and usually do not cause symptoms.

What is Arachnoid Cysts?

Arachnoid cysts are non-cancerous, fluid-filled sacs that develop between the brain or spinal cord and the arachnoid membrane, which is the middle layer of the three membranes (meninges) that protect the central nervous system. These cysts are typically filled with cerebrospinal fluid (CSF), the same fluid that cushions the brain and spinal cord.

These cysts are called "arachnoid" because they form within the arachnoid membrane, which resembles a spiderweb. They are relatively common, affecting about 1 in 100 people (1%) in the general population, though many individuals never experience symptoms. Most arachnoid cysts are primary (congenital), meaning they are present at birth. They form during the early stages of brain and spinal cord development. Less commonly, they can be secondary (acquired), developing later in life due to head injuries, infections, tumors, or complications from surgery. While arachnoid cysts can occur anywhere along the brain or spinal cord, they are most frequently found in specific areas of the brain, such as the middle cranial fossa (near the temples) and the posterior fossa (at the back of the head). They vary greatly in size, from very small to quite large, and their size and location determine if they cause any problems.

Symptoms

Many people with arachnoid cysts never experience any symptoms, and the cyst is often discovered incidentally during imaging tests for other conditions. When symptoms do occur, they depend on the cyst's size and location, often resulting from pressure on the brain or spinal cord.

If an arachnoid cyst grows large enough or presses on sensitive areas, it can cause a range of symptoms. Common symptoms include headaches, nausea, vomiting, and dizziness. Some individuals may experience seizures, which are sudden, uncontrolled electrical disturbances in the brain. In children, symptoms might include hydrocephalus (excess fluid in the brain), an abnormally large head (macrocephaly), or developmental delays. If the cyst is located on the spinal cord, it can lead to weakness or numbness in the arms or legs, back pain, or problems with balance and walking. Less common symptoms can include vision problems, hearing difficulties, or changes in behavior. It is important to remember that these symptoms are not unique to arachnoid cysts and can be caused by many other conditions. A doctor can help determine the cause of any new or worsening symptoms.

Causes & risk factors

The exact cause of most arachnoid cysts is unknown, but they are primarily considered congenital, meaning they develop before birth during the early stages of brain and spinal cord formation. Acquired cysts, which are less common, can form due to external factors later in life.

Primary (congenital) arachnoid cysts are believed to result from an abnormal splitting or duplication of the arachnoid membrane during fetal development. This creates a pocket that fills with cerebrospinal fluid. This developmental anomaly is not typically linked to genetic factors or inherited conditions. Secondary (acquired) arachnoid cysts are much rarer. They can develop as a complication of head trauma, such as a severe blow to the head, or after infections like meningitis. Other potential causes include tumors or complications following brain or spinal cord surgery. Unlike many other medical conditions, there are no clearly identified risk factors that increase a person's likelihood of developing a primary arachnoid cyst. They appear to occur randomly, affecting individuals of all ages, genders, and ethnicities without a clear pattern of inheritance.

How it's diagnosed

Arachnoid cysts are typically diagnosed using advanced imaging techniques, most commonly magnetic resonance imaging (MRI) or computed tomography (CT) scans. These tests allow doctors to visualize the brain and spinal cord and identify the presence, size, and location of any cysts.

Often, arachnoid cysts are discovered incidentally when a person undergoes an MRI or CT scan for an unrelated reason, such as a head injury or persistent headaches. If symptoms suggest a neurological issue, a doctor will usually perform a thorough neurological exam first. This exam checks reflexes, balance, coordination, and mental status. An MRI scan is generally preferred for diagnosing arachnoid cysts because it provides detailed images of soft tissues, including the brain and spinal cord, and can clearly differentiate between a cyst and other types of lesions. It can also show if the cyst is compressing surrounding brain tissue or causing hydrocephalus. In some cases, a CT scan might be used, especially in emergency situations or if an MRI is not possible. A CT scan uses X-rays to create cross-sectional images. These imaging tests help confirm the diagnosis, rule out other conditions, and guide treatment decisions.

Treatment options

Treatment for arachnoid cysts depends on whether they are causing symptoms. Many asymptomatic cysts, especially small ones, do not require treatment and are simply monitored over time. Symptomatic cysts often require surgical intervention to relieve pressure and drain the fluid.

For cysts that are not causing any problems, doctors usually recommend a "watch and wait" approach. This involves regular follow-up appointments and imaging scans to monitor the cyst's size and ensure no new symptoms develop. This is often the case for cysts discovered incidentally. When a cyst causes symptoms, grows significantly, or leads to complications like hydrocephalus, surgery is typically considered. The main goal of surgery is to drain the fluid from the cyst and relieve pressure on the brain or spinal cord. One common surgical procedure is fenestration, where a small opening is made in the cyst wall to allow the fluid to drain into the surrounding cerebrospinal fluid space. Fenestration can be performed using an endoscope, a thin tube with a camera, through a small incision (endoscopic fenestration), or through a larger opening in the skull (open craniotomy). Another surgical option is the placement of a shunt, a thin tube that drains the cyst fluid to another part of the body, such as the abdominal cavity (cystoperitoneal shunt), where it can be absorbed. Your doctor will discuss the most appropriate treatment based on your specific situation.

Recovery & outlook

The outlook for people with arachnoid cysts is generally very good, especially for those who never develop symptoms. For individuals who undergo treatment, recovery depends on the type of surgery and the severity of symptoms before the procedure, with many experiencing significant improvement.

Most arachnoid cysts are benign and do not cause long-term health problems. If a cyst is asymptomatic and remains stable in size, it may never require intervention, and individuals can live normal, healthy lives. Regular monitoring is often recommended to ensure no changes occur. For those who undergo surgery, the recovery period varies. Patients may experience some pain or discomfort at the surgical site, and it can take several weeks to fully recover. The success of surgery in relieving symptoms is generally high, particularly for headaches, seizures, and neurological deficits caused by pressure. While surgery is often effective, there is a small risk of complications, such as infection, bleeding, or recurrence of the cyst. Some individuals may require long-term follow-up or additional procedures. However, with appropriate medical care, most people with symptomatic arachnoid cysts experience a good quality of life after treatment.

When to see a doctor

You should see a doctor if you experience new or worsening symptoms that could be related to an arachnoid cyst, especially if you have a known cyst. Seek immediate medical attention for sudden, severe symptoms like intense headaches, seizures, or significant changes in consciousness.

If you have been diagnosed with an arachnoid cyst and notice any changes in your health, such as new headaches, vision problems, weakness, numbness, or balance issues, contact your doctor. These could be signs that the cyst is growing or causing increased pressure. Emergency signs that require immediate medical attention include a sudden, severe headache that is different from any you've experienced before, new onset of seizures, sudden weakness or numbness on one side of your body, difficulty speaking, or any sudden changes in your mental state or level of alertness. These symptoms could indicate a serious complication, such as bleeding into the cyst or increased pressure on the brain. Even if you have not been diagnosed with an arachnoid cyst, but experience any of these severe or rapidly worsening neurological symptoms, it is crucial to seek emergency medical care. Prompt evaluation can help identify the cause and ensure you receive timely and appropriate treatment.

Frequently asked questions

Are arachnoid cysts dangerous?

Most arachnoid cysts are not dangerous and do not cause any symptoms or problems. They are non-cancerous and often discovered incidentally. However, if a cyst grows large or presses on vital brain or spinal cord areas, it can cause symptoms that may require medical attention or surgical treatment to prevent complications.

Can an arachnoid cyst go away on its own?

Arachnoid cysts typically do not go away on their own. They are usually stable in size or may grow slowly over time. In rare cases, a cyst might spontaneously rupture or decrease in size, but this is not common. Most cysts that cause symptoms require medical intervention to resolve.

Can an arachnoid cyst cause anxiety or depression?

While arachnoid cysts primarily cause physical symptoms due to pressure on the brain, some individuals may experience mood changes, including anxiety or depression, especially if the cyst affects areas of the brain involved in emotional regulation. Living with a chronic condition or experiencing neurological symptoms can also contribute to psychological distress.

Is surgery always necessary for an arachnoid cyst?

No, surgery is not always necessary. Many arachnoid cysts are asymptomatic and do not require any treatment, only monitoring. Surgery is typically recommended only when the cyst is causing significant symptoms, growing rapidly, or leading to complications like hydrocephalus or neurological deficits.

Can an arachnoid cyst affect my memory?

Yes, depending on its size and location, an arachnoid cyst can potentially affect memory. If a cyst presses on parts of the brain involved in memory formation or retrieval, it can lead to cognitive issues, including problems with short-term or long-term memory. These symptoms usually improve after successful treatment.

What is the difference between a primary and secondary arachnoid cyst?

A primary (congenital) arachnoid cyst is present from birth, forming during fetal development due to an abnormality in the arachnoid membrane. A secondary (acquired) arachnoid cyst develops later in life as a result of external factors like head trauma, infection, tumors, or complications from surgery.

Sources

• MedlinePlus — Arachnoid Cysts
• Mayo Clinic — Arachnoid Cysts
• Cochrane Library — Arachnoid Cysts