Arachnoid Cyst — Definition, Symptoms, Epilepsy Risk, and Treatment Options

Most readers benefit from the Quick Summary plus the sections on Does the Cyst Need Treatment?, When Is Surgery Necessary?, and Relationship Between Arachnoid Cyst and Epilepsy. The remaining sections provide deeper clinical detail.

Most arachnoid cysts do not need surgery. The key question is not the size alone, but whether MRI findings match the patient’s symptoms. Surgery is usually considered when there is documented growth, hydrocephalus, progressive neurological deficit, significant mass effect with matching symptoms, or drug-resistant seizures clearly linked to the cyst.

What Is an Arachnoid Cyst?

Arachnoid cyst is a benign, cerebrospinal fluid (CSF)-filled cavity located within the arachnoid membrane, one of the three layers covering the brain and spinal cord.

The brain and spinal cord are protected by three layers called meninges: the tough outer dura mater, the delicate inner pia mater, and the middle arachnoid membrane. An arachnoid cyst occurs when this middle membrane splits or doubles over, forming a „pocket“ that traps cerebrospinal fluid (CSF). CSF is a clear, nutrient-rich liquid that usually flows freely around the brain to provide cushioning. Essentially, the cyst is a localized collection of this natural fluid contained within a duplication of the protective arachnoid tissue.

It is not a tumor.

Image: MRI scan (T2-weighted scans) showing an arachnoid cyst in the temporal region (temple area). On this scan, the cyst appears as a bright white area.

Image: Arachnoid cyst CSF-filled cavity located within the arachnoid membrane, one of the three layers covering the brain and spinal cord.

Are Arachnoid Cysts Congenital?

In most cases — yes.

Approximately 80–90% are congenital (primary cysts).

How Do Arachnoid Cysts Form?

Most arachnoid cysts are present from birth (congenital). During early fetal development, the thin middle layer covering the brain (the arachnoid membrane) may not form as a perfectly smooth sheet. Instead, it can split or fold in a small area, creating a pocket. This pocket can trap normal cerebrospinal fluid (CSF) — the clear fluid that cushions and protects the brain. In some cases, fluid can enter this pocket more easily than it can leave, creating a “one-way valve” effect, which may slowly cause the cyst to enlarge over time. When the cyst forms very early in development, it may occupy space before the nearby brain tissue has fully expanded, which is why some larger temporal cysts are associated with mild underdevelopment of the adjacent brain region. Importantly, this reflects a variation in early development rather than brain damage occurring later in life.

Most of these are primary cysts, meaning they are present from birth, and they are usually detected incidentally later in life during scans for unrelated reasons.

Secondary (acquired) cysts

10–20% are acquired and may develop after:

• head trauma
• meningitis
• intracranial hemorrhage
• neurosurgical procedures

How Common Are Arachnoid Cysts?

• Found in 1–2% of the general population
• Most large imaging studies showing a slight male predominance
• Most are discovered incidentally on MRI

Where Are These Cysts Usually Found?

Arachnoid cysts can appear anywhere in the brain or spine, but they tend to favor certain „pockets.“ Here is how their location is typically distributed:

• Middle cranial fossa (temporal region) — 50–60%: This is the most common spot, located near the temples and behind the eyes. It’s the area where the “temporal pole” we mentioned earlier is located.
• Posterior fossa — 10–15%: This is the back of the head, near the base of the skull. This area houses the cerebellum, which controls balance and coordination.
• Suprasellar region — 5–10%: Located deep in the center of the brain, just above the area where the optic nerves cross and near the pituitary gland (the “master gland” for hormones).
• Convexity or interhemispheric — 5–10%: These are found on the top or outer surface of the brain, or in the deep groove that separates the left and right halves of the brain.
• Spinal — less common: These occur along the spinal cord rather than in the brain.

How Many Become Symptomatic?

Only 5–20% of arachnoid cysts become symptomatic. However, only a minority of symptomatic cases ultimately require surgical treatment.

The majority (≈80–95%) remain asymptomatic throughout life.

Why Do Symptoms Occur? (Pathophysiological Mechanisms)

Symptoms are not caused by the cyst “being there” — but by its mass effect or CSF dynamics disturbance.

While many cysts remain small and quiet, they can sometimes cause issues through these main mechanisms:

1. Expansion (The Valve Effect): Fluid enters but cannot exit, causing the cyst to grow and increase local pressure.
2. Compression: The cyst physically pushes against brain tissue, potentially disrupting the functions controlled by that specific area.
3. CSF blockage: distorting the natural flow of brain fluid and potentially causing pressure buildup.
4. Intracranial pressure: Large cysts can increase overall pressure inside the skull, leading to chronic headaches or nausea.
5. Seizure trigger: In selected cases, a cyst located adjacent to the cerebral cortex may contribute to abnormal electrical activity. This is more likely when there is cortical thinning or structural distortion near the cyst.

Importantly:
Many cysts are stable and never enlarge.

Symptoms of Arachnoid Cyst

Symptoms depend entirely on location.

Common symptoms:

• Headache (most frequent)
• Seizures
• Focal neurological deficit may include limb weakness, coordination problems (ataxia), visual disturbances such as blurred or double vision, facial numbness, speech difficulties, or balance impairment. These symptoms occur when the cyst compresses specific functional areas of the brain.
• Balance disturbance (posterior fossa)
• Visual disturbances (suprasellar)
• Cognitive or behavioral changes (large temporal cysts)
• Hydrocephalus (rare but possible)

In children:

• Macrocephaly
• Developmental delay
• Irritability

Relationship Between Arachnoid Cyst and Epilepsy

This is one of the most misunderstood aspects.

Important principle:

Not every seizure in a patient with an arachnoid cyst is caused by the cyst.

Cysts may be:

• Incidental finding in epileptic patients
• True epileptogenic focus (rare but possible)

When is epilepsy likely cyst-related?

• Large temporal cyst
• Cortical thinning adjacent to cyst
• EEG abnormality localized to the cyst region (an electroencephalogram showing focal electrical dysfunction or „short circuits“ in the brain tissue immediately adjacent to the cyst).
• No other structural lesion on MRI

Even then, surgery does not guarantee seizure freedom.

Diagnosis

MRI: The Gold Standard

To confirm an arachnoid cyst, radiologists look for these four specific signs on your MRI:

• Matches brain fluid: The cyst looks identical to your normal brain fluid (CSF) on all types of scans.
• No contrast „glow“: It does not absorb contrast dye (no enhancement), which helps rule out tumors.
• Purely liquid: It is a hollow pocket of fluid with no solid or fleshy parts inside.
• Free movement: The fluid inside moves freely (no diffusion restriction), which distinguishes it from other types of cysts.

What a CT Scan Shows

A CT scan is often the first step in finding a cyst. Doctors look for:

• Dark pocket: The cyst appears as a dark area because fluid is less dense than brain tissue.
• Bone remodeling: In long-standing cases, the cyst can gently „reshape“ or thin the nearby skull bone. This is a sign the cyst has been there for a very long time.
• Mass effect: This is a term for the cyst „pushing“ or displacing the surrounding brain tissue due to its size.

Image: CT scan of the brain showing a right temporal arachnoid cyst.

Is an Arachnoid Cyst a Cause of Symptoms?

Finding a cyst on an MRI doesn’t always mean it is the cause of your symptoms.

Doctors use the following criteria to determine whether the cyst is truly responsible for the symptoms:

• Size vs. symptoms: A large cyst might cause zero issues, while a small one in a sensitive spot might be troublesome. Doctors check if your symptoms actually match the cyst’s size. Many middle cranial fossa (temporal) cysts smaller than approximately 2–2.5 cm remain asymptomatic, but clinical relevance depends more on mass effect and anatomical relationships than on absolute size.
• Location–symptom correlation: Doctors assess whether the cyst’s anatomical location matches the neurological findings. For example, a cyst near visual pathways should explain visual symptoms, while a posterior fossa cyst may correlate with balance or coordination problems. If the symptoms do not anatomically correspond to the cyst’s position, it is less likely to be the true cause.
• Anatomical change: Doctors look at how much the cyst is „pushing“ the brain or shifting its midline. If the brain looks comfortable despite the cyst, it is usually left alone.
• Fluid build-up (hydrocephalus): It is important to see if the cyst is blocking the brain’s drainage system. If it’s causing fluid to back up, it’s more likely to need attention.
• EEG testing: For patients with seizures, an EEG helps confirm if the electrical „short circuit“ is actually coming from the cyst area or somewhere else.
• Watching over time: Often, the best move is to do nothing and simply repeat the scan in 6–12 months to ensure the cyst isn’t growing.

Important note: Only after careful clinical and radiological correlation can a cyst reasonably be considered responsible for symptoms — and even then, surgery is not always necessary.

When is Surgery Necessary?

Surgery is not the default choice for most cysts. It is typically only considered in these specific situations:

• Worsening symptoms: If there is a clear „neurological deficit“—such as new weakness, balance issues, or loss of certain functions.
• Significant pressure: When the cyst is visibly pushing the brain out of its normal position (mass effect) and causing matching symptoms. Importantly, the clinical meaning of midline shift in arachnoid cysts is not the same as in acute hemorrhage or trauma. A few millimeters of chronic shift may be well tolerated, whereas similar numbers in an acute condition can be dangerous. Surgical decisions are therefore based on symptoms and progression — not on millimeter measurements alone.
• Documented growth: If follow-up scans show the cyst is actually getting larger over time.
• Fluid back-up (hydrocephalus): If the cyst blocks the brain’s drainage system, causing dangerous pressure build-up.
• Special cases of epilepsy: Only when seizures cannot be controlled by medication and tests prove the cyst is the exact trigger.

Arachnoid Cyst and Headaches

Headache is the most commonly reported symptom in symptomatic arachnoid cysts, occurring in roughly 50–70% of cases. Having a headache alone is usually not a reason for surgery. Headaches are very common and often have causes unrelated to the cyst. Conditions such as migraine, tension-type headache, or occipital neuralgia are far more frequent explanations. A headache needs a standard headache evaluation regardless of the cyst. Unless the headache is clearly linked to increased intracranial pressure, progressive mass effect, or documented cyst enlargement, surgery is unlikely to resolve it. Ideally, this assessment is performed by a neurosurgeon familiar with both primary headache disorders and the surgical decision-making process for arachnoid cysts.

Types of Surgical Treatment

There is no single “best” operation for all arachnoid cysts.

The choice of surgical technique depends primarily on cyst location, anatomical relationships, and the presence of symptoms such as mass effect or hydrocephalus. Endoscopic fenestration is generally preferred when the cyst can be safely connected to normal cerebrospinal fluid spaces through a minimally invasive approach. Open microsurgical fenestration (craniotomy) is considered when the cyst has complex anatomy, thick walls, internal compartments, or lacks a safe nearby fluid space for endoscopic communication. Cystoperitoneal shunting is usually reserved for selected cases in which fenestration is not feasible, has failed, or when long-term cerebrospinal fluid diversion is required. The decision is individualized and based on detailed imaging analysis.

Endoscopic Fenestration: The Preferred Method in Many Cases

When it works best:
Endoscopic fenestration is most effective when the cyst is located near natural cerebrospinal fluid (CSF) spaces that allow safe internal drainage. It is commonly used for many middle cranial fossa (temporal region) cysts, as well as selected suprasellar and intraventricular cysts where a clear pathway to the ventricles or basal cisterns can be created.

When it may not be suitable:
This technique is less appropriate for cysts with thick walls, internal partitions (multiloculated cysts), or those located in areas where there is no safe adjacent CSF space to connect them to. Some convexity cysts, deep interhemispheric cysts, or anatomically complex posterior fossa cysts may require a different surgical strategy. Most commonly, these cases are treated with open microsurgical fenestration via craniotomy, which allows a wider and more controlled opening of the cyst wall, and in selected situations partial wall excision. If fenestration is not feasible or symptoms recur, cystoperitoneal shunting may be considered.

The procedure of endoscopic fenestration: Think of this as „opening a window.“ During an endoscopic fenestration, the surgeon makes a small opening in the skull and inserts a tiny camera (endoscope). They then create a small hole in the wall of the cyst, allowing the trapped fluid to finally flow out and rejoin the brain’s natural circulation system.

Why It Is Often the Best Choice:

• Minimally invasive: It is performed through a small keyhole opening rather than a large incision.
• Lower risk: Because it doesn’t involve leaving a permanent foreign object (like a tube) in the body, it has a lower long-term complication rate.
• Shorter recovery: Patients typically spend less time in the hospital and return to their daily lives much faster.
• Common first-line option for some locations: In many middle cranial fossa (temporal) cysts, as well as selected suprasellar and quadrigeminal cysts, endoscopic fenestration is often considered the preferred initial surgical approach when safe anatomical access is possible.

Image: Endoscopic fenestration of a suprasellar arachnoid cyst. The endoscope is shown navigating through the brain’s ventricles to reach the cyst at the skull base. A small opening is made in the cyst wall, allowing cerebrospinal fluid (CSF) to flow into the natural brain cisterns.

Open Craniotomy and Fenestration

The Procedure: Unlike the minimally invasive endoscopic approach, this is a traditional „open“ surgery. The surgeon removes a small piece of the skull (a bone flap in craniotomy) to gain direct access to the cyst. Under a high-powered microscope, they carefully open the cyst walls to create a permanent drainage path into the brain’s natural fluid spaces. At the end of the procedure, the bone flap is replaced and the incision is closed.

Key Characteristics:

• Direct visualization: It provides the surgeon with the most detailed, 3D view of the cyst and surrounding blood vessels and nerves.
• For complex anatomy: This method is often chosen when the cyst has thick walls or internal partitions (multiloculated structure), or when it is located in anatomically challenging regions such as the posterior fossa (the area at the back of the brain), the deep interhemispheric region (between the two brain hemispheres), or along the brain’s outer surface (convexity), where safe endoscopic access or durable communication with cerebrospinal fluid spaces may not be feasible.
• Higher surgical burden: Because it is a more significant operation, it requires a larger incision and more time in the operating room.
• Effective but Invasive: While highly successful in permanently opening the cyst, it involves a longer recovery period compared to endoscopic methods.

Image: Craniotomy showing the bone flap created to access and treat the cyst. After the procedure, the bone flap is replaced and secured, and the incision is closed.

Cysto-Peritoneal Shunt

The Procedure: This method involves placing a permanent, thin tube (catheter) into the cyst through a tiny hole in the skull. The tube runs under the skin all the way down to the abdomen (peritoneal cavity). A specialized valve is connected to the tube to regulate how much fluid leaves the brain.

Key Characteristics:

• Fluid diversion: Instead of opening the cyst into the brain’s natural spaces, it redirects the fluid to another part of the body.
• The „backup“ plan: It is typically used when fenestration (opening the cyst) has failed or when the cyst’s location makes other surgeries impossible.
• Shunt dependency: A major drawback is that the patient may become „shunt-dependent,“ meaning they rely on the tube for life to manage brain pressure.
• Long-term risks: Because it is a foreign object in the body, there is a lifelong risk of the tube getting blocked (malfunction) or becoming infected.
• Selected cases: Due to these risks, this procedure is usually reserved for specific, difficult cases where other treatments aren’t an option.

Recovery After Arachnoid Cyst Surgery

Recovery after arachnoid cyst surgery depends on the type of operation, cyst location, patient age, symptoms before surgery, and whether there were complications such as hydrocephalus, seizures, or significant mass effect.

Recovery is usually shorter after endoscopic fenestration because the approach is minimally invasive and does not leave a permanent shunt. Open craniotomy and microsurgical fenestration usually require a longer recovery period because the operation involves a larger surgical exposure. Cystoperitoneal shunting has a different recovery pattern because the patient must also be monitored for shunt malfunction, infection, overdrainage, or later revision surgery.

In many uncomplicated cases, patients gradually return to light daily activities over several weeks, but return to work, sports, driving, and full activity must be individualized. Patients with seizures may need separate epilepsy-related restrictions.

Successful recovery does not always mean that the arachnoid cyst disappears on MRI. Symptoms may improve even when a residual cyst cavity remains visible, as long as cerebrospinal fluid communication has improved and mass effect or hydrocephalus has been relieved.

Prognosis of the Surgical Treatment

Surgical treatment is primarily aimed at relieving symptoms rather than removing the cyst completely. In carefully selected patients — particularly when symptoms are clearly related to mass effect or hydrocephalus — clinical improvement is reported in the majority of cases, most commonly in the range of approximately 70–85%. However, the cyst itself often remains visible on follow-up MRI and may not significantly decrease in size. A stable cyst on imaging does not mean that the surgery was unsuccessful, as long as adequate communication with normal cerebrospinal fluid pathways has been achieved and symptoms have improved.

Why Do Medical Opinions Differ in Arachnoid Cyst Treatment?

Different medical opinions are common because treatment decisions are not based on a single finding, but on the correlation between symptoms and imaging.

In many patients, the cyst is present, but it is unclear whether it is truly responsible for symptoms or just an incidental finding. This is the main reason why recommendations may differ.

Is the cyst truly responsible for symptoms?

Some specialists link symptoms such as headache or seizures to the cyst. Others consider the cyst incidental unless there is clear anatomical correlation between its location, size, and the patient’s symptoms.

When is surgery actually necessary?

There is no strict rule. Some recommend surgery based on imaging or symptoms, while others reserve it only for clear neurological deficit, growth, hydrocephalus, or strong symptom correlation.

Is epilepsy caused by the cyst?

In selected cases, the cyst may be considered a cause of seizures. However, many specialists require EEG correlation and exclusion of other causes, and even then, surgery does not guarantee seizure control.

Which surgical method is the best option?

Different surgeons may prefer different approaches. The choice depends on location, anatomy, and cyst structure, so both endoscopic and open surgery may be appropriate in different situations.

Should the cyst be monitored or treated?

Most cysts remain stable and do not require treatment. Observation is often recommended unless there is progression, new symptoms, or increasing mass effect.

Prognosis

• Most cysts remain stable and asymptomatic throughout life.
• After successful surgery, symptom improvement occurs in 70–85% when indication was correct.
• Seizure control is variable.
• Malignant transformation does not occur

Follow-Up Strategy

If asymptomatic:

• MRI at 6–12 months after diagnosis
• If stable → longer interval follow-up
• Often no further imaging needed after stability confirmed

If symptomatic or borderline:

• Closer radiological monitoring
• Clinical reassessment

Key Clinical Message

Arachnoid cyst is common, usually benign, and very often incidental.

The most important clinical question is not:
“Is the cyst present?”

But:
“Is the cyst responsible for symptoms?”

Those are two very different things.

It is not uncommon for neurosurgical opinions to differ regarding whether a cyst is truly responsible for symptoms — especially in cases of headache. Differences may also exist in decisions about surgery and the most appropriate surgical technique.

Frequently Asked Questions About Arachnoid Cysts