Arachnoid Cysts of the Brain
Arachnoid cysts are fluid-filled sacs containing cerebrospinal fluid (CSF) located between the delicate membranes of the brain. In most cases they are congenital, discovered incidentally on MRI or CT imaging, and remain entirely asymptomatic throughout life.
In a smaller number of children, the cyst may become clinically significant, exerting pressure on surrounding brain structures, causing headaches, hydrocephalus, or—more rarely—bleeding after minor trauma. The goal of evaluation is to determine when observation is sufficient and when surgical treatment is necessary.
What is an arachnoid cyst?
A benign, fluid-filled cavity located between the membranes of the brain.
Arachnoid cysts are collections of cerebrospinal fluid (CSF) enclosed between layers of the arachnoid membrane. They are not tumors and do not represent cancerous growth.
In most cases, these cysts are congenital, meaning they develop during fetal life. Less commonly, they may arise secondarily following trauma, infection, or hemorrhage.
How common are they and at what age?
A frequent incidental finding on brain imaging.
Large population studies show that arachnoid cysts are detected in approximately 0.3–1% of individuals undergoing brain imaging.
In children undergoing MRI for neurological symptoms, prevalence may reach 2–3%.
They are more common in males and can be identified at any age, including infancy.
Why do arachnoid cysts form?
Arachnoid cysts form when layers of the arachnoid membrane separate during early development, creating a cavity that fills with cerebrospinal fluid.
Several mechanisms have been proposed:
- Active fluid secretion by the cyst wall
- A one-way valve mechanism allowing CSF entry but not exit
- Communication with the subarachnoid space under pressure
- Osmotic gradients drawing fluid into the cyst
In most patients, the cyst remains stable throughout life. A smaller proportion may enlarge, particularly when diagnosed at an early age.
What symptoms can occur?
Most arachnoid cysts are asymptomatic and discovered incidentally. When symptoms do occur, they result from pressure on adjacent brain structures.
- Headaches or increased intracranial pressure
- Nausea or vomiting
- Seizures
- Developmental delay or learning difficulties
- Visual or balance disturbances
The type of symptoms depends largely on cyst location and size.
How is the diagnosis made?
Imaging plays a central role in diagnosis and follow-up.
Diagnosis is established primarily with:
- MRI of the brain – the gold standard
- CT scan – useful in acute settings
- Cranial ultrasound in infants with open fontanelles
Typical imaging features include CSF-equivalent signal, smooth margins, and absence of contrast enhancement.
Is it dangerous? Risk of growth or bleeding?
Most arachnoid cysts remain stable and never cause harm. However:
- Approximately 10% may increase in size over time
- About 4–9% may develop hemorrhage, usually after minor trauma
- Subdural fluid collections may occur in some cases
For this reason, appropriate follow-up and clinical monitoring are essential.
When is surgery necessary?
Treatment decisions are individualized.
- Asymptomatic, stable cysts generally require observation only.
- Symptomatic cysts causing neurological deficits may require surgery.
- Progressive enlargement or hydrocephalus are indications for intervention.
Surgical decision-making balances clinical symptoms, imaging findings, and patient age.
Surgical options
Two main surgical strategies are used:
- Endoscopic or open fenestration – creating an opening between the cyst and CSF spaces.
- Shunt placement – diverting fluid from the cyst to another body cavity.
Endoscopic techniques are often preferred when feasible, as they avoid long-term shunt dependency.
Expected outcomes
Approximately 75–80% of children experience clinical improvement following surgery.
Earlier intervention may allow better brain re-expansion and neurological recovery, particularly in younger children.
Daily life and physical activity
Most children with stable arachnoid cysts can lead normal lives, including participation in school and sports.
Contact sports may require individualized assessment depending on cyst size and location.
What are the risks of surgery?
As with any neurosurgical procedure, risks exist but are generally low in experienced centers.
- Bleeding or infection
- Incomplete decompression
- Need for revision surgery
- Shunt-related complications
Questions to ask your neurosurgeon
Helpful questions for families considering treatment.
Suggested discussion points
- Is the cyst responsible for my child’s symptoms?
- Does it require surgery or observation?
- What are the expected benefits and risks?
- What follow-up will be needed?
- How might this affect my child long-term?
At Neuroknife, treatment decisions are always individualized, based on careful clinical assessment, imaging review, and shared decision-making with families.
When should you seek expert evaluation?
If your child has been diagnosed with an arachnoid cyst or is experiencing symptoms such as headaches, seizures, or developmental concerns, a specialized evaluation can provide clarity and reassurance.
The Neuroknife team offers expert assessment, advanced imaging review, and personalized care planning.
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