ConditionBrain & nerves
Arachnoid Cyst
An arachnoid cyst is a fluid-filled sac that forms within the arachnoid membrane, one of the thin layers covering the brain and spinal cord. Most are present from birth, are not tumors, and never cause symptoms. They are often found by chance on a brain scan done for another reason. Larger cysts can press on the brain or block fluid flow, causing headaches, seizures or developmental problems, and may then need surgery.
Also called: Leptomeningeal cyst, intracranial arachnoid cyst, spinal arachnoid cyst, middle fossa cyst
At a glance
- What it is
- A benign sac of cerebrospinal fluid within the arachnoid membrane.
- How common
- Found in roughly 1 to 2% of brain MRIs, more often in males.
- Most common site
- The middle cranial fossa, near the temporal lobe.
- Symptoms
- Usually none; large cysts can cause headaches, seizures, hydrocephalus or developmental delay.
- Treatment
- Monitoring for most; surgery (fenestration or shunting) for symptomatic cysts.
When to get help
Call 911 or go to the ER
- Sudden severe headache, vomiting, drowsiness or confusion in someone with a known cyst, especially after a head injury (possible bleeding into the cyst or nearby). Call 911 or your local emergency number
- A first seizure
See a doctor within a day or two
- Increasing headaches with vomiting, especially in the morning, or a growing head size in an infant
- New vision problems, weakness, balance problems or hearing loss
Book a routine appointment
- A cyst found by chance on a scan, to discuss whether follow-up imaging is needed
- Developmental delay or behavioral changes in a child with a known cyst
- Back pain with numbness, tingling or weakness in the legs (possible spinal cyst)
What an arachnoid cyst is
The brain and spinal cord are covered by three membranes: the tough dura mater, the web-like arachnoid mater, and the delicate pia mater. Cerebrospinal fluid (CSF) normally flows in the space beneath the arachnoid. An arachnoid cyst forms when the arachnoid splits during development and fluid collects between its layers, creating a sealed pocket.
Arachnoid cysts are not tumors and are not cancerous. They do not spread.
Primary and secondary cysts
- Primary (congenital): present from birth due to a developmental anomaly. These are the vast majority.
- Secondary: develop later after head injury, meningitis, bleeding or surgery.
Locations
| Location | Approximate share | Possible effects when large |
|---|---|---|
| Middle cranial fossa (temporal) | About half | Headache, seizures, bulging of the skull |
| Suprasellar (above the pituitary) | Less common, mostly children | Hydrocephalus, vision loss, hormone problems, head bobbing |
| Posterior fossa (back of the brain) | Common | Hydrocephalus, balance problems |
| Convexity (surface) | Less common | Headache, seizures |
| Spinal | Uncommon | Back pain, leg weakness, numbness |
Symptoms
Most arachnoid cysts never cause symptoms and stay the same size for life. Symptoms occur when a cyst is large, grows, bleeds, or blocks CSF flow, causing obstructive hydrocephalus. They can include:
- headache, sometimes worse in the morning
- nausea and vomiting
- seizures
- dizziness and balance problems
- vision or hearing problems
- developmental delay or behavioral changes in children
- an enlarged or asymmetrical head in infants
- early puberty or other hormonal problems (suprasellar cysts)
- with spinal cysts: back pain, numbness, weakness, bladder problems
A difficulty for doctors is that headaches are very common, so a cyst found during a scan for headache is often unrelated to the headache.
Diagnosis
- MRI is the best test. The cyst contains fluid that looks identical to CSF, with no solid component. Special sequences help distinguish it from an epidermoid cyst or tumor.
- CT scan shows the cyst and any thinning or bulging of the overlying skull.
- Prenatal ultrasound sometimes detects cysts before birth.
- Cisternography or flow-sensitive MRI can show whether a cyst communicates with surrounding CSF.
Treatment
Observation
Asymptomatic cysts usually need no treatment. A doctor may repeat an MRI after 6 to 12 months to confirm the size is stable, especially in children. If it is stable and there are no symptoms, further scans may not be needed.
Surgery
Surgery is considered when a cyst causes symptoms clearly linked to it, hydrocephalus, or progressive growth. Options include:
- Endoscopic fenestration: a neurosurgeon uses a small camera to make openings in the cyst wall so fluid drains into normal CSF spaces. Minimally invasive and often the first choice.
- Microsurgical (open) fenestration: through a small opening in the skull.
- Cyst-peritoneal shunt: a tube drains the cyst into the abdomen. It is effective but the shunt may need revision later.
Success rates are generally good, though cysts can recur and not all symptoms, especially headaches, resolve.
Complications
- bleeding into or around the cyst (subdural hematoma), usually after head trauma; the risk is low but higher with large middle fossa cysts
- hydrocephalus
- surgical complications, including infection and CSF leak
Some neurosurgeons advise people with large cysts to avoid high-impact contact sports. This should be discussed individually.
Outlook
The outlook is excellent for most people. Asymptomatic cysts rarely change. Symptomatic cysts usually respond well to surgery, and children treated for hydrocephalus or developmental effects often improve.
Frequently asked questions
Is an arachnoid cyst dangerous?
Usually not. Most arachnoid cysts are harmless, present from birth, and never cause symptoms. Large cysts can press on the brain, block fluid flow or bleed after an injury. Your doctor will decide whether monitoring is needed based on size, location and symptoms.
Can an arachnoid cyst cause headaches?
Large cysts can cause headaches by raising pressure or blocking fluid flow. However, many cysts are discovered during scans for headaches that have a different cause. A neurologist or neurosurgeon assesses whether the cyst and headache are truly linked.
Do arachnoid cysts grow?
Most stay the same size for life. Some grow, mainly in young children. Rarely, cysts shrink or disappear. A repeat MRI after several months is often done to check for change.
Is an arachnoid cyst a brain tumor?
No. An arachnoid cyst is a pocket of normal brain fluid, not a growth of abnormal cells. It is not cancerous and does not spread. MRI can usually tell it apart from tumors and other types of cysts.
Sources
This guide was written from current patient and clinical guidance, including:
- NINDS: Arachnoid cysts
- American Association of Neurological Surgeons: Arachnoid cysts
- NIH Genetic and Rare Diseases Information Center: Arachnoid cysts
- Boston Children's Hospital: Arachnoid cysts
- Johns Hopkins Medicine: Arachnoid cysts