An arachnoid cyst is a fluid-filled sac that forms between the brain (or spinal cord) and a thin membrane called the arachnoid, one of the protective layers surrounding the central nervous system. The fluid inside is cerebrospinal fluid, the same clear liquid that normally cushions the brain. These cysts affect roughly 1 to 2.6% of the population, and the vast majority never cause problems or require treatment.
Most arachnoid cysts are discovered by accident, showing up on a brain scan ordered for an unrelated reason. If you or your child has recently been told about one, understanding what it is, whether it’s likely to grow, and what symptoms would actually matter can save a lot of unnecessary worry.
How Arachnoid Cysts Form
The arachnoid membrane is one of three layers (called meninges) that wrap around the brain and spinal cord. During fetal development, a small pocket can form in this membrane, trapping cerebrospinal fluid inside. Over time, the pocket may slowly fill with more fluid and become a visible cyst. Most arachnoid cysts are congenital, meaning they’re present at birth even if they aren’t detected until years later.
Some cysts stay the same size indefinitely. Others can slowly enlarge through a one-way valve mechanism: a slit-shaped opening in the cyst wall allows fluid to flow in but not back out, creating a net accumulation over time. Cysts with round, open connections to the surrounding fluid space tend to remain stable because fluid can move freely in both directions. Only about 3 to 5% of arachnoid cysts change in size at all, whether growing or shrinking.
Where They’re Most Commonly Found
The most common location is the middle cranial fossa, the bony compartment just behind the temple, outside the temporal lobe. Cysts can also develop near the back of the brain, around the base of the skull, or along the spinal cord, though spinal arachnoid cysts are considerably rarer. In the middle cranial fossa, cysts are sometimes classified by size: small ones confined to the front of the fossa, medium ones that extend along the natural groove between brain lobes, and large ones that fill most of the fossa and push against the brain hemisphere.
Symptoms and When They Matter
Most arachnoid cysts cause no symptoms at all. Problems only develop when a cyst grows large enough to press on brain tissue, nerves, or the spinal cord. Even then, symptoms depend heavily on location and size.
When symptoms do appear, the most common ones include headaches, dizziness, nausea, fatigue, seizures, and muscle spasms. In children, the signs can be subtler. A child with a large cyst may take longer to reach developmental milestones like walking or talking. In infants, a cyst can cause the head to grow faster than expected because the skull bones haven’t yet fused and can be pushed outward by the expanding fluid. A large cyst may also block the normal flow of cerebrospinal fluid, leading to a buildup of pressure inside the skull.
If a cyst becomes very large, more serious symptoms can develop: hearing loss, vision changes, facial weakness or twitching, difficulty swallowing, or weakness in the eye muscles. Spinal cysts may cause back pain, curvature of the spine, or urinary problems.
How They’re Diagnosed
Arachnoid cysts are typically found on MRI or CT scans. On imaging, they appear as sharply defined, fluid-filled pockets that push the surrounding brain tissue aside rather than growing into it. The fluid inside looks identical to normal cerebrospinal fluid on all standard scan types. One distinguishing feature is that the cyst displaces nearby blood vessels and nerves rather than wrapping around them. Over time, a cyst can even thin and reshape the overlying skull bone.
Radiologists sometimes need to distinguish an arachnoid cyst from an epidermoid cyst, a different type of growth that can look similar on basic scans. The key difference shows up on specialized MRI sequences: epidermoid cysts appear bright on diffusion-weighted imaging, while arachnoid cysts do not. Epidermoid cysts also tend to wrap around blood vessels and nerves instead of pushing them aside.
Long-Term Outlook Without Treatment
For the majority of people with an asymptomatic arachnoid cyst, the outlook is excellent. In a study following 300 patients with serial imaging, only one person developed new neurological symptoms over the monitoring period. The risk of a cyst growing or causing new problems is low enough that observation with periodic imaging is the standard approach for cysts that aren’t causing symptoms.
A small but real risk worth knowing about: arachnoid cysts, particularly large ones in the middle cranial fossa, are associated with a higher risk of developing a type of bleeding called a chronic subdural hematoma. This can happen spontaneously or after even minor head trauma. Patients with chronic subdural hematomas have about five times the prevalence of arachnoid cysts compared to people without them. The bleeding likely occurs because the cyst stretches the small bridging veins near the brain’s surface, making them more vulnerable to tearing.
When Surgery Is Considered
Surgery is reserved for cysts that are clearly causing symptoms, particularly those producing increased pressure inside the skull, blocking cerebrospinal fluid flow, or causing developmental delays in children. The decision to operate on cysts associated with common symptoms like headache remains debated, since headaches are so common in the general population that attributing them to a cyst can be difficult.
Asymptomatic cysts, even large ones, are generally left alone. There is broad consensus that operating on a cyst simply because it exists on a scan does more harm than good.
Surgical Options and What to Expect
When surgery is needed, there are three main approaches. Endoscopic fenestration involves inserting a small camera and creating an opening in the cyst wall so fluid can drain into the brain’s normal fluid pathways. Microsurgical fenestration accomplishes the same goal through a traditional open approach with a larger incision. The third option, a shunt system, uses a thin tube to drain cyst fluid into the abdominal cavity where the body can absorb it.
In a study of 61 pediatric patients, clinical improvement immediately after surgery was high across all three methods, ranging from about 89 to 100%. However, the long-term picture differed significantly. Endoscopic fenestration had the lowest rate of needing a second operation (about 17%), compared to 33% for microsurgical fenestration and 60% for shunts. For cysts located near the temporal lobe, 89% of patients treated endoscopically remained problem-free afterward, compared to 71% after microsurgery and just 20% after shunting.
Shunts carry the least immediate surgical risk but create a long-term dependence on a mechanical device that can malfunction, block, or become infected. For this reason, shunting has become less common as endoscopic techniques have improved. Endoscopic fenestration is now widely considered the preferred approach when anatomy allows it.
Arachnoid Cysts in Children
Children are more likely than adults to have symptomatic cysts. The most common presenting signs in the pediatric population are symptoms of increased pressure inside the skull (persistent headache, vomiting, irritability), rapid head growth, developmental delays, and seizures. In rare cases, a cyst pressing on certain brain structures can trigger precocious puberty or hormonal disruptions.
Because a young child’s skull is still growing, a cyst can reshape the skull as it expands. This makes head circumference measurements during routine pediatric checkups an important screening tool. If a child’s head is growing along a normal curve and they’re meeting developmental milestones, even a known cyst is typically just monitored over time.