An arachnoid cyst is a fluid-filled sac that forms between the brain (or spinal cord) and the thin membrane called the arachnoid, one of the layers that covers the central nervous system. The fluid inside is essentially the same cerebrospinal fluid that normally bathes the brain. Most arachnoid cysts are congenital, meaning they develop before birth, and the vast majority never cause any trouble at all. They show up on about 1.4% of brain MRI scans, which makes them one of the more common incidental findings a radiologist will flag, often to the surprise of the person who went in for an unrelated headache or a routine check.
How Common Are Arachnoid Cysts
In a large study that reviewed over 48,000 brain MRIs, arachnoid cysts appeared in about 1.4% of patients, with men significantly more likely to have them than women.1PubMed. Prevalence and natural history of arachnoid cysts in adults That percentage is consistent across other reviews, confirming that these cysts are a frequent incidental finding on both MRI and CT.2PubMed Central. Intracranial arachnoid cysts: Review of natural history and proposed treatment algorithm Because so many people walk around with an arachnoid cyst and never know it, the discovery often happens when imaging is done for something else entirely. This creates a common scenario in clinical practice: a patient has a scan for headaches or dizziness, the scan reveals a cyst, and the question becomes whether the cyst is responsible for the symptoms or is simply sitting there as a bystander.
Children are also frequently diagnosed. Arachnoid cysts are considered developmental anomalies most often identified in childhood, sometimes because of head-size concerns or neurological symptoms, but increasingly because pediatric imaging has become more routine.3PubMed Central. Intracranial arachnoid cysts in children: a review of pathogenesis, clinical features, and management The middle cranial fossa, the depression in the skull near the temple, is by far the most common location, though cysts can appear in other spots around the brain and along the spine.
What Causes Them
Most arachnoid cysts are congenital. During fetal development, the arachnoid membrane can split or fold abnormally, creating a pocket that fills with cerebrospinal fluid. The exact mechanism is not fully understood, but it is thought to involve a failure in the normal formation of the meninges during early gestation. There may also be underlying differences in how the nearby brain tissue develops, particularly in the temporal lobe.3PubMed Central. Intracranial arachnoid cysts in children: a review of pathogenesis, clinical features, and management
A smaller proportion are secondary, meaning they form later in life as a result of some insult to the brain or spinal cord. Trauma, infection, bleeding, and prior surgery can all give rise to a secondary arachnoid cyst.4PubMed Central. Secondary Arachnoid Cyst – A Late Complication of Craniotomy: A Case Report and Review of Literature A systematic review of secondary spinal arachnoid cysts found that the most common causes were prior surgical procedures, trauma, and subarachnoid hemorrhage, with the cysts appearing months to years after the triggering event.5PubMed. Symptomatic secondary spinal arachnoid cysts: a systematic review Secondary cysts tend to be more clinically relevant than congenital ones because they often develop in a context that is already causing neurological problems.
Do Genetics Play a Role
The vast majority of arachnoid cysts appear to be sporadic, with no clear family pattern. But they are not always random. Nearly 40 families have been reported in the medical literature where multiple members had arachnoid cysts, and the cysts also show up with increased frequency in certain genetic syndromes, including Chudley-McCullough syndrome, acrocallosal syndrome, and autosomal recessive primary ciliary dyskinesia.6PubMed Central. Familial and syndromic forms of arachnoid cyst implicate genetic factors in disease pathogenesis Among familial cases, mutations in the FOXC2 gene have been reported most often, and there may be a broader association with changes on chromosome 16.7PubMed. Familial arachnoid cysts: a review of 35 families
This does not mean you should assume your child will develop an arachnoid cyst because you have one. The genetic contribution is real but probably small in absolute terms. Researchers are still working out which genes are involved and how much they raise the risk. If multiple family members have been diagnosed, it is worth mentioning to a neurologist, but population-wide genetic screening for arachnoid cysts is not something anyone is seriously recommending.
Symptoms in Adults
Many arachnoid cysts sit quietly for an entire lifetime. When they do cause symptoms, the presentation depends on the cyst’s size, location, and whether it is pressing on surrounding structures or blocking normal fluid circulation. In adults who undergo surgery for symptomatic cysts, headaches are the most common complaint, reported by roughly two-thirds of patients, followed by dizziness in about half and seizures in more than a third.8PubMed. Clinical and radiological outcomes of surgical treatment for symptomatic arachnoid cysts in adults
The difficulty is figuring out whether the cyst is actually causing the headaches or whether the two are unrelated. Headaches are extremely common in the general population, and the coincidence of a headache complaint and an incidentally discovered cyst does not prove a causal connection. Seizures, focal neurological deficits (like weakness or numbness on one side), and signs of increased pressure inside the skull are more convincing evidence that the cyst is the culprit. Surgeons tend to be more confident about operating when the symptoms have a clear anatomical explanation. A large cyst in the left temporal fossa in a patient with language difficulties, for example, is a more compelling case than a small cyst and vague headaches.
Symptoms in Children
Children can present differently. Younger children may show an enlarging head circumference, developmental delay, or irritability. In some cases, arachnoid cysts near the base of the brain have been associated with hormonal problems. A study of children with arachnoid cysts found cases of precocious puberty, growth hormone deficiency, and even panhypopituitarism with diabetes insipidus.9Karger. The Endocrine Spectrum of Arachnoid Cysts in Childhood These endocrine effects are uncommon but worth knowing about, especially because early puberty or unexplained growth delays might be the first clue that a cyst is present near the pituitary gland. Older children may present with headaches and seizures similar to adults.
Effects on Thinking and Mood
Beyond the classic neurological symptoms, there is evidence that arachnoid cysts can subtly affect cognitive function even when the person does not have obvious neurological complaints. A study comparing patients with arachnoid cysts to matched controls found that cyst patients performed worse on tests of verbal knowledge, mental flexibility, problem-solving, and planning. Patients with left-sided temporal cysts did particularly poorly on complex verbal tasks. After surgical decompression, many of these deficits improved significantly, and patients were no longer distinguishable from controls on several tests, though some measures of verbal fluency remained lower.10PubMed Central. Intracranial arachnoid cysts: impairment of higher cognitive functions and postoperative improvement
Quality of life is another underappreciated dimension. Patients with arachnoid cysts report lower quality of life and a markedly higher prevalence of anxiety compared to the general population, and men seem to be more affected than women.11PubMed. Quality of life in adult patients with primary intracranial arachnoid cysts Whether this anxiety comes from the cyst itself exerting pressure on brain structures, from the psychological burden of knowing you have a brain cyst, or from some combination of the two, is not entirely clear. But it is something clinicians are paying more attention to when deciding whether a “technically asymptomatic” patient might still benefit from intervention.
How Arachnoid Cysts Are Found and Diagnosed
Most arachnoid cysts are discovered on CT or MRI scans performed for other reasons. On imaging, they appear as well-defined, smooth-walled fluid collections that follow the same signal characteristics as cerebrospinal fluid. MRI is the preferred tool because it gives better detail about the cyst walls, the surrounding brain tissue, and any associated abnormalities.
One important job for the radiologist is distinguishing an arachnoid cyst from other fluid-filled lesions, particularly epidermoid cysts, which can look similar on standard MRI sequences. Both tend to appear dark on one type of MRI weighting and bright on another. The key differentiator is a specialized MRI sequence called FLAIR, on which arachnoid cysts follow cerebrospinal fluid and appear dark, while epidermoids become bright. If there is still doubt, diffusion-weighted imaging resolves it: epidermoids light up brightly, and arachnoid cysts do not.12PubMed. Radiologic differentiation of intracranial epidermoids from arachnoid cysts Newer computational approaches analyzing the texture of CT scans can also distinguish the two with high accuracy, though these are not yet part of everyday practice in most hospitals.13PubMed Central. Computerized Tomography Texture Analysis in the Differential Diagnosis of Intracranial Epidermoid and Arachnoid Cysts
When to Watch and When to Treat
The large majority of arachnoid cysts remain stable over time and do not need surgery. The standard approach for an asymptomatic cyst is periodic follow-up imaging, typically a repeat MRI at intervals decided by the treating physician, to make sure the cyst is not growing.14PubMed Central. A Comprehensive Review of Arachnoid Cysts This conservative strategy is well supported by data: in one study, the cyst size stayed the same or decreased in over 99% of patients who had follow-up imaging, and the single cyst that did enlarge remained asymptomatic.15PubMed. Natural History of Intracranial Arachnoid Cysts
Surgery is reserved for cases where the cyst is causing clear problems, particularly hydrocephalus (a buildup of fluid that increases pressure in the skull), seizures that are attributable to the cyst, or focal neurological deficits like weakness or vision loss. Patients without signs of increased intracranial pressure or focal deficits are generally managed conservatively, because surgery itself carries risks.16Clinical Neurology and Neurosurgery. Intracranial arachnoid cysts: Current concepts and treatment alternatives The hardest decisions involve patients whose symptoms are real but nonspecific. Headaches and dizziness are uncomfortable, and if they are truly coming from the cyst, surgery can help. But if they are coincidental, putting someone through a cranial procedure will not fix the problem and will expose them to unnecessary risk.
Surgical Options
When surgery is indicated, three main approaches are used for intracranial arachnoid cysts:
- Endoscopic fenestration: A small camera and instruments are used to make an opening in the cyst wall so that fluid drains into the normal cerebrospinal fluid pathways. This is the least invasive option and tends to have the lowest revision rate.
- Microsurgical fenestration: An open surgical approach to create a window in the cyst. It allows the surgeon direct visualization but involves a larger craniotomy.
- Cystoperitoneal shunting: A tube is placed to drain the cyst fluid into the abdominal cavity. This consistently reduces cyst volume but comes with the long-term burden of having an implanted shunt, which can malfunction or become infected.
A pediatric study comparing these three methods found that clinical improvement immediately after surgery occurred in nearly all patients regardless of technique. However, the revision rate told a different story: 60% of shunted patients eventually needed a second procedure, compared to about a third of microsurgical patients and roughly one in six endoscopic patients.17PubMed Central. Intracranial arachnoid cysts: What is the appropriate surgical technique? A retrospective comparative study with 61 pediatric patients For cysts located near the sylvian fissure (the groove between the temporal and frontal lobes, where most arachnoid cysts sit), endoscopic fenestration had the best event-free outcomes. These results are broadly consistent with trends across the literature: fenestration, whether endoscopic or microsurgical, has become the preferred approach, with shunting reserved for cases where fenestration has failed or is technically difficult.
When surgery is performed for symptomatic cysts, quality-of-life improvements tend to hold up over time. In a prospective study with long-term follow-up, about 82% of patients reported improvement in headaches and about 68% reported improvement in dizziness, with gains sustained years after the procedure. Multiple dimensions of quality of life improved significantly and stayed improved at long-term follow-up.18PubMed. Surgical decompression of arachnoid cysts leads to improved quality of life: a prospective study-long-term follow-up
Spinal Arachnoid Cysts
Not all arachnoid cysts are in the head. Spinal arachnoid cysts are fluid-filled sacs that form around the spinal cord, most often in the thoracic region. They can sit outside the dura (extradural), within it (intradural), or rarely inside the spinal cord itself. Symptoms depend on whether the cyst is compressing the spinal cord or nerve roots and can include back pain, radiating pain, numbness, weakness, or problems with bladder and bowel function.19PubMed Central. Minimally invasive management of a spinal arachnoid cyst with ultrasound-assisted catheter placement: illustrative case
When spinal cysts cause significant neurological deficits, early surgery is recommended to prevent permanent damage. The surgical approach typically involves removing a portion of the vertebral arch to access the cyst, then fenestrating or resecting it and sealing any connections to the normal fluid space.20PubMed Central. Review/Perspective On the Diagnosis and Surgical Management of Spinal Arachnoid Cysts Results are generally good, with sensory deficits improving in about 80% of patients, pain in about three-quarters, and motor function in about two-thirds. Complications are usually mild to moderate, and about 12% of patients need a second operation for local recurrence.21The Spine Journal. Long-term outcomes following surgical treatment of spinal arachnoid cysts: a population-based consecutive cohort study Recurrence remains the major concern with spinal cysts. Even with successful initial surgery, the cyst can re-form, sometimes many years later.22PubMed Central. Recurrence of a symptomatic spinal intradural arachnoid cyst 29 years after fenestration
Head Injury and the Risk of Rupture
One concern that comes up frequently, especially for parents of children diagnosed with an arachnoid cyst, is whether a blow to the head could rupture the cyst and cause a dangerous bleed. This does happen, but it is rare. The mechanism involves stretching or tearing of bridging veins around the cyst during a head impact, which can lead to bleeding into or around the cyst, forming a subdural hematoma. Even small cysts can serve as a risk factor for this complication after mild head injury in young people.23PubMed. Arachnoid cyst is a risk factor for chronic subdural hematoma in juveniles: twelve cases of chronic subdural hematoma associated with arachnoid cyst Case reports describe previously healthy teenagers developing worsening headaches days after head trauma, with imaging revealing a ruptured arachnoid cyst accompanied by subdural bleeding.24PubMed Central. Intracranial arachnoid cyst associated with traumatic intracystic hemorrhage and subdural haematoma
The practical question this raises is whether people with known arachnoid cysts should avoid contact sports. The evidence does not support a blanket ban. A review of arachnoid cysts in athletes found no basis for an absolute rule advising against sports participation in general. However, certain regulatory bodies take a stricter stance: the British Boxing Board of Control, for instance, bars participation if any arachnoid cyst is found, including incidental asymptomatic ones. Other combat sports organizations differ. The recommended approach for most athletes is an individual review by a neurosurgeon, ideally one with experience in sports-related concussion, rather than a one-size-fits-all prohibition.25PubMed Central. Arachnoid Cysts in Athletes with Sports-Related Concussion: A Case Series and Literature Review
A Brief History of the Diagnosis
The term arachnoid cyst has not always existed in the medical vocabulary. The first documented cases trace back to 1831, when the physician Richard Bright described two cysts found during autopsies. For decades after, arachnoid cysts were considered curiosities associated with chronic, asymptomatic processes, and the leading theories about their origin blamed trauma or inflammation. It was not until the emergence of neurosurgery in the early twentieth century that symptomatic cases began to be recognized, and congenital and developmental theories gained ground as possible explanations.26Arachnoid Cysts: State-of-the-Art. History and Etymology of Arachnoid Cysts The real explosion in diagnoses came with the introduction of CT scanning in the 1970s and MRI in the 1980s. Suddenly clinicians could see these cysts in living patients, and the category shifted from a rare autopsy finding to a common incidental discovery. That shift is largely responsible for the modern clinical dilemma: how to handle a finding that is almost always harmless but occasionally requires urgent intervention.