Arachnoid cyst found by chance on a brain scan

What the report means

What it is. The arachnoid is one of the thin layers (membranes) around the brain and spinal cord. An arachnoid cyst is a sac inside this layer filled with the clear fluid that bathes the brain. It is benign, and most are thought to be present from birth. (Wang 2025)
Found by chance. Many are found on a scan done for an unrelated reason, often only in adulthood, and many never cause symptoms. (Wang 2025) MRI uses magnets, radio waves and a computer to make detailed pictures of soft tissue. (MedlinePlus)
Where it sits. The report may name a place. In a US hospital series of adults, the most common sites were the middle fossa (beside the temple, 34%), behind the cerebellum at the back of the head (33%) and over the surface of the brain (14%). (Al-Holou 2013) In a Norwegian population study, 53% were over the brain surface. (HUNT MRI 2016)
Common findings on brain scans. In a review of brain MRI studies, an arachnoid cyst was the single most common unexpected finding. (Morris 2009)

How common it is in adults, by age and sex

Ages 50 to 66. In the HUNT MRI study of 1,006 people from the general population in Norway, 3.6% had an arachnoid cyst, the most common unexpected finding. Earlier MRI studies reported 0.6% to 2.6%; the authors link the higher figure to sharper scans and counting cysts of any size. (HUNT MRI 2016)
Ages 70 and over. In 1,235 people aged 70 and over in Sweden who had a brain CT, 2.3% had an arachnoid cyst, at the same rate in men and women. (Rabiei 2016)
In hospital scans. Among 48,417 adults who had a brain MRI at a US hospital over 12 years, 1.4% had an arachnoid cyst, more often in men than women. (Al-Holou 2013)
Overall. A 2025 review puts the figure at about 1% to 2% of people and reports that cysts are found 2 to 4 times more often in males. (Wang 2025)

Why most never change

Adults followed over years. Of 203 adults followed for an average of 3.8 years, 2.3% of cysts grew and 0.9% shrank. Only 2 people had new or worse symptoms. (Al-Holou 2013)
A UK series. Of 485 people with an arachnoid cyst, 87.6% had no symptoms from it. Of 148 with a repeat scan, 99.3% were stable or smaller, and the one cyst that grew still caused no symptoms. (Hall 2019)
Growth is more of a childhood pattern. The review notes that the cysts that enlarge are mainly in younger patients. (Wang 2025) In the UK series, blocked brain fluid from a cyst was seen only in children. (Hall 2019)
Symptoms often unrelated. In the Swedish study of people aged 70 and over, those with a cyst had headache, dizziness, memory problems and low mood as often as those without. The authors advise caution in blaming symptoms on a cyst found by chance. (Rabiei 2016)

Symptoms that need a check

When a cyst does cause symptoms. Headache is the most common. A cyst that blocks the flow of brain fluid can bring headache with nausea, vomiting and changes in vision. Cysts at the back of the head can affect balance, hearing or swallowing, and cysts near the pituitary gland can affect vision. (Wang 2025)
Location matters. In the US series, cysts in and above the pituitary area were more likely to be judged as causing symptoms, and cysts beside the temple less likely. (Al-Holou 2013)
Size alone is not the answer. Some people with large cysts have no symptoms, while a small cyst in a tight spot can cause problems. (Wang 2025) Ask what the radiologist's report suggests for your scan.
Worry is real. See after an abnormal screening result.

How it ties to your age-based checks

An arachnoid cyst found by chance is a good moment to make sure your routine checks are up to date:

Other brain scan findings have their own pages: pineal cyst found by chance, small meningioma found by chance, brain shrinkage (atrophy) on a brain scan and sinus thickening on a head scan.

Who it applies to

Adults told that a brain CT or MRI done for another reason shows an arachnoid cyst, with no symptoms from it. Children, cysts in the spine and people with fluid build-up in the brain (hydrocephalus) are outside this page.

The NHS advises an urgent GP appointment or NHS 111 for a headache with vision or eye problems, a headache made worse by coughing, sneezing or bending down, or a headache with vomiting. Call 999 (911 in the US) for a headache with a seizure, numbness or weakness, sudden extreme pain, trouble speaking, balancing, walking or remembering, drowsiness or confusion, or loss of vision. (NHS)

Routine age-based screening

A finding on a brain scan does not replace the routine screenings for your age. See screenings by age and gender.

Where they differ:

Repeat scans. A UK series found so little change that it supports not routinely scanning cysts that cause no symptoms. (Hall 2019) A 2025 review describes MRI every 6 to 12 months at first, then less often once the cyst is stable. (Wang 2025)
Men and women. A US hospital series and a 2025 review found cysts more often in men (Al-Holou 2013)(Wang 2025), while a Swedish population study of people aged 70 and over found the same rate in men and women. (Rabiei 2016)
How common. Figures range from 0.5% in a pooled review of MRI studies to 3.6% in a Norwegian population study, depending on the scan and how small a cyst is counted. (Morris 2009)(HUNT MRI 2016)

Your doctor decides what your result means for you.

Questions people ask

How common is an arachnoid cyst?

Common. In a Norwegian population study of 1,006 people aged 50 to 66, 3.6% had one, and it was the most common unexpected finding. Among 48,417 adults who had a brain MRI at a US hospital, 1.4% had one. (HUNT MRI 2016) (Al-Holou 2013)

Is an arachnoid cyst a tumour?

No. It is a fluid-filled sac in one of the thin layers that cover the brain. It is benign and most are thought to be present from birth. (Wang 2025)

Will it grow?

Most stay the same. In 203 adults followed for an average of 3.8 years, 2.3% of cysts grew and 0.9% shrank. In a UK series, 99.3% of cysts with a repeat scan were stable or smaller. (Al-Holou 2013) (Hall 2019)

Is the cyst causing my headaches?

Often not. In 1,235 people aged 70 and over, those with an arachnoid cyst had headache and dizziness as often as those without. The authors advise caution in blaming symptoms on a cyst found by chance. Your doctor decides. (Rabiei 2016)

Do I need a repeat scan?

Sources differ. A UK series supports not routinely scanning a cyst that causes no symptoms, while a 2025 review describes MRI at 6 to 12 months at first, then less often if it is stable. Your doctor decides. (Hall 2019) (Wang 2025)