Arachnoid Cyst on Brain MRI: Is It Serious and When Does It Need Treatment?

An arachnoid cyst on brain MRI is a sac filled with cerebrospinal fluid, the clear fluid that surrounds and protects the brain. It is not a tumour and is not cancer. Most are present from birth, cause no symptoms and are found by chance, and small cysts without symptoms often need no treatment.

The US National Institute of Neurological Disorders and Stroke (NINDS) describes arachnoid cysts as the most common type of brain cyst. They form where the arachnoid membrane, one of the three layers covering the brain and spinal cord, splits and fluid collects inside.

This guide is for people reading an existing scan report. It explains common wording and questions to ask; it cannot judge what your own cyst means.

Call your local emergency number or go to an emergency department if any of these happen. They are emergency signs whether or not a cyst is involved, so do not wait for a planned scan or appointment.

• a seizure (fit), especially a first one, one lasting more than 5 minutes or one followed by not waking up properly
• a headache that starts suddenly and is extremely painful
• new numbness or weakness in the face, arm or leg, or difficulty speaking, balancing or walking
• loss of vision, new drowsiness or confusion
• a headache after a recent head injury

Ask for an urgent appointment, or use a local urgent advice service, if you have a headache with vomiting, a headache with vision or eye problems, or a headache triggered or made worse by coughing, sneezing, bending down or exercise. In a child, a headache that is getting worse or wakes them at night also needs prompt advice. See a doctor if headaches keep coming back or are not improving.

Cleveland Clinic notes that a cyst can occasionally rupture, which is a medical emergency, and advises getting help immediately for sudden changes in how you move, think, see or feel.

Usually, yes. NINDS explains that most arachnoid cysts are primary cysts, present from birth because of differences in how the brain or spinal cord developed in the early weeks of pregnancy. Some are secondary cysts that develop later after a head injury, infection, tumour or surgery.

They are most often found in the middle cranial fossa, an area at the base of the skull near the temporal lobe, close to the ears. Cleveland Clinic adds that they can also occur behind the eyes or lower down at the base of the skull, and less often near the spinal cord. NINDS reports that males are about four times more likely than females to have one.

Often not. According to NINDS, small cysts that do not disturb surrounding tissue and do not cause symptoms may not need treatment. Cleveland Clinic says most arachnoid cysts never grow large enough to cause symptoms.

Monitoring plans vary. NINDS recommends that cysts are watched over time to make sure they do not grow or start causing symptoms, and Cleveland Clinic notes that providers may suggest repeat MRI or CT scans. Your team will decide whether and when a repeat scan is useful for you, so ask what is planned and who arranges it.

An arachnoid cyst matters mainly when it is large enough to press on the brain, spinal cord or nerves, or blocks the normal flow of fluid. Location and size decide whether symptoms appear. Possible symptoms include headache, nausea and vomiting, seizures, hearing or vision problems, vertigo, and trouble with balance and walking.

Age is also relevant. People born with a cyst who develop symptoms usually do so early: NINDS notes children often develop symptoms within the first year of life, and most people who will develop symptoms do so before age 20. A cyst first noticed in an adult who has no related symptoms is often a long-standing, stable finding.

NINDS also notes that head trauma near a cyst can make it grow or bleed. Tell your clinician about any significant head injury.

Surgery is considered when a cyst is causing symptoms or pressing on important structures. The usual aim is to let the trapped fluid drain. One common approach, called fenestration, opens the cyst wall so the fluid can be absorbed; this can be done endoscopically or through open surgery. A shunt, a tube that diverts the fluid elsewhere in the body, is used only rarely.

NINDS reports that treatment usually makes symptoms go away or improve, although a cyst can come back in rare cases. A neurosurgeon can explain the options, benefits and risks for a specific situation.

Bring the full MRI report and, if possible, any earlier scans so the team can compare them.

• Where exactly is the cyst, and how large is it?
• Does it press on the brain or affect fluid flow, or is it simply sitting in that space?
• Do you think any of my symptoms come from the cyst, or is another cause more likely?
• Is a repeat scan recommended? If so, when, and who arranges it?
• Which symptoms should make me seek help sooner, and should I see a neurologist or neurosurgeon?

On SameCase, patients and verified doctors share de-identified cases. Seeing how others with an arachnoid cyst on brain MRI described their reports, symptoms and follow-up plans can help you organise your own history and decide which questions matter most to you.

Other people's cases cannot tell you whether your own cyst is significant, and we make no promises about outcomes. Use them to prepare, and rely on your clinician for interpretation. If headaches continue despite reassurance, the guide on headaches after a normal brain MRI may also help.