Health information to help you understand symptoms. It isn't a diagnosis.About OwieClear
Check symptoms

Arachnoid cyst (fluid sac on the brain)

Also called: Arachnoid cyst, intracranial arachnoid cyst

A fluid-filled sac on the brain or spinal cord, usually present from birth and found by chance on a scan. Most cause no symptoms and never need treatment.

Found byBrain scan for another reason
MostNeed no treatment
Symptoms if largeHeadaches, seizures

Overview

Large cysts can press on the brain and cause symptoms.

A head injury can rarely cause bleeding into a cyst.

Symptoms

Common signs:

  • Arachnoid cyst found on a scan

Less common

  • Headaches
  • Seizures
  • Dizziness

Look up a symptom: Headache

When to get medical care

Call 911A seizure, sudden severe headache, or after a head injury with vomiting.
Get care todayNew headaches with vomiting.
See a doctor soonA cyst found on a scan.

Causes

Present from birth.

Does it spread or run in families?

  • It isn't contagious. You can't catch it from someone else.

Who is more likely to get it

  • Boys

Diagnosis

An MRI shows it.

Questions you may be asked

  • Headaches, seizures

The exam

Nerve exam.

Tests

Tests, if needed:

  • MRI:

Treatment

Usually only watching.

Self-care

  • Report new headaches or symptoms
  • Keep follow-up scans
  • Wear a helmet for sports

Surgery to drain large cysts with symptoms.

Prevention

It can't be prevented.

  • There is no known way to prevent it. Early checkups help find it sooner.

Which doctor treats this

  • Start with your primary care provider (family doctor or internist).
  • They may send you to a neurologist (brain and nerve doctor).

Children, pregnancy, and older adults

  • Children: Most are found in childhood.

Pregnancy and breastfeeding

  • If you're pregnant or planning to be, tell your prenatal provider about this condition.
  • Don't start or stop any medicine on your own. Some are safe in pregnancy and some need to be changed.
  • Ask before taking any pain reliever. The FDA advises avoiding ibuprofen and naproxen after 20 weeks unless your doctor says to.
  • Breastfeeding: Most medicines are fine, but ask. The NIH LactMed database lists what's known about each one.

Outlook

Most are harmless.

Most never need surgery.

Getting ready for your visit

Bring a list of your symptoms and medicines.

Questions to ask

  • Do I need follow-up scans?
  • Is it safe to play contact sports?

Research studies

Research studies test new ways to find and treat health problems. Some are looking for volunteers. Talk with your doctor before joining one.

Search ClinicalTrials.gov for studies on Arachnoid cyst (fluid sac on the brain)

Sources

Last updated 2026-11-12.