What it means
The arachnoid is one of the thin membranes that wrap the brain and spinal cord. An arachnoid cyst forms when this layer splits and the gap fills with cerebrospinal fluid (CSF), the clear liquid that bathes the brain. On CT and MRI the cyst looks exactly like that fluid. It is not a tumor and does not turn into one.
When it matters
Most arachnoid cysts cause no symptoms, do not change and are left alone. A newly found one may be checked once to confirm it is stable. Doctors look more closely when a cyst is large, presses on the brain or blocks fluid flow, which can cause headaches or other symptoms. In those cases a neurosurgeon may consider draining it or opening it into the normal fluid spaces.
Questions to ask your doctor
- Is the cyst pressing on the brain or blocking fluid flow?
- Could it be causing any of my symptoms, or is it incidental?
- Does it need a follow-up scan, or can it be left alone?
