An arachnoid cyst is a fluid-filled sac sitting between the brain and its thin covering. Most stay silent, but if they grow or press on nearby parts they start to whisper clues.

The first clue is a dull headache that feels like pressure from the inside. It may come and go, often worse after bending over or coughing.

Nausea can ride along. You feel queasy without fever and might throw up once, then feel fine for days.

Vision may blur at the edges. You notice you bump into door frames or miss words on the left side of a page, like a curtain sliding over one eye.

Some people hear an internal “whoosh” matching their pulse when lying flat. Raising your head on two pillows can quiet it.

Seizures can appear out of nowhere. A blank stare, a brief hand jerk, or a full fall may be the first real alarm.

Large cysts in kids can speed up head growth or delay milestones like walking or talking.