Papilledema is the swelling of the optic nerve head where it enters the eyeball, caused by an increase in intracranial pressure (pressure within or around the brain). Cerebrospinal fluid (CSF), which surrounds the brain and spinal cord, normally protects the brain and maintains balanced pressure. However, when intracranial pressure rises, this fluid compresses the optic nerve, causing it to swell towards the back of the eye. The optic nerve connects the back of each eyeball to the brain, and because its entire surface is surrounded by CSF, it is highly sensitive to changes in intracranial pressure and serves as an important indicator of brain health.