Normally the optic nerve disc is flush with the retina. In papilledema, intracranial pressure (pressure in the brain cavity) is so high that the optic nerve protrudes into the eye.
Increased intracranial pressure can occur if there are space-occupying lesions in the brain, such as tumors. There is a condition called pseudotumor cerebri where the intracranial pressure is high but there is no space-occupying lesion. The patient often suffers from headaches and ringing in the ears. It often occurs in young, overweight females. This was a patient I saw this week who came in for a routine eye examination.
A space-occupying brain lesion needs to be ruled out with imaging and often a lumbar puncture is performed to confirm the increased intracranial pressure and rule out other possible problems