These are calcium crystals that form in the optic nerve and give the illusion of optic nerve swelling. The optic nerve is the large nerve that connects the eyeball to the brain. This nerve enters the back of the eye and this insertion can be seen inside the eye as the optic disk. Some people have natural calcium crystal deposits that form inside their optic disk. These calcium deposits are bulky, taking up space and giving the illusion of optic nerve swelling (papilledema). Swelling is a big deal as dangerous conditions like pseudotumor cerebri (high pressure inside the skull) can cause true optic nerve swelling. It is not always easy to differentiate between optic nerve drusen and true edema. In adults, drusen are easy to diagnose as the crystals glow and look like clusters of shiny rocks embedded in the nerve. In younger people the diagnoses is harder as the crystals are buried deeply and not as discernable. To truly detect optic nerve drusen, a number of tests can be performed. We can ultrasound the eye and look for the crystals – as the drusen are made of calcium, sound waves bounce off them and show up brightly on an ultrasound picture. A more invasive CAT scan can also be obtained as the crystals light up just like calcium-filled bones do. Another detection method is with a fluorescein angiogram. This test involves injection of a yellow dye into the blood stream to look for leakage inside the retina and optic nerve – calcium crystals don’t leak fluid, while true optic nerve swelling does. While usually harmless, optic nerve drusen can cause problems with the peripheral vision, so a visual field test is sometimes obtained to establish a good baseline.