This is bilateral swelling of the optic nerves secondary to high intracranial pressure. The brain sits inside the skull and is suspended by membranes, floating in fluid. This fluid is the cerebral spinal fluid (CSF) and is similar to the aqueous fluid in the eyeball itself. If the pressure of this CSF is high, such as in pseudotumor cerebri, this can cause headaches and neurologic changes. Unfortunately, it is very hard to actually measure the pressure inside the skull. One could always drill a hole through the skull and see how fast the water comes out, but for obvious reasons this is not feasible. Instead, a spinal tap is performed. A needle is inserted in the lower back. Since the fluid in the spine is connected to the brain, a neurologist can estimate the brain pressure by measuring the “opening pressure” during the spinal tap. This is not fun either, but is really the only way to truly know the brain pressure. Fortunately, an eye doctor can sometimes estimate the CSF pressure by dilating the eye and looking at the optic disk. The eyeball is a direct extension of the brain … and the pressure from the brain fluid will travel down the optic nerves and be visible as swelling inside the eye. This swelling is called papilledema. If the pressure is high enough, the nerves will swell and elevate like a volcano, causing blurring of the normally crisp nerve margins.