This is a method for checking the pressure inside the eye. The eyeball is a closed ball of fluid so that there is no good way to measure the internal pressure of the eyeball directly. However, we can estimate the eye pressure by pushing on the surface of the eye and feeling “how hard” it seems. This is similar to kicking a car tire to estimate the air pressure inside. With applanation tonometry, a flat probe is pushed onto the surface of the cornea (the clear window that makes up the front of the eye). The probe is pushed hard enough to flatten a small round area of the cornea. By looking at the size of the circle flattened, and examining the amount of pressure used, the internal eye pressure can be calculated. The machine we use for this is called a Goldmann Applanation Tonometer. It is attached to the slit lamp microscope and it looks like a blue light when in use. You don’t feel this measurement as we use anesthetic drops ahead of time.