In regards to the eye, “pressure” usually refers to the intraocular pressure – the pressure inside the eyeball itself. Eye pressure can be measured a number of ways, but all of the methods involve pushing on the eye to estimate the pressure within. This is akin to kicking a car tire with your foot in order to estimate the internal tire pressure. In a doctor’s office, pressure is usually measured with applanation tonometry. This is a blue light attached to a weighted mechanism that pushes on the surface of the cornea. Topical numbing drops are used, so the process is quick and painless. Another method to measure pressure is with a hand-held Tono-Pen device. This is an electronic gadget that looks like a large pen or marker and is used to measure the pressure electronically. The Tono-Pen is not terribly accurate, but sometimes it is the only way to check pressure in patients who can’t get into the microscope or are bedridden. The final way to check pressure, and the one you may be most familiar with, is the “air puff.” This device puffs air at the cornea and measures the surface distortion caused by the shockwave. The air puff is used in some eye offices because it requires little skill to learn and is easy to keep sterile. It is seldom used in medical ophthalmology, however, as the puff is uncomfortable and the measurements may not be as accurate as applanation tonometry (though this is highly debated by some). On our scale, “normal” eye pressure is 10-21, with distribution being skewed toward the higher pressures. High eye pressure is associated with glaucoma and can also be present with inflammation or bleeding inside the eye (from traumatic or after surgery). Low pressure can be seen with wound leaks after surgery and sometimes with internal inflammation like uveitis.