You have a patient with a corneal thickness of 600 microns, and measured pressure of 15. What is the actual pressure?

Normal corneal thickness is about 540 nm, thus this patient has thick corneas. The measured pressure, therefore, is likely higher than his actual pressure. Thus, our reading is overestimating – his actual eye pressure may be closer to 13.

This is because we check eye pressure using Goldman applanation tonometry. This device works by pushing on the cornea and determining how much force it takes to flatten a predetermined area of corneal surface. This is comparable to checking the air pressure in a car tire by kicking it. If you have a truck with very thick rubber tires, then it feels “harder when you kick it” even though the air pressure inside is exactly the same.