Corneal Thickness Test

Normal corneal thickness is about 540 microns (half of a millimeter). Thickness is checked with a handheld ultrasound device called a pachymeter.

We check corneal thickness mainly for two reasons:

1. People with thin corneas (500 or less) are at a higher risk for having glaucoma. The mechanism for this is not entirely clear.

2. Cornea thickness allows us to calibrate our applanation tonometer readings. The Goldman tonometer (the device with the blue light on the microscope) checks eye pressure by pushing on the cornea. This is analogous to kicking a car tire to estimate the air pressure. However, if you drive a truck with a thick rubber tread on the tire, the tire “feels” harder when you kick it. Likewise, if you have a thick cornea, the pressure reading seems higher than it really is. There are many other uses for corneal thickness, such as determining/following corneal edema and evaluating patients for refractive surgery.