During a comprehensive eye exam, the eye doctor may check to see if you have evidence of glaucoma, a sight-threatening disease caused by increased eye pressure. The doctor will use a tonometer to measure intraocular pressure (IOP) in millimeters of mercury while you look into a light. The instrument releases a puff of air which flattens your cornea before it can calculate your IOP. This puff of air is essential to your annual eye exam because if detected early, glaucoma can be managed to avoid a loss of vision.