An increase in eye pressure may be a sign of glaucoma. There are two ways to test the pressure. One uses eye drops to numb your eyes for a short while and the other involves blowing at least three puffs of air at the front of each eye. Neither test hurts.
Your eye needs a certain amount of pressure to keep the eyeball in shape so that it can work properly. In some people, the damage is caused by raised eye pressure. Others may have an eye pressure within normal limits but damage occurs because there is a weakness in the optic nerve. In most cases both factors of high pressure and weakness in the optic nerve are involved but to a varying extent.
If the optic nerve comes under too much pressure then it can be injured. How much damage there is will depend on how much pressure there is and how long it has lasted, and whether there is a poor blood supply or other weakness of the optic nerve. A really high pressure will damage the optic nerve immediately. A lower level of pressure can cause damage more slowly, and then you would gradually lose your sight if it is not treated.
You should ask the question of your optometrist and they should explain everything to you clearly and what risks you need to be aware of whilst they are doing the test.