There are various types but the main type that your optician will screen for using a tonometer is primary open-angle glaucoma or chronic glaucoma. This type is characterised by a gradual increase in the intraocular pressure.

Other types of glaucoma include acute glaucoma, which occurs when there's a sudden blockage of flow of the intraocular fluid; secondary glaucoma, which is a rise in the pressure of the fluid in the eye as a consequence of another eye condition; and developmental glaucoma. This last kind is extremely rare and occurs when a baby's eyes are not formed correctly within the womb.