The most common form of the disease occurs when the ocular drainage canals become clogged over time. The inner eye pressure (also called intraocular pressure, or IOP) rises when the correct amount of fluid can’t drain out of the eye. With the most common form of glaucoma, the entrances to the drainage canals are clear, and should be working correctly. However, the clogging problem occurs farther inside the drainage canals. This is like a clogged pipe below the drain of a sink.