By numbers alone, the majority of those with glaucoma have the open angle type. In the eyes with open angle glaucoma, the structures that make aqueous humor and the channels out of which aqueous leaves the eye are normal in appearance when the doctor examines them. The problems that lead to poor aqueous outflow are microscopic—too small to detect in a clinical exam. Open angle glaucoma can happen at any level of eye pressure, and half of those with open angle glaucoma have normal levels of eye pressure. This often surprises those who think of glaucoma as always due to high eye pressure. Glaucoma can occur at normal levels of eye pressure due to weaknesses in the eye and its structures that are enough to kill ganglion cells. Some of these weaknesses have been identified at the microscopic, genetic and biochemical levels. We will deal with these in more detail in the next section How did you get glaucoma?

For the other half of those with open angle glaucoma, the pressure that damages their vision is actually above normal. In these persons, various abnormalities lead the eye pressure to be high, and, if the eye and ganglion cells are susceptible to the effects of such higher pressure, glaucoma ensues. There are different theories for why the pressure is above normal in open angle glaucoma eyes with higher pressure. The best explanation is that there is a problem that prevents fluid from leaving the angle rapidly enough. The most likely reason for this is premature death of the cells in the aqueous outflow area of the angle. Another probable factor is blockage of the outflow channels by very tiny collections of material that shouldn't be there. Because the primary cause of disease in this group of open angle glaucoma patients is poor flow through the meshwork, some new potential treatments for open angle glaucoma try to improve this flow. Since lowering eye pressure helps all persons with glaucoma, this could help both those with the higher and the lower pressure types of open angle glaucoma.