Why do people with a "normal" eye pressure still get glaucoma?

Take Home Points

As described in previous sections, open angle glaucoma is no longer defined by the level of eye pressure. Studies of large populations in every continent on earth show that this disease affects many people with normal pressure as often as it does those with higher than normal pressure. In Japan, glaucoma occurs at normal pressure in more than 80% of cases. But, until the 1970s, open angle glaucoma was known as the disease of “elevated” eye pressure and many experts thought that having it with “low tension” was a distinct disease. Not only was so-called low tension or normal tension glaucoma thought to be quite different, patients were often told that treatment by lowering pressure wouldn’t work. Many patients still tell us that they have been told: “I’ve got that dread thing, normal tension glaucoma, and unfortunately none of the treatments will work. I’m going blind.” But, we now know that among European-derived persons, 40-50% of those with open angle glaucoma have normal pressure levels when their disease is discovered. It isn’t that their pressure is irrelevant. Persons without glaucoma have eye pressures between 10 and 20—let’s call this the “normal” range. When we study open angle glaucoma patients with pressures in this range, the eyes that are higher than 15 are more likely to be more damaged than those below 15. So, the higher the pressure in the normal range, the worse is the glaucoma. And, for those with glaucoma and pressure in the “normal” range, lowering the pressure with treatment is just as beneficial as lowering pressure in “high tension” glaucoma. There are only very modest differences between how glaucoma affects the optic nerve head structure and the visual field between those with lower and those with higher pressures.

Even more interesting, the most recent data show that those with open angle glaucoma at lower pressure (which we consider the right way to talk about this group) actually have a more slowly progressive disease on average than those with higher pressure open angle glaucoma. In other words, all other things equal, you’d rather have “low tension” than “high tension” glaucoma. Some risk factors for low tension glaucoma include: snoring (called “sleep apnea”), vasospastic disorders such as migraine headaches, Raynaud’s phenomenon or having low diastolic blood pressure (bottom number on the equation). But, there are still many people with glaucoma and pressure in the normal range who do not have any of those risk factors, and still get nerve damage.

If you are found to have a higher than normal high eye pressure in an eye exam, that will properly identify you as a glaucoma suspect. But, a lot of people with normal eye pressures have glaucoma, too, and they get missed if all that is done is pressure measurement. We need to include a thorough examination of the optic nerve to check for glaucoma. This underscores the importance of getting a comprehensive eye exam each year, which includes dilation and examination of the optic nerve even if you have a normal eye pressure.

Among the most disturbing things that doctors used to do (and unfortunately sometimes still do) is to assume that a person with clearcut glaucoma-type damage to the nerve head and visual field loss might not have glaucoma because their pressure is normal. So, they order testing to find some other disease that might affect the optic nerve, like a brain tumor or multiple sclerosis affecting vision. On extremely rare occasions, a very atypical glaucoma-like set of findings turns out to be one of these disorders instead. But, good eye doctors know the features that define glaucoma, and those that define brain tumors affecting the optic nerve, and the differences are generally so clear that this additional testing is totally unnecessary. In fact, several studies have been done to show how silly it is to take people with typical glaucoma findings and subject them to the cost and inconvenience of brain imaging, lumbar punctures, and neurological consultations. The worst part of this is that it scares the patient in a way that they may never get over. If you were told that you “might” have a brain tumor or multiple sclerosis, hearing that they couldn’t find anything on the imaging would never erase the “might” from your mind.

Over a decade ago, Dr. M. Roy Wilson, one of the most respected glaucoma doctors in the world wrote: “With respect to normal tension glaucoma, there is no such disease entity—distinct from primary open angle glaucoma—and it serves no useful purpose to continue to perpetuate this term.” Yet, every year, doctors publish papers that are said to study “low tension” glaucoma. What could be done in such studies is to study everyone with open angle glaucoma and to treat the pressure level not as normal or abnormal, but as a continuous number from low to high.

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