What is Glaucoma?

You’d think this would be an easy question to answer. After all, when your doctor has just finished her exam and tells you that she believes you have glaucoma shouldn’t you expect her to be able to answer the question that immediately drops from your mouth, “What is glaucoma?”

The term “glaucoma” has been around for centuries. Prior to the Victorian Era the term “glaucoma” could have described many different eye diseases from cataracts to corneal disease, to actual glaucoma. It was not until the 1800s, however, that the term “glaucoma” was limited to describe a condition thought to be related to elevated eye pressure.

What Is Intraocular Pressure (IOP)?

“How,” you might ask, “is the eye pressurized?” The eye is filled with a fluid called the aqueous fluid. The purpose of this fluid is to nourish the lens and other structures in the eye. It also keeps the eye “inflated”. It turns out that just like a soccer ball the eye needs to maintain a minimum pressure to work well. Too low a pressure and the eye shrivels up like a raisin (this is called phthisis). Too high a pressure and damage to the eye is possible.

The pressure exerted by the fluid inside your eye is called intraocular pressure (IOP). This is usually measured in millimeters of Mercury (or mmHg), similar to the way your car’s tire pressure is measured in pounds per square inch (PSI) – it’s simply a unit of measurement.

If the the right amount of fluid is produced and the eye’s drainage is working properly, then the IOP should be within the “normal range”. Even so, IOP will vary throughout the day – this is called diurnal fluctuation.

If the eye’s drainage system becomes clogged, excess fluid is not removed. As the fluid builds up, the eye pressure increases. This happens in most types of glaucoma. Elevated pressure may damage the optic nerve, resulting in irreversible vision loss.

So It’s High Pressure in the Eye?

For many years it was taught that high eye pressure would inevitably lead to optic nerve damage and that was called glaucoma. It is now thought that an elevated IOP is just one risk factor for glaucoma rather than its sole cause. However, IOP is the only risk factor that can be modified by currently available prescription or surgical treatments to prevent progressive nerve damage. Until such time as we develop therapies to treat glaucoma without lowering the IOP, this is all we’ve got.

It’s Not Just Intraocular Pressure (IOP)

Mounting evidence suggests that factors other than intraocular pressure (IOP) contribute to the development of glaucoma. One suspected factor is blood flow to the eye. Other implicated factors include inflammation and oxidative cellular damage. This may explain why some people with “normal” IOP still develop loss of vision typically associated with higher IOPs – this is called Normal Tension Glaucoma.

References
  1. Sommer A, Tielsch JM, Katz J, et al. Relationship between intraocular pressure and primary open angle glaucoma among white and black Americans. The Baltimore Eye Survey. Arch Ophthalmol 1991;109:1090-1095.
  2. Flammer J, Orgül S. Optic nerve blood-flow abnormalities in glaucoma. Prog Retin Eye Res 1998;17: 267-289.
  3. Wang N, Chintala SK, Fini ME, Schuman JS. Activation of a tissue-specific stress response in the aqueous outflow pathway of the eye defines the glaucoma disease phenotype. Nat Med 2001;7:304–309.
  4. Alvarado J, Murphy C, Polansky J, Juster R. Age-related changes in trabecular meshwork cellularity. Invest Ophthalmol Vis Sci 1981;21:714–727.
    Alvarado J, Murphy C, Juster R. Trabecular meshwork cellularity in primary open-angle glaucoma and nonglaucomatous normals. Ophthalmology 1984;91:564–579.

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