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Eyes Under Pressure

A Look at the Many Faces of Glaucoma – Part 1.

manDr. Megan Polack, Cowichan Eyecare

Many people have heard the term “glaucoma” before, some may know that it causes loss of peripheral vision, a few will know that it involves increased pressure inside the eye…But, did you know there are over 10 different types of glaucoma, each with different signs but few with any noticeable symptoms until it’s too late? This article will explore some of the more common types of glaucoma. One type of glaucoma, narrow-angle glaucoma, was discussed in detail in a previous article.

Open Angle Glaucoma: This is the most common form of glaucoma. The “open angle” part refers to an angle inside the eye formed between the cornea and the iris. At the apex of this angle is the location of the drainage system for the fluid that is produced inside the eye. This should not be confused with the tears that lubricate the outer surface of the eye.

Eye diagram showing fluid exitsThis fluid inside the eye, called aqueous humour, needs to drain at the same rate that it is produced or else the pressure inside the eye (intraocular pressure) rises and can damage the nerve fibers in the retina. In open-angle glaucoma, the angle is anatomically open but the aqueous humour is not draining well enough.

Glaucoma is essentially accelerated damage to the nerve fibers at a rate faster than normal aging changes. The optic nerve is like the cable where all the thousands of nerve fibers of the retina meet up and it carries visual information to the brain.

If nerve fibers die off, there is more empty space in the middle of the optic nerve (the “cup”) and a thinner surrounding layer of healthy nerve fibers (the “rim” around the cup). Damage to the nerve fibers results in loss of peripheral vision and can eventually lead to blindness.

Open angle glaucoma is more common in older patients and in those with a family history. Other risk factors include myopia, sleep apnea, diabetes and African American race. It usually has no symptoms and there can be quite significant vision loss before patients notice that something is wrong. Intraocular pressure checks are part of all adult routine eye exams.

Narrow Anterior Chamber Angles and Angle Closure Glaucoma: This has to do with the same angle discussed above and the fact that there is anatomically not much space between the iris and cornea.

Narrow angles are at risk of closing off and blocking the drainage of aqueous out of the eye. The worst consequence of the angle closing is an episode of “acute angle closure glaucoma” which causes a red and very painful eye, headache, nausea and blurry vision.

This is an ocular emergency and patients experiencing these symptoms should immediately see their Optometrist or go to Emergency.

Glaucomatous_field_defectsNormal Tension Glaucoma: The “normal” range for intraocular pressure readings is between 10 and 21 millimeters of mercury. Having pressures higher than 21 may be cause for concern and your Optometrist will monitor your pressures closely to determine your risk for glaucoma.

However, some patients have completely normal or even low intraocular pressures and still develop optic nerve damage and vision loss from glaucoma, hence the term “normal tension”. Research is ongoing but it is believed that patients with normal tension glaucoma have factors that decrease the blood flow to the optic nerve and its nerve fibers, leading to damage even with normal intraocular pressure.

Patients with normal tension glaucoma are more likely to have other circulatory and vasospastic disorders, such as migraines, Raynaud phenomenon (very cold hands and feet), ischemic vascular diseases and low blood pressure.

Therefore, in addition to checking your intraocular pressure at every eye health exam, it is important that your Optometrist have a thorough look at your optic nerve and retina and also perform a screening of your peripheral vision (visual field test).

A special imaging instrument called an OCT (optical coherence tomography) can measure the thicknes of the optic nerve fibers and determine if there is any premature thinning and thus risk of glaucoma.

It is also important that you mention medical conditions, even ones you may not think are related to your eyes (like migraines and Raynaud), to your Optometrist during the history taking.