Every time you visit the eye doctor you get your eye pressure checked and asked questions about a family history for glaucoma. What exactly is glaucoma and why are eye doctors so obsessed with it?
Ocular Anatomy
Before we dive into understating glaucoma, it will be helpful to have a basic understanding of how the eye works.
For proper formation of vision, light rays must travel through the eye and come to a focus on the backmost structure of the eye—the retina.
The retina is a thin layer of tissue compromised of several different cells, with the most important being photoreceptors.
Photoreceptors are specialized light detecting cells. When light lands on a photoreceptor, it is stimulated, sending an electrical impulse signal down a chain of several other unique cells. These signals are transmitted via thin membranes called axons.
With there being millions of photoreceptors within the retina, there are millions of axons. The axons meet near the center of the retina at a point called the optic channel to create the optic nerve.
Essentially, the millions of axons combine to create one thick nerve—the optic nerve. It is easiest to think about this like millions of little cables coming together to form a fiber optic cable.
The optic nerve then exits the eye and travels to the back of the brain where the cumulative responses from photoreceptors are processed to form an image.
Thus, in order for information to properly reach the brain for processing, it must reach the retina and then travel to reach the brain without interference. Axons must be healthy and unaltered.
The eye is filled with two major fluids—aqueous humor and vitreous humor.
Aqueous humor fills the front half of the eye. It is constantly created via ultrafiltration of blood and enters the eye near the equator. It then flows forward toward the front of the eye where it drains through a series of channels to re-enter the systemic venous system.
Aqueous humor is important because it provides the front half of the eye with needed nutrition. Once the nutrients have been depleted, it flows into the venous system.
Aqueous humor is therefore responsible for eye pressure. If too much aqueous humor is made, pressure will increase. If the drainage channels become clogged or do not drain efficiently, pressure will increase. Sometimes there is a combination of both—aqueous is being produced quickly but not draining fast enough, once again leading to an increase in eye pressure.
The vitreous humor, on the other hand, fills the back half of the eye. The vitreous humor is produced during development. It is contained in a thin membrane and used primarily for structural support.
What is Glaucoma?
Glaucoma is a disease in which, for whatever reason, axons die off and vision becomes restricted.
This means that glaucoma is a disease affecting the optic nerve. The underlying etiology is death of axons.
The most common cause of axonal death is increased eye pressure resulting in compression of the axons. Glaucoma is often mistaken as a disease of elevated eye pressure—although this is not always the case. Many individuals may have elevated eye pressure without necessarily having a diagnosis of glaucoma (yet).
Extended periods of time with elevated eye pressure drastically increases the risk of developing glaucoma.
If eye pressure increases, the vitreous humor can be compressed backwards into the retina, thus putting pressure and stress on the axons.
Enough pressure and stress on axons will lead to axonal death. Death of axons means information from the corresponding photoreceptors can no longer travel to the brain. If the brain does not receive information from a given axon, it will perceive that area in your vision as “black”—hence a visual defect or blind spot is created.
In glaucoma, the peripheral vision axons are typically affected first, resulting in constricted peripheral vision.
Vision loss in glaucoma is usually very slow, meaning individuals may not even notice their peripheral vision is changing until it is too late.
Your eye doctor will want to check the health of your eyes yearly to watch for changes to the optic nerve and monitor eye pressure, as these can be the earliest signs of glaucoma.
When it comes to glaucoma, once damage has occurred it cannot be repaired. Treatment for glaucoma consequently focuses on prevention. Often times this means reducing eye pressure in an attempt to alleviate some of the strain put on the axons and preserve their integrity for as long as possible.
This is not to say, however, that elevated eye pressure is the only cause of glaucoma. There are situations in which axonal death and optic nerve changes are observed with normal eye pressures. Causes of this type of glaucoma is unknown.
Treatments for Glaucoma
The most common treatment for glaucoma is eye pressure lowering drops. There are many different types of eyedrops that can be used to lower eye pressure. Sometimes a combination of multiple eye drops may be necessary to get your eye pressure to the desired pressure range.
Surgeries are also potential options for glaucoma management.
Some surgeries focus on creating artificial drainage channels to remove excess aqueous humor from the eye.
Other surgeries use lasers to “open-up” the drainage system of the eye, making it more effective at naturally draining the aqueous humor from the eye.
If you or someone you know has glaucoma, it is important to have regular eye exams to monitor the condition. Most eye doctors will want to see patients with glaucoma every 6 months to check eye pressures and perform scans and tests to monitor for changes to the optic nerve.
Individuals with a first-degree family member (parent or sibling) are at increased risk for developing glaucoma themselves. As noted before, prevention is key with this disease.
Yearly eye exams can pick up minute changes and treatment can be initiated before vision loss ever occurs. Or, in other cases, perhaps treatment will be initiated in early stages to prevent further, more serious vision loss from occurring.
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