What Causes Glaucoma?

 

 

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Glaucoma occurs as a result of damage to the optic nerve. Gradual deterioration of the nerve will lead to the formation of blind spots in a person’s visual field (area of vision). Usually a person’s peripheral vision (side vision) is affected first, and the changes are so small they may not be detected. Over time, central vision is also lost. The primary cause of glaucoma is increased fluid buildup within the anterior chamber (front portion) of the eye that puts extreme, and potentially damaging, pressure on the optic nerve, leading to vision loss. It is important to note that some people can also develop glaucoma and damage to their optic nerve with eye pressure that is within normal limits.

The eye constantly produces fluid, called aqueous humor, which is secreted from the ciliary epithelium, a structure supporting the lens at the front of the eye. As fluid is produced, an equal amount of fluid should drain out through draining pathway structures, ‘drainage angles’ or ‘trabecular meshwork’. If fluid is produced too quickly or fluid does not drain adequately, there becomes a build up of pressure inside the eye (increased intraocular pressure – IOP).

As pressure inside the eye rises, over time it puts a strain on the optic nerve fibers leading to significant damage. It is not fully understood if the optic nerve damage occurs because of the mechanical compression of the nerve fibers from the increased pressure, or if the pressure restricts necessary blood flow to the optic nerve. As nerve fibers become damaged, they are unable to transmit light from the retina to the brain, meaning that blind spots may begin to occur in the field of vision.

The type of glaucoma usually indicates the cause and dictates the type of treatment and monitoring plan required. Treating the cause, not only the symptoms, can help to preserve vision for people with glaucoma.

What are the causes of glaucoma?

Glaucoma causes may include:

• Overproduction of fluid (aqueous humor)

• Insufficient drainage of fluid (aqueous humor)

  • Blockage

    • Pigment blockage

    • Exfoliation blockage

    • Blockage by abnormal blood vessel growth (neovascularization)

  • Trauma

• Genetics

• Medications

• Uveitis inflammation and medication

• Optic Nerve damage with normal intraocular pressure

• Birth defect

Overproduction of Fluid

The normal eye is constantly producing fluid (aqueous humor) in a structure called the ciliary body. If the eyes are producing abnormally high levels of fluid, the normal drainage channels may be unable to manage this extra volume and pressure in the eye may rise (increased intraocular pressure – IOP).

Insufficient Drainage

The cornea is the clear front part of the eye that covers the iris (colored part of eye), pupil, and anterior chamber. The angle where the iris and cornea meet is where fluid normally drains from the eye. It drains through a mesh-like tissue called the trabecular meshwork. If the openings are not able to adequately drain the fluid due to obstruction or closure of the angle, this buildup can cause an increase in eye pressure.

Insufficient Drainage due to blockage

When the drainage angle formed by the cornea and iris remains open, but the trabecular meshwork is partially blocked, this is referred to as open-angle glaucoma, the most common form of glaucoma. Open angle glaucoma causes a very gradual increase in the eye pressure. Resulting damage to the optic nerve can happen so slowly that someone may experience permanent partial vision loss before they even realize they have a problem.   

When the iris tilts or bulges forward it can narrow or even close the normal drainage angle between the iris and cornea, restricting fluid from draining. This may lead to angle-closure glaucoma or closed-angle glaucoma. Some people just simply have drainage angles that are narrower than others, so they are at a higher risk for angle-closure. Angle-closure glaucoma may occur suddenly (acute angle-closure glaucoma) which is a medical emergency, or more gradually (chronic angle-closure glaucoma).

Pigment blockage

Pigmentary glaucoma occurs when pigment granules in the iris, that give your eyes their color, become built up in the drainage pathways of the trabecular meshwork. This pigment granules can obstruct or slow normal drainage of fluid. The granules can also move, often during activities like running, and cause intermittent pressure buildup.   

Exfoliative or pseudoexfoliative blockage

Exfoliative glaucoma can occur when fragments flake off from the outer layer of the lens and these dandruff-like pieces collect in the drainage canals between the cornea and iris. Like pigment granules, these exfoliative bits can obstruct or slow the normal drainage of fluid from the eye, causing an increase in pressure. As noted earlier, fluid buildup may occur not as a result of obstructed trabecular meshwork or a closed angle, but rather from increased aqueous humor fluid production.

Abnormal blood vessel growth (neovascularization)

Neovascular glaucoma can develop when new, abnormal blood vessels form and grow on the iris. These abnormal vessels can grow over and through the eye’s trabecular meshwork (drainage pathways), obstructing adequate fluid drainage. The formation of these vessels is always associated with other conditions or diseases, such as diabetes or central retinal vein occlusion (CRVO). [2]

Uveitis inflammation and medication

The uvea of the middle layer of the eye and its role is to provide blood supply to the retina. Uveitis refers to swelling and inflammation of the uvea. Glaucoma associated with uveitis is one of the most serious complications of intraocular (inside the eye) inflammation. Approximately 20% of uveitis patients in the United States develop glaucoma, either due to the inflammation or the medication (steroids) used in its treatment. Although corticosteroids have been shown to effectively reduce inflammation, prolonged use can result in elevated eye pressure by decreasing fluid (aqueous humor) outflow.

Damage to the Optic Nerve

There are many other factors, disorders and diseases that lead to the development of glaucoma. Certain nutritional deficiencies, reduced blood flow, early nerve cell damage, autoimmune disease, and trauma may damage the optic nerve, resulting in glaucoma. Disease such as diabetes or having previous surgeries may also cause glaucoma.

If optic nerve damage occurs, despite having eye pressure that is within normal limits, it is called normal-tension glaucoma. This type of glaucoma often occurs due to reduced blood flow to the optic nerve and can be caused by different conditions that cause poor circulation.  

Genetics

Glaucoma tends to run in families and researchers have been able to identify some genes that are related to the development of elevated eye pressure and optic nerve damage. Those at higher risk for normal-tension glaucoma have a family member that also has the condition.    

Ethnicity can also result in an increased likelihood of developing glaucoma. Individuals at greater risk for glaucoma include African-Americans over the age of 40 and all men and women aged 60 or older. Individuals of Asian descent may be at greater risk of angle closure glaucoma due to the structure of their eye. People of Japanese ancestry also are at higher risk for normal-tension glaucoma.

Trauma

Traumatic injury to the eye can cause traumatic glaucoma, a form of open-angle glaucoma. This type of glaucoma usually involves one of two types of trauma; a blunt trauma that causes bruising of the eye, or injuries that penetrate the eye. Trauma can cause immediate changes to the mechanics and eye structure leading to increased eye pressure, including direct injury to the trabecular meshwork or changes to the angle between the cornea and iris, or later due to development of inflammation or a hyphema (pooling or collection of blood inside the space between the cornea and the iris). [3] Traumatic glaucoma can occur immediately after the injury or develop years after.

What causes glaucoma in children?

Glaucoma can occur in children, referred to as congenital glaucoma, pediatric, or infantile glaucoma. Most times, there is no specific identifiable cause and optic nerve damage can be related to blocked drainage pathways or an underlying medical condition.

Congenital glaucoma is present at birth and often occurs because there are underdeveloped or incorrectly developed drainage pathways in the infant’s eye. This is quite rare and may be related to an inherited condition.

Can my medication cause glaucoma?

Cold and allergy medications, steroids or cortisone, and several other medications have been shown to increase the risk of developing glaucoma. [1]

If you have glaucoma or know that you are at risk for developing glaucoma, it is important to read the labels and package inserts for all cold and allergy medication. Some medications explicitly state that they are unsuitable for people with glaucoma. The main reason is that the medications may cause pupils to dilate, causing the angle between the cornea and iris to narrow or close, resulting in acute angle-closure glaucoma, which requires immediate medical attention or potentially permanent vision loss will occur.

Steroids and cortisone products are commonly used drugs that have an important role in controlling inflammation and swelling throughout the entire body. Overuse or inappropriate use of steroids, usually drops or inhaled forms, can cause increased eye pressure and potentially open angle glaucoma. It is important to take these products only as prescribed, and if it is recommended to take for longer than 10 days, consult your physician to see if your eye pressure should be closely monitored.   

Medications used to treat various conditions such as depression, migraines, and Parkinson’s disease may also cause the pupils to dilate, leading to acute angle closure glaucoma. Some dietary supplements, such as those containing pseudoephedrine for weight loss, have also been reported to induce dangerous attacks of acute angle closure glaucoma.

Medications used to treat epilepsy and help prevent migraines, Topamax® or topiramate, may cause acute glaucoma. These instances usually occur within the first 6 months of drug use then tend to taper off.

 

References:

[1] Ah-Kee EY, Egong E, Shafi A, Lim LT, Yim JL. A review of drug-induced acute angle closure glaucoma for non-ophthalmologists.  Qatar Med J. 2015 May 10;2015(1):6.  https://www.ncbi.nlm.nih.gov/pubmed/26535174

[2] Havens SJ, Gulati V.  Neovascular Glaucoma. Dev Ophthalmol. 2016;55:196-204.  https://www.ncbi.nlm.nih.gov/pubmed/26501989

[3] Milder E., Davis K. Ocular trauma and glaucoma. Int Ophthalmol Clin. 2008;48(4):47–64.  https://www.ncbi.nlm.nih.gov/pubmed/18936636

[4] Siddique SS, Suelves AM, Baheti U, Foster CS. Glaucoma and uveitis. Surv Ophthalmol. 2013 Jan-Feb;58(1):1-10.  https://www.ncbi.nlm.nih.gov/pubmed/23217584