What are the causes of glaucoma?

While most people who have glaucoma may have no idea they have it, and, as a result, the optical nerve is damaged over the years, there are some forms of glaucoma that are far more acute.

Angle-closure glaucoma, also called narrow-angle glaucoma, causes intraocular pressure to increase so rapidly that the onset of debilitating symptoms is swift and prompt treatment is needed.

Unlike open-angle glaucoma, in which the eye-nourishing aqueous humor cannot drain through the trabecular meshwork located at the angle where iris and cornea meet, with angle-closure glaucoma, the aqueous humor never even reaches that draining trabecular meshwork, because the angle is blocked or closed. This rapidly increases intraocular pressure, resulting in very noticeable symptoms like nausea, headaches and blurred vision.

One of the causes of this is thought to be the angle where the iris and cornea meet.

In most people, that is a 45 degree angle. But in some people, that angle is narrower and the iris is very close to the trabecular meshwork, which drains the aqueous humor. But in addition to a narrower angle, angle-closure glaucoma can also be triggered by pupil dilation, because the lens and iris are then in close contact, potentially blocking the trabecular meshwork. This is the reason angle closure glaucoma often occurs in a dark room or when someone is undergoing stress. The pupils dilate in both cases.

Clear liquid called aqueous humor circulates inside the front portion of the eye. To maintain a healthy level of pressure within the eye, a small amount of this fluid is produced constantly while an equal amount flows out of the eye through a microscopic drainage system. (This liquid is not part of the tears on the outer surface of the eye.)

Because the eye is a closed structure, if the drainage area for the aqueous humor — called the drainage angle — is blocked, the excess fluid cannot flow out of the eye. Fluid pressure within the eye increases, pushing against the optic nerve and causing damage.

The cause of glaucoma appears to be an abnormality in the composition of the supportive structures of the eye. Specifically' structural changes reflecting poor collagen integrity and function are the hallmark features of glaucoma. These changes lead to blockage in the flow of the aqueous humour and result in elevated IOP readings. These changes can occur with aging and medication use' including of corticosteroids.

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The eye is made from soft, but strong tissues. However, like a camera, the eye needs to maintain its shape so that it can focus light accurately. This is achieved by using secretions which keep the eye firm and expanded, similar to a balloon. These fluids include the vitreous humor (or vitreous body), a thick, gel-like fluid that fills most of the spaces in the eyes, and the aqueous humor, a clear fluid that fills the anterior chamber (the space between the cornea and iris). The aqueous humor carries oxygen, sugars, vitamins, minerals, and other essential nutrients for the eye.

The production of the aqueous humor is a constant process, and its removal is just as important. The aqueous humor filters through a spongy meshwork called the trabecular meshwork, and then drains out of the anterior chamber through a complex drainage system that leads through structures (vessels) called the canal of Schlemm, then exits out of the eye. The balance between production and drainage of the aqueous humor determines the intraocular pressure or IOP (the fluid pressure in the eye). Most individual's IOPs fall between 10 and 21 millimeters of mercury (mmHg). However, some individual's eyes can tolerate higher pressures than others.

Most, but not all, forms of glaucoma are characterized by high intraocular pressure. In most types of glaucoma, fluid cannot flow effectively through the trabecular meshwork and this causes an increase in intraocular pressure causing damage to the optic nerve and leading to vision loss.

Blurry vision and "seeing stars" are warning signs of increased intraocular pressure, which can result from overproduction of aqueous humor or damage to the drainage system, among other causes. This is glaucoma, and uncorrected, it can damage the eye, causing injury to the retina or optic nerve and eventual blindness.

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The exact cause of glaucoma isn't known. Experts think that increased pressure in the eye (intraocular pressure) may cause the nerve damage in many cases. But some people who have glaucoma have normal eye pressure.

Get more information on eye anatomy and function.

Open-angle glaucoma

In open-angle glaucoma (OAG), fluid in the eye (aqueous humor) doesn't drain well. When this happens, the fluid builds up. This buildup increases the intraocular pressure (IOP) and may damage the optic nerve.

Up to half of the people with OAG don't have higher-than-normal IOP. This is called normal- or low-tension glaucoma.

Closed-angle glaucoma

• The colored part of the eye (iris) and the lens block the movement of fluid between the chambers of the eye. The blockage of fluid causes pressure to build up in the eye and makes the iris press on the eye's drainage system ( trabecular meshwork ). The increased pressure can cause damage to the optic nerve, leading to vision loss and possible blindness.
• You have a defect in your iris or another problem that causes the iris to fall forward and block the drainage angle.
• You have scar tissue between the iris and the cornea, and it blocks the eye's drainage system.

Congenital and infantile glaucoma

Glaucoma that is present at birth ( congenital glaucoma ) or that develops in the first few years of life (infantile glaucoma) is often caused by certain birth defects. A birth defect may occur because of an infection in the mother during pregnancy, such as rubella, or because of an inherited condition such as neurofibromatosis.

Secondary glaucoma

Some people get glaucoma after an eye injury or after eye surgery. A cataract and some medicines (corticosteroids) that are used to treat other diseases may also cause glaucoma. Glaucoma caused in these ways is called secondary glaucoma.