For reasons that doctors don’t completely understand, increased pressure within the eye (intraocular pressure) is usually associated with the optic nerve damage that characterizes glaucoma. This pressure comes from a buildup of aqueous humor, a fluid naturally and continuously produced in the front of your eye.
Aqueous humor normally exits your eye through a drainage system at the angle where the iris and the cornea meet. When the drainage system doesn’t work properly, the aqueous humor can’t filter out of the eye at its normal rate, and pressure builds within your eye.
In primary open-angle glaucoma, the drainage angle formed by the cornea and the iris remains open, but the microscopic drainage channels in the angle (called the trabecular meshwork) are partially blocked, causing the aqueous humor to drain out of the eye too slowly. This leads to fluid backup and a gradual increase of pressure within your eye. Damage to the optic nerve is painless and so slow that a large portion of your vision can be lost before you’re even aware of a problem. The exact cause of primary open-angle glaucoma remains unknown.
Angle-closure glaucoma, also called closed-angle glaucoma, occurs when the iris bulges forward to narrow or block the drainage angle formed by the cornea and the iris. As a result, aqueous fluid can no longer reach the trabecular meshwork at the angle, so the eye pressure increases abruptly. Angle-closure glaucoma usually occurs suddenly (acute angle-closure glaucoma), but it can also occur gradually (chronic angle-closure glaucoma).
Many people who develop closed-angle glaucoma have an abnormally narrow drainage angle to begin with. This narrow angle may never cause any problems, so it may go undetected for life.
If you have a narrow drainage angle, sudden dilation of your pupils may trigger acute angle-closure glaucoma. Pupils become dilated in response to darkness, dim light, stress, excitement and certain medications. These medications include antihistamines, such as desloratadine (Clarinex) and cetirizine (Zyrtec); tricyclic antidepressants, such as doxepin and protriptyline (Vivactil); and eyedrops used to dilate your pupils for a thorough eye exam.
Another form of the disease, poorly understood but not uncommon, is low-tension glaucoma. In this form, optic nerve damage occurs even though eye pressure stays within the normal range. Why this happens is unknown. Some experts believe that people with low-tension glaucoma may have an abnormally sensitive optic nerve or a reduced blood supply to the optic nerve caused by atherosclerosis – an accumulation of fatty deposits (plaques) in the arteries – or another condition limiting circulation. Under these circumstances, optic nerve damage can occur even with normal pressure.
Pigmentary glaucoma, a type of glaucoma that can develop in young to middle-aged adults, is associated with a dispersion of pigment granules within the eye. The pigment granules appear to arise from the back of the iris. When the granules accumulate on and in the trabecular meshwork, they can interfere with the outflow of aqueous humor and cause a rise in pressure. Physical activities, such as jogging, sometimes stir up the pigment granules, depositing them on the trabecular meshwork and causing intermittent pressure elevations. This type of glaucoma can usually be easily diagnosed by your ophthalmologist.
This form of secondary open-angle glaucoma occurs when a flaky, dandruff-like material peels off the outer layer of the lens within the eye. The material collects in the angle between the cornea and iris and can clog the drainage system of the eye, causing eye pressure to rise. Pseudoexfoliative Glaucoma is common in those of Scandinavian descent. Treatment usually includes medications or surgery.
The arrows in the photo point to pseudoexfoliation material on the front of the lens
Injury to the eye may cause secondary open-angle glaucoma. Traumatic glaucoma can occur immediately after the injury or years later. It can be caused by blunt injuries that bruise the eye (called blunt trauma) or by injuries that penetrate the eye. In addition, conditions such as severe nearsightedness, previous injury, infection, or prior surgery may make the eye more vulnerable to a serious eye injury.
The abnormal formation of new blood vessels on the iris and over the eye’s drainage channels can cause a form of secondary open-angle glaucoma.
Neovascular glaucoma is always associated with other abnormalities, most often diabetes. It never occurs on its own. The new blood vessels block the eye’s fluid from exiting through the trabecular meshwork (the eye’s drainage canals), causing an increase in eye pressure. This type of glaucoma is very difficult to treat.