One of the most frequently asked questions in an eye doctor's office is "What is glaucoma?" Glaucoma is a group of conditions in which tissues of the eye become damaged. The optic nerve is most frequently affected, but other structures can be affected as well.
Glaucoma results in vision loss, which can occur with or without elevated intraocular pressure. The vision loss is due to damage to the optic nerve. When examining the eye, the anterior chamber angle is formed by the iris and the cornea. Between the iris and cornea is the trabecular meshwork. Aqueous humor is a clear liquid that fills the front part of the eye and drains out of the eye through the trabecular meshwork. The fluid flows through the trabecular meshwork to Schlemm's canal where it is then collected by the bloodstream. Glaucoma can be classified into two groups of disorders: those with open angles and those with closed angles.
In open-angle glaucoma, the mechanism of raised intraocular pressure is impaired outflow of aqueous humor within the drainage system; the drain itself is open. Imagine a sink with the drain open, but water cannot leave the sink due to a clog within the drainage pipes. However, not all forms of open-angle glaucoma have elevated intraocular pressure. In low-tension glaucoma, intraocular pressure plays a small role in the progression of the disease, and the pressure may actually be below normal. In the open-angle and low-tension glaucomas, the patient has no symptoms until damage to the optic nerve is marked. The vision loss is slowly progressive and painless.
Primary open-angle glaucoma is the most common form of glaucoma in America and Europe. The risk factors for developing open-angle glaucoma are advanced age, family history of glaucoma, elevated intraocular pressure, race, myopia (nearsightedness), and systemic diseases such as diabetes, hypertension, and cardiovascular disease. Secondary forms of open-angle glaucoma exist as well in which the drainage system becomes clogged with various substances that can be identified.
In acute primary angle-closure glaucoma, the flow of aqueous humor out of the eye is physically blocked. The iris becomes adherent to the trabecular meshwork. This results in elevated intraocular pressure, and the patient typically has excruciating pain. Blindness can occur in a few hours. There are also several forms of secondary angle-closure glaucoma, where iris tissue blocks the angle as a consequence of another preexisting ocular disease.
Elevated intraocular pressure is still considered a key feature of glaucoma even though it is not considered essential to its diagnosis. Intraocular pressure remains the one risk factor that can be modified, and glaucoma treatment is aimed at lowering the intraocular pressure. This can be accomplished through medications, laser treatment, or surgery. Medications work to decrease the intraocular pressure by either decreasing the amount of aqueous humor that is produced inside the eye or by increasing the ability of the aqueous humor to flow out of the eye. Laser can be used to treat both the open-angle and angle-closure forms of glaucoma. In open-angle glaucoma, laser spots are directed at the trabecular meshwork to facilitate flow through the drainage system. Laser can be used to treat angle-closure glaucoma by making a hole in the iris. This enables the pressure to equalize on both sides of the iris and allows the iris to move away from the trabecular meshwork, opening the angle. Surgery to bypass the drainage system is recommended in cases that do not respond to medical treatment.
A careful history and examination are important in the detection of glaucoma. There are generally no reported symptoms, and an elevated intraocular pressure or suspicious-appearing optic nerve may be found on routine eye examination. Further testing can be performed to detect any loss of peripheral vision or defects in the nerve fiber layer of the retina.
The management of glaucoma is best left to the ophthalmologist, but the importance of detection calls for the cooperation and assistance of all medical personnel. Careful periodic evaluation of the optic nerve and measurement of intraocular pressure are of great importance in the follow-up of glaucoma patients. Proper identification and management of glaucoma would lead to fewer patients losing vision as a result of this blinding disease.