What is Glaucoma?
Glaucoma is a group of eye diseases that cause deterioration of the optic nerve with associated loss of visual function.
The optic nerve carries visual information from the eye to the brain, much like the cable from a computer screen to the computer.
The simplest definition of glaucoma is a condition of the eye where the intraocular pressure (pressure inside the eye) is elevated, causing damage to the optic nerve. In most patients, a normal intraocular pressure is between 8 and 21 mmHg.
However, there are some individuals who have “normal” pressures (usually defined as below 21 mmHg) who develop glaucomatous damage to their optic nerve. This is called low-tension glaucoma.
Conversely, there are other people with significant elevation in intraocular pressure who sustains no optic nerve damage, even after a protracted period. This is called ocular hypertension.
When the optic nerve is damaged or injured, transmission of visual information from the eye to the brain is limited, and hence the visual image is impaired.
Fortunately, ophthalmologists can usually detect glaucomatous optic nerve damage before there is noticeable vision loss by examining the back of the eye. Often, damage to the nerve can be detected before there is a change in vision by specialized tests.
Causes of Glaucoma – Optic Nerve and Pressure
In most cases, glaucomatous optic nerve damage occurs from high eye pressure. Nerve damage can usually be stopped or slowed by lowering the eye pressure. The goal of glaucoma treatment is to lower the eye pressure by medicines, lasers, or surgery. Although vision lost from glaucoma cannot be restored, treatment can prevent or slow down further vision loss. Uncommonly, optic nerve damage continues despite having the lowest possible eye pressure. Intensive research around the world is now directed at understanding the cause of the damage in these patients and to develop new treatments to preserve the optic nerve.
Many different eye disorders can cause high eye pressure. After measuring the eye pressure, your ophthalmologist attempts to determine the cause of the elevation. It is believed that some form of “clogging” or blockage of the drainage of fluid within the eye (aqueous humor) causes increased eye pressure. Since the eye continually produces aqueous humor, obstruction of its drainage causes the eye pressure to increase. Nearly any eye disorder associated with aging, inflammation, bleeding, injury, tumors, or even birth defects can raise the eye pressure. However, in most cases of glaucoma, examination reveals a normal appearing drainage system and no specific eye abnormalities are found. These patients are described as having “primary open-angle glaucoma.” In other cases, various abnormalities cause a partial or complete blockage of the drainage system. In closed-angle glaucoma, the drainage system is blocked instead of just being clogged.
At least fifty different mechanisms have been described that can raise the eye pressure, but all produce similar damage of the optic nerve.
Ongoing assessment of glaucoma depends upon regular examination of the optic nerve, checking intraocular pressure, and specialized testing.
The well-informed glaucoma patient asks not, “What is my eye pressure, doctor?”, but rather, “How is my optic nerve?”
Optic Nerve Cupping in Glaucoma
Damage to the nerve fibers is visible in the appearance of the optic disc. The mechanism(s) by which elevated intraocular pressure eventually results in nerve fiber degeneration is a matter of intense debate.
Microscopic examination of the optic nerve head reveals damage along the length of the nerve fibers and to the mesh-like structure at the optic nerve head that surrounds and supports them.
There is a typical sequence of change that occurs to the optic disc in uncontrolled glaucoma.
Optic Nerve Head Cupping (1–4, left to right; C/D = Cup-to-Disc Ratio)
- Normal optic nerve head with small central physiologic cup, C/D ratio ~ 0.2
- Concentric enlargement of the central cup, C/D ratio ~ 0.5
- Irregular enlargement of the cup, especially inferiorly due to loss of inferior neural rim tissue, C/D ratio ~ 0.7
- Marked glaucoma cupping with high degree of central atrophy and loss of inferior rim, C/D ratio ~ 0.9