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Glaucoma is a term applied to a group of eye diseases that cause a characteristic and recognisable deterioration of the optic nerve. The optic nerve carries visual information between the eye and the brain, much like the cable between a computer screen and the computer. In the brain, this information is assembled into a visual image that we recognise as sight (Figure 1). |
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When the optic nerve is damaged or injured, the transmission of visual information is altered and, hence, the visual image is impaired. Since patients with glaucoma may be unaware of any change in their vision until late in the course of the disease, they may not seek medical attention until there is substantial permanent damage. Fortunately for patients who do have regular examinations, ophthalmologists can usually detect glaucomatous optic nerve damage before there is noticeable visual loss by examining the back of the eye. Sometimes, damage to the nerve can be observed even before specialised tests detect a change of vision.
Why Should Your Optic Nerve Be Examined?
For these reasons, examination of the optic nerve and its ability to transmit the visual message is an essential part of the examination for glaucoma. By determining whether or not the optic nerve has been damaged and whether the damage is getting worse, your eye doctor can diagnose glaucoma or determine whether it is progressively deteriorating.
How Is the Optic Nerve Examined?
 In most patients, the optic nerve can be readily examined. It can be seen directly inside the eye with an instrument called the ophthalmoscope. Although many different diseases affect the optic nerve, the damage from glaucoma has a characteristic appearance that permits your ophthalmologist to recognise whether glaucoma is present. The optic nerve exits directly through the back of the eye. The nerve is made up of about one million fibres which originate in nerve cells located in the retina, the light sensitive film coating the inside of the eye. When looking into the eye, the optic nerve is seen on end (optic nerve head), and the nerve fibres are seen to fan out onto the retina.
The optic nerve fibres as seen to fan out onto the retina from the optic nerve head.The pattern looks like a fingerprint.
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What Is the Appearance of the Optic Nerve Head in Glaucoma?
In the normal state, the optic nerve head looks much like a doughnut, with the outer ring consisting of the nerve fibres. The hole (called the optic cup) is the space which remains after the nerve fibres turn to fan out onto the retina.
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| In glaucoma, the nerve fibres are damaged and erode away, leaving a larger cup (or hole of the doughnut) (Figure 3). |
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| Figure 3. Diagram of optic nerve viewed on end and in profile.The optic nerve has one million fibres.The optic nerve head looks like a doughnut,with the outer ring consisting of nerve fibres. |
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Especially when the degree of enlargement is different between the two eyes, a doctor can diagnose early glaucoma from this appearance alone (Figure 4).
New methods of detecting damage to the optic nerve
appearance using computers are now available, but standard examination with conventional instruments can often detect even mild damage. Newer methods that may permit even earlier detection also are being developed to examine and measure the nerve fibres as they spread into the retina.
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| Figure 4.The normal optic nerve (left) looks like a doughnut with a small cup (hole) in the center.In glaucoma (right),it appears that a bite has been taken from the center of the doughnut and there is an enlarged cup (hole) in the center. |
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Visual Field Testing
Another important tool used
to examine and measure the optic nerve is the visual field test. This measures how well the optic nerve functions in carrying visual information the brain. Most visual field tests measure the ability of the patient to recognise light at each area of the retina. Many new visual field tests also are being developed, using coloured (blue and yellow) light, flickering lights, and special targets (such as rings).
Testing of the visual field and examination of the optic nerve are the most important determinants of whether or not there is glaucoma. If glaucoma is present, these tests also allow your eye doctor to determine whether it is stable or becoming progressively worse.
Most patients dislike having their visual field tested. The test is often long, tiring and boring. It is not uncommon to
feel that you have performed
badly. Most modern visual
field machines measure at
each location until the testing
light is not seen. This is done
in order to determine the
dimmest light detectable.
Thus, even someone with a
healthy eye often will not see
the testing light during the
course of the examination. In
other words, not seeing the
light may be perfectly normal,
and should not cause anxiety.
What Causes Glaucoma?
It is not known what causes
glaucoma. In some, but not
all, the optic nerve is
damaged by high eye
pressure. Nerve damage can
usually be stopped, or slowed,
by lowering the eye pressure.
With medicines, laser, or
conventional surgery, glaucoma treatment is designed
only to lower the eye
pressure. Some eyes with
glaucoma optic nerve damage
continue to deteriorate despite
having the lowest possible eye
pressures. It is not known
why this happens. 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 conditions can cause high eye pressure. For you to be treated correctly, it is important that the cause of the elevation is determined. Nearly always, it is caused by some form of “clogging” or blockage of the drainage of internal fluid within the eye (aqueous humour). Since the eye continually produces this fluid, blockage of the drainage causes the eye pressure to increase.
Keys to Understanding Glaucoma
- In glaucoma, the optic nerve fibres are damaged and then die.
- Once the nerve fibres are damaged, they do not recover.
- Damaged nerve fibres lead to loss of vision.
- Eye pressure which is too high is a leading cause of optic nerve fibre damage.
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Almost any eye disorder associated with aging, inflammation, bleeding, injury, tumour or even birth defects can raise the eye pressure. However, in most cases of glaucoma, no specific abnormality is recognised; this condition is known as primary open-angle glaucoma. In other cases, the eye may be unusually small or exhibit other minor shape abnormalities that cause closed-angle glaucoma. In closed-angle glaucoma, the drainage system is totally 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 to the optic nerve. All methods of treatment are designed to lower the eye pressure to a level that will prevent further optic nerve damage.
"How's My Optic Nerve, Doctor?"
Ongoing assessment of glaucoma depends upon regular examination of the optic nerve and visual field in addition to the eye pressure. The well -informed glaucoma patient asks not, ÒWhat is my eye pressure, doctor?Ó but rather, "How is my optic nerve?"!
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