How Glaucoma Affects the Optic Nerve
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What is the optic nerve?
This is the part of the eye that carries visual information from the eye to the brain. It is located at the very back of the eye just to the nose side of center. It is also the part of the eye that gets injured when someone has glaucoma.
What comprises the optic nerve?
It is made up of about 1 million small individual thread-like nerve fibers that come from the retina. The fibers bend about 90 degrees as they leave the retina and enter the front of the optic nerve (known as the optic nerve head).
Normally, there is a small crater-like depression seen at the front of the optic nerve head. This depression is known as the cup. Its diameter is smaller than the diameter of the optic nerve.
In the old days, when a doctor looked at the nerve with a monocular magnification device, the nerve head looked like a cup on a saucer (or disc). This started a whole group of optic nerve descriptors (cup to disc ratio, cupping, cupped, etc.).
How is optic nerve damage detected?
The normal cup to disc ratio (the diameter of the cup divided by the diameter of the whole nerve head or disc) is about 1/3 or 0.3.
There is some normal variation here, with some people having almost no cup (thus having 1/10 or 0.1), and others having 4/5ths or 0.8 as a cup to disc ratio. If someone has a cup/disc ratio larger than 1/3, then doctors get suspicious that the cup could be getting larger than it used to be.
Glaucoma can cause the cup to enlarge (actually little nerve fibers are being wiped out along the rim of the optic nerve in glaucoma). Some doctors refer to an enlarged cup/disc ratio as cupping or a cupped nerve. Glaucoma typically causes the cup to get bigger in a vertical oval type pattern.
To discern whether a large cup is glaucomatous or normal requires the doctor to pay close attention to the rim of the nerve on the temporal side (the side closest to the temple or ear side of the head).
Stereo photos of the optic nerve are extremely valuable for documentation of the nerve shape and for future comparisons. If the temporal rim of the optic nerve is very thin or is sloped, then glaucoma is more likely.
The doctor also pays close attention to the color of the optic nerve because some other diseases of the optic nerve can cause enlarged cups but also cause the nerve to look pale (multiple sclerosis, brain tumors, strokes to the nerve or brain, etc.).
What does the visual field test tell my doctor?
Finally, the visual field test is used to help decide if an unusual looking optic nerve is glaucomatous or not (or whether the known glaucoma nerve is getting worse).
Visual field abnormalities occur with many optic nerve diseases. The glaucoma pattern of loss is typically on the nasal side of the field and usually is more dense at the top or bottom of the field.
Article by Bradley L. Schuster, MD, a full time glaucoma specialist in Denver, Colorado at Kaiser Permanente, and Assistant Clinical Professor at the University of Colorado, Health Sciences Center.
Last reviewed on March 05, 2012