Introduction to Glaucoma
We have spoken about the importance of the way the front of the optic nerve looks like a disc (Optic Nerve Head). I would like to get into a little bit more detail about the relationship of the retina to the optic nerve. What we are addressing is the Nerve Fiber Layer that originates in the retina and becomes the optic nerve as the fibers go to the brain.
How the Eye Processes Reflected Light
The reason we can see, is that light reflected from objects is focused onto the retina. The retina changes light to electrical signals. The signals travel from the bottom of the retina (cones and rods) up through various layers of the retina, ending up stimulating cells called ganglion cells. These cells then transmit the signal through long fibers (Retina Nerve Fibers) that travel on the surface of the retina then turn sharply as they converge and make up the optic nerve.
The way the nerve fibers make their way to the optic nerve is very important to understanding what happens in glaucoma.
The Path of the Nerve Fibers from Retina to Optic Nerve
Nerve fibers from the retina travel in an arc to reach the optic nerve, both in the upper and lower retina. A line can be drawn through the middle of the optic nerve and the middle of the macula to the outer retina, which is like a “property line.” Fibers above this line come together in the upper optic nerve. Fibers from below this line come together in the lower optic nerve.
Damage to the nerve fibers is seen most in the upper and lower optic nerve, hence vision damage shows most in the upper and lower field of vision on the nose side. This damage is known as nasal steps.
How the Optic Nerve Changes in Appearance from Glaucoma Damage
Damage to the upper and lower nerve fibers results in the cup getting progressively bigger in the upper and lower direction, hence it elongates vertically. A cup that is longer in the horizontal (side to side) direction argues against glaucoma.
The Strength of Evidence in Cupping
Changes in the cup are not always clear-cut, and often we are not sure what the cup looked like years prior to the present evaluation. When changes are not definite enough for us to say whether glaucoma is present or not we have to get evidence from other sources such as imaging of the Nerve Fiber Layer (RNFL) and from visual field testing.
Documenting the appearance of the optic nerve in stereo (three dimensional) with digital photography is, to me, a very important part of glaucoma care. Orion Eye Center has the very latest digital fundus camera.
The Importance of Eye Pressures
Eye pressure is a very important part of glaucoma, not so much in diagnosis as in monitoring the effectiveness of treatments. However, eye pressure is only one part of the treatment of glaucoma, the other components being changes in the Retina Nerve Fiber Layer, the Visual Fields, and the appearance of the optic nerve.
Lowering eye pressure, hopefully throughout the day and night, is the only treatment option open to us at present, hence the somewhat misplaced importance of focusing on eye pressure.
Treatment of Glaucoma
Treatment is focused on lowering the eye pressure. Several ways are available to lower eye pressure, which in main are:
1. Various medications
We will discuss these various treatments in a future article.
We will also discuss how glaucoma damage manifests in the appearance of the optic nerve, the Visual Field, and Retinal Nerve Fiber Layer.