ASSIGNMENT 1: OCULAR BLOOD FLOW & GLAUCOMA

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NOTE 2: FOR MORE INFORMATION REGARDING GLAUCOMA READ ASSIGNMENT #3

DEFINING GLAUCOMA

Glaucoma is a type of optic nerve disease that affects the visual field. It does so by destroying the axons of the ganglion cells that take information from the retina to the brain. The axonal death is clinically seen as a change in the appearance of the optic disk. Intra ocular pressure is considered to be an important risk factor, but it is not an etiologic factor.

OPTIC NERVE & THE OPTIC DISK -STRUCTURAL OVERVIEW

The optic nerve is the second cranial nerve. It is an afferent nerve that takes the visual signal to the brain. The optic nerve is divided into several parts depending on its location. The part within the brain is called the intra-cranial part, the part in the optic canal is the intra canalicular-part. The part in the orbit is the intra-orbital part.

The part of the optic nerve that is visible to the eye when examining the fundus is called the optic disk, or the optic nerve head (ONH)

The optic disk, so called because of disk like appearance can be structurally divided into two basic parts, as reflected in the diagram:

The Optic Cup:

The central "white" portion of the optic disk (indicated by black bars) is called the optic cup or simply the cup. Anatomically the cup is composed of glial tissue which supports the central retinal artery and vein as they travel through the optic nerve. There are no axons of the ganglion cells in the central cup. The cup usually occupies anywhere between 30% to 45% of the total disk size. In the picture the cup is about 30% (3/10 ) of the total size of the disk (The white lines)

The Disk Rim:

The pink portion of the optic disk is called the optic disk rim, or simply the rim (indicated by green bars). This lies between the margins (boundary) of the central cup and the margins of the disk (indicated by the white horizontal bars). This is the part that carries the axons of the ganglion cells to the brain. It appears pink as the axons are transparent and allow light reflected from vessels in the disk to pass. The size of the rim varies from 70% to 65% of the total disk size. In the picture the size of the rim is about 70% of the disk size

OPTIC NERVE & THE OPTIC DISK -STRUCTURAL CHANGES IN GLAUCOMA

Disk changes are very characteristic of glaucoma. They changes seen are rarely ever witnessed in any other disease of the eye. The death of axons in the optic disk cause the area occupied by the rim to decrease. This area is then over taken by the cup. Think of this as a battle between two forces. "White" (the cup) and "Pink" (the rim). Pink is loosing troops and thus the area occupied by it decreases. This area is then over taken by the invading army of the "White".

The picture shows the changes that take place in the disk due to glaucoma. Notice how the "invading" white has nearly over run the dying "pink". The cup now occupies nearly all of the disk (nearly 90% or 9/10). The rim is only left with 10% of the area (or 1/10).

YOUR VISUAL FIELDS: OVER-VIEW

Visual fields are extensively defined in your study guide. (Appendix I: Over-view of an Eye examination). Here I'll briefly go over visual fields and how they are affected by glaucoma.

THE "NORMAL" VISUAL FIELD

Visual field is everything you see around you. If you are reading this text, you can also see the image above and some of the text above the image. But you can only read the lines you are "directly" looking at. The lines above are a blur. You know they are there, but you can't really make out what they say. This is in effect the two parts of your vision. The central part is the text you are reading and is tested using visual acuity charts (Look under "Videos" on this site and Appendix I in your study guide for more info on visual acuity charts). The rest of the vision is tested using visual fields.

Essentially your visual field is a bowl like area in front of eye in which you can see objects. Anything to your rear and some of the area to your sides are hidden from you. The best definition of a visual field is an island of vision in a sea of darkness. When you test a visual field you are essentially doing two things:

I. Identifying the perimeter of the field:

This helps figure out whether the field's boundaries are affected

II. Identifying problems with-in the field:

This helps to find out if there are areas with-in the perimeter where you can't really see well. Everyone has at least one area of defect, called the (physiologic) blind spot. This corresponds to the location of the optic disk. The disk has no photoreceptors, thus is not sensitive to light. We are not aware of our blind spot as the vision from the other eye helps cover this "defect". Though if you close one of your eyes, it is possible to see this. (You can test your own blind spot by following the instructions given here

There are several ways of testing a visual field, from simple confrontational to bowl perimeters to computerized perimeters.

The picture shows a visual field record from a bowl (or arc) perimeter of the right eye. You will do this test yourself and will gain a better understanding into the specifics then. How do I know this is the right eye? The site of the "blind spot" (thus the optic disk) shows the side of the visual field. The fovea is located in the center of the field (fovea is the part of your retina which allows to recognize letters). The perimeter is the boundary of your field. the markings on the chart show how many degrees away from the fovea is the limit of your field. Thus superiorly the limit is about 50 degrees (the highest point superiorly).

VISUAL FIELD CHANGES IN GLAUCOMA

From the discussion above (optic disk), the death of axons (of ganglion cells) is responsible for change in disk appearance. The same axonal loss is responsible for changes in the visual field. The photoreceptors (rods & cones) help convert light to electrical signals which are eventually carried away to the brain by the axons of ganglion cells. Think of this arrangement as a light bulb assembly. If the wire connecting the bulb to the switch is damaged, the bulb won't work, even if it is okay. Similarly if the axon is dead the signal from photoreceptors will not reach the brain, even if the photoreceptors are working perfectly. This will creates areas within the perimeter of the visual field where a person who suffers from glaucoma will not be able to see anything. These non-seeing areas are called Scotomas.

As the appearance of optic disk changes characteristic of glaucoma, so are the visual field changes.

This is a visual field of a patient who suffers from glaucoma. See the black arc like defects? They reflect axonal damage. They are called "arcuate" scotomas as they are shaped like an arc and are very characteristic of glaucoma. This person can not see anything in the black areas.

BLOOD FLOW VARIATIONS & THE NEW FACE OF GLAUCOMA

For a very long time it was thought that the intra ocular pressure compresses the axons and eventually cause their death. However recent evidence suggests that it might not be the intra-ocular pressure at all, but the blood flow dynamics of the optic disk that is responsible for the changes seen in this disease. The optic disk is supplied by ciliary circulation and has elements of auto regulation

The task at hand is to look for evidence that proves the role of blood flow dynamics of the optic disk in this disease. Perhaps it is the auto-regulation that is central to solving this riddle?