Understanding Vision Loss from Glaucoma

The complex images that we see are transmitted to the brain for processing by way of the optic nerve. Approximately 1.2 million nerve fibers, or axons, make up each human optic nerve (Figure 1-2). The optic nerve travels from the back surface of each eye and joins together to form the optic chiasm. The nerve fibers that entered the chiasm as part of the optic nerves, travel from the chiasm to the lateral geniculate body but are now called optic tracts. Finally, visual input is transmitted from the lateral geniculate body and travel as optic radiations to an area of the brain called the occipital lobe. Visual images from the retina (the “film” of the eye) travel through the optic nerve, optic tract, and eventually to the visual part of the brain. There the images are processed and interpreted by the brain. Any disease process which affects the optic nerve could disrupt this input, leading to visual loss.

Figure 1-2. This is a healthy optic nerve. Notice the healthy rim of nerve tissue with a central cup.

CUPPING

The term cupping of the optic nerve describes the appearance of the optic nerve to the examining eye doctor. When the nerve is viewed through the pupil, it looks like a cup seen from above. The cup is really an empty space in the middle of the optic nerve surrounded by optic nerve fibers. With the loss of nerve fibers from glaucoma, the cup becomes progressively larger because there is less space occupied by the remaining nerve fibers. A healthy optic nerve has many nerve fibers traveling through it (approximately 1.2 million fibers), so there is usually a small cup. As progressively more nerve fibers are damaged, less nerve fibers remain. Consequently, the cup enlarges. The amount of cupping is often described by the eye doctor as the “cup-to-disc ratio” (C/D ratio).

Figure 1-5. The optic nerve is divided into tenths and the cup is compared to the entire optic nerve (optic disc) to obtain the cup-to-disc ratio. This C/D ratio here is 0.4.

Figure 1-6. Normal optic nerve - 0.3 C/D ratio

Figure 1-7. Glaucomatous optic nerve – 0.9 C/D ratio

The C/D ratio attempts to quantify the extent of axonal (nerve fiber) loss. It compares the diameter of the cup to the entire diameter of the optic nerve head (disc). The normal C/D ratio is typically less than 0.5 (meaning that the diameter of the cup is 50% the diameter of the optic nerve head). This number is not a uniform number, because eyes with glaucoma tend to have more cupping in the vertical orientation than the horizontal orientation due to a difference in axonal support structures. Therefore, a measurement in just one orientation may not fully describe the extent of cupping. Eye doctors will often draw pictures and take photographs to accurately document the appearance of a glaucomatous optic nerve. Increases in cupping or nerve fiber loss indicate poorly controlled glaucoma.

In glaucoma the position of the blood vessels within the optic nerve can shift with the progressive cupping, and this can be an important clue that the glaucoma is continuing to cause optic nerve damage. Other exam findings suggestive of glaucoma include hemorrhages (bleeding) on or near the optic nerve. This is commonly seen in poorly controlled glaucoma and is indicative of ongoing optic nerve damage or unstable glaucoma.

If glaucoma affects only one eye, there may be asymmetry of the optic nerve cupping between the two eyes. The unaffected optic nerve will look normal while the affected eye will look cupped. Asymmetry of the optic nerves is another clue that can aid in the diagnosis of glaucoma.

Figures 1-8, 1-9. There is a slight asymmetry of the cup between these eyes. The right eye (pictured at left) has a slightly larger cut than the left eye (pictured at right).

In end stage disease, the nerve may be completely cupped, with no nerve fibers left. In this case, the nerve may appear pale and vision would be poor. While most patients that receive treatment will not progress to end-stage glaucoma, those that do may eventually become totally blind.

It is rare that other optic neuropathies result in cupping of the optic nerve. Most of the time, optic nerve head cupping is caused by glaucoma, and the amount of vision loss corresponds to the extent of cupping.

Understanding Vision Loss from Glaucoma

Central vision is the fine vision people use to read and recognize faces, while peripheral vision is the side vision that is used for navigating obstacles in the environment (like doorways and coffee tables) and for detecting oncoming vehicles from a side street. The diagnosis of glaucoma is often made late in the disease course, because early stages of glaucoma are usually characterized by loss of peripheral vision and sparing of central vision. Patients often fail to notice peripheral vision loss until it has progressed towards the center of vision. Unless the patient happens to be examined by an eye doctor, they could be unaware that they have glaucoma. The only way to be diagnosed with glaucoma in early stages is to be examined by an eye doctor and undergo an eye exam and visual field testing, which measures the amount of vision loss from optic nerve damage (Figures 1-10 to 1-14).

	Figure 1-10. Normal Humphrey visual field test. This is a side vision test which is performed to diagnosis glaucoma. There are no dense black spots indicating vision loss.
	Figure 1-11. Superior arcuate visual field loss from glaucoma – as tested by Humphrey visual field device. The black areas represent abnormal blind spots corresponding to areas of vision loss.
	Figure 1-12. Superior and inferior visual field loss from glaucoma causing tunnel vision. Despite the peripheral vision loss, central vision is still intact and the vision may be 20/20.
	Figure 1-13. End stage glaucoma with dense superior and inferior visual field defects – as tested by Humphrey visual field device. Central vision is now gone and there is severe visual impairment.