The third important exam method used to identify glaucoma is looking at the retina and optic nerve head for signs of glaucoma damage. In this exam, called ophthalmoscopy, doctors look for signs that structural loss of ganglion cells and their fibers has already happened. Ganglion cells are scattered all over the retina, and their fibers converge on the optic nerve head like roads coming into a big city. The layer of fibers gets quite thick just at the nerve head and as the fibers pile up and dive into the opening. The nerve head, often called the disc, is mostly filled with fibers. There is left over space in the middle of the nerve head called the cup, and the fibers are grouped around the cup in the rim. We all want to have lots of fibers, so we want a big rim (and by subtraction, a small cup). When we compare the size of the cup to the size of the whole disc, we talk about a “cup to disc ratio” (Figure 13). The bigger the ratio, the more empty space there is in the nerve head. That space may be left behind when nerve cells die. The average cup to disc ratio is about 0.4, and ratios of 0.7 or greater happen only 2.5% of the time, so cups this big raise our suspicion that glaucoma might already have started (Figure 13).

Figure 13: Optic nerve exam. (Left) Drawings of how the doctor estimates glaucoma damage from optic nerve head examination. The outer margin of the disc determines the outer circle and the cup size determines the inner circle. The ratio of cup size to nerve head size is the cup/disc ratio. The upper left drawing shows a smaller, more normal cup/disc ratio and the bottom left shows a larger cup/disc ratio due to loss of fibers in the rim of the nerve head. Top right: Photograph of a normal nerve head, with small cup to disc ratio; Middle right: nerve head with larger than normal cup and loss of upper rim of disc; Bottom right: large cup/disc ratio due to substantial glaucoma damage.

Glaucoma causes loss of fibers in groups that run together, so in looking at the nerve head, there is sometimes a tell-tale notch or local loss of fibers in one area of the rim, typically at the top or bottom (see Figure 13 and Figure 14). Big differences between the two eyes in the size of the cup are also a way to identify glaucoma that has started in the eye with the bigger cup. And, occasionally, bleeding in little splinter-like streaks happens on the nerve head to indicate that glaucoma is active.

Figure 14: Optic nerve photographs. Photographs of nerve heads with glaucoma. Top left: cup extends abnormally to inferior rim of nerve head (arrowhead); Top right: bleeding on the inferior disc rim, seen as a flame-shaped red area (arrowhead); Bottom left: disc with unusual shape and larger than normal cup/disc ratio.

If the changes in the nerve head by ophthalmoscopy identify glaucoma, then why are some cases not diagnosed early enough? Perhaps the biggest problem is that the structure of the nerve head varies a lot among people. Many human features have a wide normal range. There are short people, 5 feet tall, and 7 footers, so just being outside the “normal” range doesn’t mean that a disease is present. The cup can be big in some discs just because the disc itself is big and there’s lots of left over space after the fibers go through. That leads to over-diagnosis of glaucoma in persons with big discs. Small discs that start with no cup space can have real damage when the cup to disc ratio is in the middle of the average normal range. And, some discs have a tilted or distorted shape (Figure 14, bottom left) that makes judging the cup to disc ratio very hard, even though the eye sees just fine. Having a disc size or shape that is different from normal isn’t a disease in and of itself and does not predispose a person to having glaucoma. However, it may make a person’s nerve seem more suspicious and therefore require more careful monitoring than someone with a disc that is of normal size and shape. There is some influence of race in disc size, for example, African-derived persons on average have larger discs.

To add information during ophthalmoscopy, eye doctors started looking just outside the nerve head at the layer of nerve fibers as they came toward the disc to see if they were missing. This helps quite a lot with the unusual shaped discs to tell if there is fiber loss. But, this nerve fiber layer exam itself has variations and is harder in persons with light pigmentation than in darker individuals. The Wilmer Glaucoma Center of Excellence did a landmark study that showed one could predict functional glaucoma damage 5 years earlier by doing this nerve fiber layer exam.

For many years, it was most common to record how the nerve head looked by taking color photographs (as in Figure 13 and Figure 14), often doing pairs of them to allow a stereoscopic view. These were repeated at intervals and the appearance of the nerve head as the doctor looked at it in the office was compared to previously taken photographs to see if the eyes were the same or had gotten worse. Color film gave way to digital photographs during the last decade in these cameras. But, these photographs needed to be taken with very bright flashes of light and with dilated pupils. The patient’s vision is quite dimmed for a few minutes after this procedure and it is hard to drive home or see to read for some time afterward. There is still some role for this type of photography.

Both the need to have quantifiable information and the lack of convenience of color photographs has stimulated the development of digital imaging methods to assess glaucoma progress and stability. We now can measure the thickness of the nerve fibers with several such instruments, more comfortably and with greater ability to measure for change. Some of these instruments concentrate on the topography of the nerve head—kind of like measuring the slope in and out of a volcano’s cone (Figure 15). Other instruments are good at estimating the number of nerve fibers and ganglion cells by making an optical thickness measurement (Figure 15). The true strength of these instruments has not been fully achieved yet, as their sensitivity is still being improved. The maximum help they give will be realized when we can depend on them to tell us if an eye’s fibers have decreased quantitatively. That is an important measure of whether the patient is doing well (staying the same) or getting worse (losing fibers). There is considerable research being done to see which instruments allow us to judge how patients are doing. Artificial intelligence methods may soon assist the experienced eye doctor to interpret these tests.

Figure 15: Optic nerve imaging. Images taken from the digital imaging instruments for ophthalmoscopy called "Optical Coherence Tomography" or for short, "OCT". The images provide an estimate of the thickness of nerve fibers of ganglion cells around the nerve head. Courtesy of Dr. Elyse McGlumphy, MD.