by Jeffrey Henderer, MD
Ken Parker, PhD

Glaucoma specialists at Wills and elsewhere are always looking for better ways to determine if an individual definitely has glaucoma and, if so, what kind of glaucoma it is. It is only when they have answers to these questions that they can suggest the best treatment for a particular individual. The only way they can get answers to these questions is to actually examine the optic nerve to see if and how it has been damaged.

Measuring Optic Nerve Damage

The typical optic nerve damage that occurs in glaucoma is known as "cupping." As the cells making up the nerve die, due at least in part to a pressure inside the eye that is too great for that particular eye to tolerate, they die and disappear. When sufficient numbers of these cells are gone, they leave behind a small "crater" or "cup" in the nerve. A portion of the nerve then appears to have been "scooped out." So one important thing doctors look for when they examine the optic nerve is the presence and extent of the "cup," how deep and wide it is.

Glaucoma specialists can get a good idea of the amount of cupping in an optic nerve by looking at it with an instrument known as an ophthalmoscope. They can get an idea of whether the cupping is remaining stable or worsening by taking a series of photographs over time. But these methods have an important limitation. They can only suggest how big the cup is in the same way that an ordinary aerial photograph of a crater could can give us only a rough idea of how deep the crater is. We can get a much better idea of the depth of the cup or crater by taking a stereoscopic photograph. This would allow us actually to measure just how much the optic nerve has been damaged.

The HRT and How It Works

Glaucoma Service doctors are now examining patients with an instrument that can give more detailed information about the 3-dimensional structure of the cup -- the Heidelberg Retina Tomograph (HRT). The HRT uses a special laser to take 3-dimensional photographs of the optic nerve and surrounding retina.

This laser, which is not powerful enough to harm the eye, is first focused on the surface of the optic nerve and captures that image. Then it is focused on the layer just below the surface and captures that image. The HRT continues to take images of deeper and deeper layers until the desired depth has been reached. Finally, the instrument takes all these pictures of the layers and puts them together to form a 3-dimentional image of the entire optic nerve.

You can imagine your optic nerve as a stack of pancakes and you are looking at the stack from above. First, you can only see the top cake. An ordinary photograph taken from the same angle of course also would capture only the top pancake. In order to see or photograph the next pancake, we would have to remove the top cake. But using laser light, we have only to change the focus from the top cake to the cake just below it.

The HRT takes 32 layer-by-layer pictures from the surface of the optic nerve to from 0.5 mm to 4.0 mm deep into the ocular structures. The computer then piles all the slices together in a reconstructed paper printout that looks like a map drawn to represent the hills and valleys of a geographical area. By color coding areas of elevation and depression, the HRT provides a two-dimensional representation of what the original, three-dimensional, stack looks like.

The HRT image can be used to compute things such as the area of the optic disc (the part of the optic nerve at the back of the eye), the volume of the cup, and the area of the rim around the cup as well. These numbers can then be used in two ways. First, they might show measurements different enough from normal to help in diagnosing glaucoma. As changes in the optic nerve are often the first sign of glaucoma and can precede visual field changes, one might be able to diagnose the disease earlier. Second, the measurements can be followed over time by taking a series of tests - much like taking a series of visual fields. Changes in depth are then computed.Various changes might indicate a worsening or mprovement in the disease.

Problems with the HRT

Despite the apparent advantages of images obtained with the HRT, like any new method of observation, several other factors need to be considered. Is it easy to do? Is it accurate? Is it any better than current techniques?

One problem is that, even though the test only takes a couple seconds to perform, any patient movement (including moving the eye, blinking, or moving the head) will disrupt the laser's path, impairing the quality of the image. Likewise, if the patient does not focus on the same location from test to test, the angle of the image will change, and that will affect the measurements greatly.

Second, the images created by the HRT must be reproducible. That is, different images taken at about the same time should be nearly identical. Limited information available to date suggests that they are. But the range of "normal" optic nerves is sufficiently broad that finding measurements that definitively indicate "early glaucoma" has been very difficult. No test is 100% accurate at distinguishing normal from abnormal, and the HRT is no exception. Several studies have been done applying various formulas to the optic nerve picture, and in the right setting the machine can separate a "normal" optic nerve from a "glaucoma" optic nerve with reasonable accuracy. But, again, the variability among individual eyes is so great that it sometimes remains difficult to know for sure from an HRT image whether the optic nerve is in fact damaged. This is why comparison for change is so important.

Third, the machine has not been used long enough to prove that it is any better than if your doctor took a series of optic nerve photos and carefully examined them. However, the HRT may provide more objective comparison over time, and research at Wills is ongoing on this subject.

Instruments for Measuring Blood Flow in the Eye

In addition to imaging the optic nerve, several instruments have been developed to measure the blood flow to various portions of the eye. The idea is that in addition to increased pressure within the eye decreased blood flow can also result in damage to optic nerve cells. One such instrument, closely related to the HRT, is the Heidelberg Retinal Flowmeter, or HRF. The HRF is similar to the HRT in that both employ a scanning laser to acquire images. The HRF however, is used to look at the blood flow in the small capillaries near the optic nerve head. This is done by detecting changes in the frequency of sound reflected by flowing blood. The velocity of flow that is measured has been reported to be altered in glaucoma. Note that this is not total blood flow, just the velocity. In addition, the area of the retina that is being measured may not have much to do with the blood supply to the optic nerve. But despite these limitations, we are investigating the HRF as a potential test in evaluating patients with glaucoma.

Another imaging instrument we are investigating is the Ocular Blood Flow Analyzer. Ocular blood flow has been reported to be reduced in glaucoma patients with normal intraocular pressure. This analyzer is a small device that uses light to calculate the choroidal blood flow of the eye. It can also be used to measure intraocular pressure simultaneously. This instrument is being used in several clinical studies to determine the pattern of blood flow throughout the day in patients undergoing a Glaucoma Service Diagnostic Laboratory evaluation and before and after glaucoma surgery in patients to see if blood flow has changed.

Despite their limitations, these instruments clearly are steps ahead in diagnosing and managing glaucoma. The Glaucoma Service feels that these devices, and even more so the next generation models, have great potential. We are conducting studies to learn how they can be best be used to detect and prevent further damage from glaucoma.