The HRT: Examining the Optic Nerve in Glaucoma

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.