THE OPTIC NERVE EXAM
In the back of the eye lies a structure approximately 2 millimeters in size, without which there would be no vision at all. The optic nerve is the physical connection between the eye and the brain, the “cable” that carries information back to be interpreted by the gray matter somewhere in the back of our head where images are formed into recognizable concepts and objects that we commonly call the gift of sight. In glaucoma, the optic nerve is damaged in a particular way that is recognizable and slowly becomes more damaged over time.
I spend an enormous amount of time examining the optic nerve in people who are suspicious for glaucoma. In fact, when you come for your routine eye examination, I will probably spend as much time examining the optic nerve as I do examining the rest of the eye. In other words, if you have macular degeneration or cataracts, we examine you in the way that would normally occur in a general ophthalmologist’s office. Then we do a detailed examination of the optic nerve.
To examine the optic nerve, over time, requires diligence, persistence, and an enjoyment of meticulous detail. I often draw the optic nerve in detail and then compare the way the optic nerve looks to the drawings that were made previously. Photographs are also taken. The comparison between old and new photographs and drawings, tells me if the optic nerve is stable or slowly getting worse. It often takes many years of examination in a marginally controlled case to see the accumulation of subtle changes that may have been occurring for quite some time. What I actually look for is a change in the shape of the three dimensional structure. In early cases of glaucoma, this examination is even more important, because usually the first changes occur in the way the optic nerve looks, even before the visual field is damaged. For this reason, one can detect glaucoma sooner by looking at the optic nerve than any other exam technique. Aggressive action taken at this point can be very effective in stopping the disease before it has a chance to damage the vision at all.
The erosion of vision that occurs if glaucoma is allowed to progress is due to damage to the optic nerve. Therefore, careful and periodic evaluation of the optic nerve is very important in diagnosing and following the course of glaucoma. Depending upon the severity of the disease, this evaluation should take place anywhere from once every three months to once a year. In order to examine your optic nerve, we employ special light sources and lenses that allow us to view the back of your eye.
We use a number of instruments. One such instrument, the Direct Ophthalmoscope, has the advantage of not requiring a dilated pupil although dilation is helpful. However, the image is flat and two dimensional which does not give a good three dimensional impression of the contours of the nerve.
For a three dimensional image, we use either a hand-held lens called a 60, 78 or 90 Diopter lens with the patient seated at the biomicroscope or a lens mounted on the biomicroscope called a Hruby lens or a special contact lens held up to the patient’s cornea. Although convenient, the image through the 90 Diopter lens is inverted and flipped left to right. The Hruby Lens and contact lens do not suffer this disadvantage, but are somewhat less convenient to use.
New technologies have become available to, hopefully, make optic nerve evaluation more precise and objective. These instruments quantify images of the optic nerve and the surrounding retina. A computer analysis has the potential to more accurately diagnose early glaucoma damage. It may allow a more accurate assessment of changes in these tissues over time. Each of these new technologies have their own inherent sources of error and, therefore, false positive and negative results.