University of California, San Francisco, UCSF Medical Center
Optic nerve imaging provides an objective test to diagnose and follow the progression of glaucoma, and the technology has undergone tremendous advancements over the last decade.
When an eye doctor evaluates a patient for glaucoma, one very critical aspect of the office visit is the careful examination of the optic nerve. The optic nerve is composed of approximately 1.5 million axons that connect the retina to the visual targets in the brain. Axons can be thought of as long “cables” or “extensions” of the retinal ganglion cells, which are the cells that are damaged in glaucoma. When the axons and the retinal ganglion cells are damaged, there is loss of function of the optic nerve, which results in visual field deficits. While the visual field tests are very important for glaucoma diagnosis and for tracking glaucoma over time, it is a subjective test. Factors such as alertness, noise or distractions in the surroundings, or how the test is setup can affect how well someone performs on a field of vision test. Therefore, it is very important to also have an objective test to diagnose and follow glaucoma, and the technology of optic nerve imaging has undergone rapid advancements over the last decade.
A very important type of optic nerve imaging involves optic nerve photographs, or stereo disc photography. These photographs are useful because they give detailed documentation of the appearance of the optic nerves, and they can be used as a baseline for future comparisons. Furthermore, a good set of color optic nerve photographs will be forever useful even as technology advances. Stereo disc photography allows the eye doctor to detect if there has been any change in the optic nerves since baseline photographs were taken. Taking a set of stereo disc photographs does require the eyes to be dilated and for the eye doctor to have specialized equipment. If you believe you have previously had optic nerve photographs taken, it is worth your effort to call the doctor’s office and obtain digital copies. This way if you move or change eye doctors, a review of your prior photographs can help the new provider determine if there has been progression or stability of your glaucoma.
More recently, the introduction of more advanced optic nerve imaging techniques have allowed the quantification of certain parameters related to the optic nerves and retina. While optic nerve photographs provide an objective documentation of the appearance of the optic nerves, it does not provide quantitative information. This is one major advantage of the advanced techniques.
A young woman with glaucoma preparing to have an optic nerve imaging test using optical coherence tomography.
The next section lists some of the more common optic nerve imaging techniques. The goal of all of these techniques is to measure the thickness of the nerve fiber layer.
The nerve fiber layer is the part of the retina that contains the axons of the retinal ganglion cells. In glaucoma, there is thinning of the retinal nerve fiber layer. Thus, eye doctors can use it both for diagnosing glaucoma, as there are characteristic areas in which the retinal nerve fiber layer thins first, as well as for monitoring the stability of the disease over time. For example, the test can indicate whether the retinal nerve fiber layer thickness is stable on repeat examination, or if there is progressive thinning.
There are nuances between the instruments, but they all serve the purpose to quantitatively analyze the nerve fiber layer thickness and measure certain parameters of the optic nerve head and macula. The lasers are all safe for the eye, and the test is quick and painless. During the test, the chin is placed on chin rest and the forehead presses against a bar, similar to a normal eye examination with an eye doctor. The technician or eye doctor tests each eye individually, and instructions are given with regard to where to focus and when it is all right to blink.
Most cases of severe vision loss and blindness can be prevented with early detection. Optical Coherence Tomography, or 'OCT' is a powerful new test that can help your doctor identify and manage conditions early in the disease process. Most eye diseases that can lead to irreversible blindness, such as glaucoma, macular degeneration, and diabetic retinopathy, tend to progress without any pain, or vision symptoms, in the early stages. Age is a major risk factor for the development of many eye conditions, because they typically develop over the course of one's life time. This makes routine eye exams, a vital step for detecting eye conditions, even in the absence of any vision problems. The later an eye condition is diagnosed, the more difficult it may be to treat. The OCT test uses light, to scan the structures in the back of the eye. Once the scan is complete, this test generates a highly-detailed, cross-sectional image of the back of the eye. The OCT test can also help doctors detect changes in the eye, caused by diabetes and macular degeneration. The test can help doctors quantify abnormal thickening of the center of the retina, known as macular edema, or help identify subtle, less common retinal conditions such as macular holes. The OCT test can be used to monitor changes to the back of the eye, as a result of drug or laser therapy. These changes can affect a doctor's decision regarding the need for additional therapy. The OCT procedure is brief, painless, and completely safe. Let the OCT test provide you and your doctor valuable information today... that can help preserve your vision for tomorrow.
In summary, optic nerve imaging is one of several tests and examination findings that an eye doctor will use to diagnose and monitor glaucoma. It is an important addition to eye care, and has the advantage of providing an eye doctor with objective data regarding the optic nerves and macula.
Yvonne Ou, MD, is a board certified ophthalmologist who specializes in glaucoma, including medical, laser and surgical therapies; cataract evaluation and treatment including combined cataract and glaucoma surgery; glaucoma filtering and implant surgery; and newer procedures.
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