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 tremendous 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. 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.
Advanced Imaging Techniques
More recently, the introduction of more advanced optic nerve imaging techniques have allowed the quantification of certain parameters related to the optic nerves. 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.
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.
Optical Coherence Tomography (OCT) OCT measures the reflection of laser light similar to the way that ultrasound measures the reflection of sound. Using this device, a 3D reconstruction of the optic nerve can be created.
Heidelberg Retina Tomograph (HRT) HRT is also a laser that can produce a 3D representation of the optic nerve.
Nerve Fiber Analyzer (GDx) GDx uses laser light to measure the thickness of the nerve fiber layer.
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. 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.
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.
Yvonne Ou, MD Assistant Clinical Professor of Ophthalmology Glaucoma and Cataract Surgery UCSF Department of Ophthalmology
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