What Is GDx?

Question: What Is GDx?

My doctor has ordered a GDx test for me. What does that mean? Is it necessary?

Answer: Your doctor has ordered a test called the "GDxTM Nerve Fiber Analyzer" (GDx). The GDx test is a relatively new test that has proved its usefulness in the diagnosis and management of glaucoma. The GDx is a tool that uses laser to determine the thickness of the nerve fiber layer. Older glaucoma tests have centered around measuring eye pressure or measuring the effect that glaucoma has on your overall visual field.

Although these tests are extremely important in the treatment and management of glaucoma, it would be helpful to measure or test early what damage glaucoma can cause to the nerve fiber layer in the back of the eye.

The nerve fiber layer consists of millions of individual fibers called "axons" that surround the optic nerve and spread out over your retina. In many patients with glaucoma, considerable nerve fiber layer damage may have already occurred by the time any vision loss is noticed. Glaucoma is known as the "sneak thief of sight," because many patients are not aware they have a problem until significant vision has been lost.

The GDx uses a type of scanning laser polarimeter to measure the thickness of the nerve fiber layer. The procedure is painless and is usually performed on an undilated pupil. The thickness of the nerve fiber layer is then compared with the nerve fiber layer of normal eyes.

The GDx maps the nerve fibers and compares them to a database of healthy, glaucoma-free patients. A thinning of the fibers indicates glaucoma. This information is then made available to your doctor in the form of pictures, graphs and statistical data that indicate the probability of glaucoma.

The GDx test is particularly powerful in early detection as studies show that more than 50% of individuals with glaucoma aren't aware they have it.

It is also very useful for managing glaucoma over the years, because it detects very small changes when compared with previous GDx data. This information is helpful to your doctor so that he or she can decide if you truly have glaucoma or should only be considered a "glaucoma suspect." While nerve fiber analysis as a standalone test does not make for a definitive diagnosis of glaucoma, it does provide your doctor with data that will add to your overall case so that better decisions may be made regarding your treatment.

Although a GDx is still a good test, it is fast becoming an outdated instrument. The development of the GDx and other instruments lead to the production of an OCT: Optical Coherence Tomography.

What is an OCT?

Optical coherence tomography (OCT) is a noninvasive imaging technology used to obtain high resolution cross-sectional images of the retina. OCT is similar to ultrasound testing, except that imaging is performed by measuring light rather than sound. OCT measures the retinal nerve fiber layer thickness in glaucoma and other diseases of the optic nerve.

Learning more about OCT

Optical Coherence Tomography, also referred to as OCT, is a way for optometrists and ophthalmologists to image the back of the eye including macula, optic nerve, retina and choroid.

During an eye examination, optometrists and ophthalmologist can view the back of the eye and its anatomy. However, sometimes doctors need more detail or need to inspect detail right below the surface which is difficult to view with standard techniques. In some ways, it can be compared to doing an "MRI of the retina."  Some describe it as an optical ultrasound because it images reflections between tissues to provide doctors with cross-sectional images.The detail that can be visualized with an OCT is at such high resolution that doctors are seeing things never been seen before in a living human eye.

Source:

Devine, Norma. Glaucoma Service Foundation to Prevent Blindness, 23 Aug 2000.

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