NERVE FIBER ANALYSIS
Since the axons of nerves cannot be measured accurately in a live eye, indirect measures of axon “counting” must be used. Optical coherence tomography (OCT), Heidelberg Retinal Tomography (HRT), and the GDx are currently used in clinical practice. Our practice currently uses the GDx as a measure of the retinal nerve fiber layer.
The GDx is a type of scanning laser polarimeter. The GDx uses a diode laser in the near infrared region to measure nerve fiber layer thickness and 65,536 retinal points. The polarized light passes through the retinal nerve fiber layer. The axons in this layer have a birefringent property that causes the polarized light to undergo a “phase shift.” The amount of phase shift is directly proportional to the thickness of the nerve fiber layer.
The procedure is performed in an undilated pupil. Three images are obtained for each eye and the images are then averaged for a baseline reading. The GDx comes with computer software that allows the physician to interpret the results, comparing them to normal eyes. The GDx provides the physician quantitative information and is a useful adjunct along with visual field testing (which is more user dependent).
The GDx is currently used in our practice as a baseline exam. The test provides important information that is useful in following your optic nerve status throughout the years. The GDx also helps the physician decide whether you truly have glaucoma or could be considered a glaucoma suspect. The GDx test, alone, does not make the diagnosis of glaucoma; therefore, the physician must use all of the baseline and follow-up data of each patient to make a decision regarding treatment.