Nerve fiber layer analysis with sophisticated laser scanning and digital image processing techniques are used by Glaucoma Associates of Texas to assist in the clinical evaluation of the optic nerve and the retinal nerve fiber layer.
The optic nerve is composed of over one-million individual nerve fibers/axons. Since the axons of nerves cannot be measured accurately in a live eye, indirect measures of axons (axon “counting”) must be used. Optical coherence tomography (OCT), the GDx Analyzer (short for Glaucoma Diagnosis Analyzer), and Heidelberg Retinal Tomography (HRT) are used by glaucoma specialists for optic nerve fiber analysis. Both OCT and GDx, the more common analyzers, are available at Glaucoma Associates of Texas.
Optical Coherence Tomography (OCT)
OCT technology is a non-invasive clinical tool used to get an in-vivo (live snapshot) analysis of the optic nerve and retina. OCT uses near-infrared light which acts as an ultrasound, to give a quick and accurate analysis. Light emitted from the OCT machine travels through the eye and is reflected back by the retina and nerve fiber layer. This reflected light is then analyzed via various algorithms to provide cross-sectional and 3-dimensional data. The current OCT used at Glaucoma Associates of Texas takes more than 20,000 A-scans per second, and provides a resolution accurate within 5 micro-meters (0.0002 inches!).
At Glaucoma Associates of Texas, we most often perform OCT on the retinal nerve fiber layer (rNFL or NFL). The nerve fiber layer, the innermost layer of the retina, coalesces to form the optic nerve. It is this layer that is damaged in glaucoma as depicted in the image below.
(OCT of the Right and Left eyes — Note the pie charts at the bottom of the image. The left eye (shown on the right side of the image) is completely green, which indicates a healthy nerve fiber layer. The right eye (shown on the left side of the image) has mild superior nerve fiber layer thinning (shown in yellow) and severe inferior thinning (shown in red). This patient has severe glaucoma in the right eye due to trauma, and a healthy left eye.)
GDx Nerve Fiber Analyzer (GDx)
The GDx (short for Glaucoma Diagnosis) 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.
(GDx Analysis – the above images depicts a normal nerve fiber layer in both eyes.)
Both procedures can be performed in an undilated pupil and come with computer software that allows your physician to interpret the results, comparing them to normal eyes. OCT and GDx provide the physician with objective, quantitative information and are useful adjuncts along with visual field testing (which is more subjective and user dependent).
OCT and GDx provide important information that is useful in following your optic nerve status throughout the years. They will also help your physician decide whether you truly have glaucoma or could be considered a glaucoma suspect (borderline glaucoma). Neither test alone makes 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.
Please contact the Glaucoma Associates of Texas if you have questions about the treament for glaucoma.