*2.6. Optical Coherence Tomography of the Retinal Nerve Fiber Layer*

In the past, serial optic nerve head photography was the only way to document structural changes indicative of glaucoma. In recent years, however, optical coherence tomography (OCT) has taken over as a more precise and objective tool in the diagnosis and monitoring of glaucoma. OCT relies on near-infrared light to obtain a cross-sectional image of the retina and optic nerve head. Different layers of the retina can be isolated in the scan, as there is an increased reflectivity in the nerve fiber layer and the plexiform layers and decreased reflectivity in the cell and nuclear layers. OCT performs a retinal nerve fiber layer (RNFL) analysis by measuring the RNFL thickness at a circle, 3.4–3.5 mm in diameter, centered around the optic nerve head. In the presence of a glaucoma, areas of RNFL loss are seen, typically in the superotemporal and inferotemporal regions of the circle. The OCT compares the RNFL thickness pattern with that of a normative database of the same age group and flags any areas below the reference range with a color-coded scheme. The OCT is a powerful tool in the early detection of glaucoma, as RNFL thinning can be seen, even before peripheral visual field loss occurs. Furthermore, when serial OCT scans are taken, the RNFL thickness of each region can be plotted on a trend line to assess for disease progression over time.

OCT is a desktop device only available in clinic. Companies are working to develop a portable version, but the closest to that at this time is the Envisu C2300 (Leica; Wetzlar, Germany), which consists of a handheld OCT scanhead attached to a cart of equipment. The Envisu C2300 is particularly useful for patients who cannot sit up to reach a desktop device. In the future, a portable home OCT machine for glaucoma patients may be available, given that Notal Vision (Manassas, VA, USA) has developed one for the detection of neovascular age-related macular degeneration (AMD) progression at home.
