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OCT offers refined method of detecting, tracking glaucoma progression
The advent of optical coherence tomography has changed the landscape for detection of structural damage of the optic nerve head and retinal nerve fiber layer in glaucoma assessment.
Although color optic disc stereo photography and red-free retinal nerve fiber layer (RNFL) photography remain essential tools in the armamentarium of glaucoma evaluation, relative lack of objectivity and reproducibility have limited their performance to reliably diagnose glaucoma and its progression. In contrast to confocal scanning laser ophthalmoscopy and scanning laser polarimetry, OCT is unique in offering reproducible measurements of the RNFL, optic nerve head and macular thicknesses. These measurements are widely adopted in clinical practice and research laboratories for investigation of glaucoma.
From time-domain OCT to spectral-domain OCT, RNFL thinning has been shown to be a predictive factor for development of visual field defects in glaucoma suspects and patients with ocular hypertension. RNFL thickness is a sensitive biomarker of retinal ganglion cell damage in glaucoma. Spectral-domain OCT can construct three-dimensional topology of the RNFL, facilitating the visualization of different patterns of RNFL defects in the RNFL thickness map. In some cases, RNFL defects evident in the OCT may occur before detectable neuroretinal rim loss. Longitudinal studies are ongoing to investigate the sequence of changes at the optic nerve head region during the course of glaucoma progression.
Choosing an appropriate reference plane to discriminate the neuroretinal rim from the optic cup is difficult. The use of the Bruch’s membrane opening as a landmark to identify the disc margin in OCT images provides a stronger anatomical basis to define and measure the neuroretinal rim compared with an arbitrary reference plane. Spectral-domain OCT has improved our understanding of the optic nerve head anatomy, offering insights into the stress and strain response of the deep optic nerve head structures in glaucoma.
The macula is a suitable target to evaluate glaucoma because it has the highest density of retinal ganglion cells in the retina. A few commercially available OCT models are equipped with software that can segment and measure the ganglion cell and inner plexiform layers. It is worth noting that the RNFL, optic nerve head and macular changes in glaucoma likely represent different areas of retinal ganglion cell damage (proximal axons, distal axons, ganglion cell soma and dendrites, respectively). Integrating information collected from OCT may further enhance our ability to detect and track glaucoma. In this issue, we have insightful comments from five glaucoma experts discussing the future of OCT applications and the pearls and pitfalls in interpretation of results.
For more information:
Dennis S.C. Lam, MD, FRCOphth, can be reached at State Key Laboratory in Ophthalmology, Sun Yat-Yen University, 54 South Xianlie Road, Guangzhou 510060, People’s Republic of China; +852-3997-3266; fax +852-3996-8212; email: dennislam.gm@gmail.com.
Disclosures: Leung has received research support/speaker honorarium from Carl Zeiss Meditec, Optovue, Tomey, Heidelberg Engineering and Topcon. Lam has no relevant financial disclosures.