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Abstract

Introduction


Cup disc ratio measurements are one of the most popular methods not only for early detection but also for monitoring progression of glaucoma. Especially when monitoring the treatment effect on glaucoma, accurate and reproducible measurements of disc are necessary.1 Disc evaluation is important because clinically detectable glaucomatous damage in the optic disc may precede the onset of white on white visual field defects by several years.2-4 Clinical evaluation of optic disc morphology can be made with direct ophthalmoscope, slit lamp biomicroscopy with 78D and 90D and with indirect ophthalmoscope.5 Clinical evaluation of the optic nerve head is notoriously subject to variation between observers though agreement can be substantial given the right conditions. The development of imaging and measurement techniques has brought the prospect of greater objectivity in the measurement of optic disc features, and therefore better agreement between observers. Confocal Scanning Laser Ophthalmoscope is the most advanced method in current use that provides objective, reproducible, quantitative and three-dimensional analysis of the optic disc. Image analysis on the HRT II has been shown to produce optic disc measurements, such as the estimation of cup/disc area ratio, which are comparable to those made by clinician. Heidelberg Retina Tomograph, software version 1.5, Heidelberg Engineering, Germany, uses a Confocal Scanning Laser Ophthalmoscope (Diode 670 nm) to produce a topographic map of optic nerve head. The light reflected from the surface of retina is imaged. Each image is formed by tiny squares called pixels. Totals of 32 images are taken. The first of these 32 is parallel to the surface of the retina, just anterior to blood vessels as they emerge from the optic cup. Subsequent images are acquired by advancing the focal plane posterior to the lamina cribrosa. A plane is chosen as the reference plane from which the parameters of interest can be calculated. Computer converted three-dimensional reconstruction, can be used to evaluate disc damage in glaucoma.6-8The HRT software computes a set of stereo metric parameters that quantitatively describe the shape of the optic nerve. Only the vertical and horizontal cup disc ratios were taken from these parameters.Although more objective measurements can be achieved by Heidelberg retina tomograph II, but it does not mean that this is more accurate in estimating disc measurements. This device uses the clinician induced contour line to define disc margin, whereas in slit lamp examination with 90D lens and in stereoscopic photographs, one evaluates the cup disc ratio by observing the three dimensional shape of cup, including the bending of small blood vessels in the optic disc and the disc border is much more delineable then in tomograph.