Computerised Three-Dimensional Analysis of Optic Nerve Head Topography in Normal and Glaucomatous Eyes
Abstract: Evaluation of the optic nerve head topography is one of the most important methods to diagnose and monitor patients with glaucoma. Until recently, optic nerve head topography has most often been judged by either ophthalmoscopy or from optic disc photographs. These methods are subjective and examiners do not have access to absolute measurements of the optic disc size and cup depth. The result of such analysis has been shown to vary substantially even among expert observers. Using two different instruments for three-dimensional imaging, scanning laser tomography and raster tomography, we have studied the optic nerve head topography in a large and representative group of normal subjects (450 eyes) and in a large groups of patients with manifest glaucoma (296 eyes). Measurements of the optic nerve head topography varied widely among normal eyes, but were not influenced by age or gender. Multiple regression analysis demonstrated that up to 80% of this variability of neuroretinal rim area measurements could be explained by variations in disc size, cup depth and cup shape. The distribution of global optic disc parameters like the neuroretinal rim area or cup area showed a large overlap between normal and glaucomatous eyes, which seriously hampered the discrimination between the two groups. Single sector analysis varied widely between different regions of the optic disc. Only measurements of sectors located close to the vertical meridian of the disc (superior and inferior pole) yielded acceptable discrimination between normal and glaucomatous eyes, achieving combinations of specificity and sensitivity of over 91%. If sectors located in the temporal region of the disc were included in the analysis, the discrimination deteriorated significantly. The best single sector located close to the inferior pole of the disc achieved better separation between normal and glaucomatous eyes than any global parameter. Although scanning laser and raster tomography apply completely different imaging principles, both instruments gave very similar results. This implies that results of analysis with one method may have more general validity.
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