Quantitative Morphologic and Functional Evaluation of the Optic Nerve Head in Chronic Open-Angle Glaucoma

Abstract

Glaucoma leads to morphologic changes of the optic nerve head and to functional defects. Morphologic changes in the three-dimensional surface structure of the optic nerve head at its entrance site into the globe can be examined by laser scanning tomography. The standard technique for evaluating functional defects in glaucoma is static computerized perimetry. We compared these two techniques to determine which is more sensitive for follow-up of glaucomatous damage of the optic nerve head. If decreased function is presumed to precede imminent cell death, visual field analysis should be the more sensitive method, as cell death results in absolute defects of the visual field. However, the neuronal networks do not necessarily function in this way. In the case of loss of individual elements in the neuronal network, the complex linkages, even at the retinal level, are able to maintain functions and compensate for loss of function, which means that visual field defects would not be prominent. If the damage increases with time and is accompanied by a progressive loss of ganglion cells, however, compensation is eventually no longer possible, and the functional defects then become measurable by visual field analysis. Thus, morphologic absolute changes may be more prominent than visual field defects in the early stages of glaucoma. To evaluate the quantitative relationship between morphometrically measurable defects of the optic nerve head and measurable functional defects, we first examined the visual field with static computerized perimetry and then evaluated the surface structure with a laser scanning tomograph in 90 patients with chronic open-angle glaucoma, 10 patients with ocular hypertension, and 10 patients without any eye disease. Based on the 95th percentile of the standardized rim/disk area ratio, we calculated the relative rim area loss and correlated this with the mean defect in visual field analysis. The scatterplot shows an exponential curve. In the early stages of glaucoma, visual field defects were less prominent than morphologic absolute changes; 40% of the neuroretinal rim area is lost by glaucomatous optic nerve damage before first defects in visual field analysis appear. In late stages of glaucoma, changes in perimetry are more prominent than those observed with biomorphometry. These results show that in the follow-up of patients with early stages of glaucomatous damage, special attention should be given to morphologic absolute changes. In patients with advanced glaucoma, progress of the damage should be observed by repeated functional, rather than morphologic, examinations. It is important to keep in mind, however, that the sensitivity of any method is dependent on technology. One reason why functional tests may not be as sentitive as morphologic examination in observing patients with early stages of glaucoma may simply be that functional tests are not yet sensitive enough to detect early damage.