Comparison of measurements of neuroretinal rim area between confocal laser scanning tomography and planimetry of photographs

Background-To compare neuroretinal rim area measurements by confocal scanning laser tomography and planimetric evaluation of optic disc photographs. Methods-For 221 patients with primary and secondary open angle glaucoma, 72 subjects with ocular hypertension, and 139 normal subjects, the optic disc was morphometrically analysed by the confocal scanning laser tomograph HRT (Heidelberg retina tomograph) and by planimetric evaluation of stereo colour optic disc photographs. Results-Absolute rim area and rim to disc area were significantly (p<0.0001) larger with the HRT than with planimetric evaluation of photographs. Differences between the two methods were significantly (p<0.01) larger in normal eyes with small cupping than in normal eyes with large cupping, and differences were significantly (p<0.01) larger in glaucomatous eyes with marked nerve damage than in glaucomatous eyes with moderate nerve damage. Coefficients of correlations between rim measurements of both methods were R-2=0.60 for rim to disc area and R-2=0.33 for absolute rim area. Planimetric measurements of rim area correlated significantly (p<0.05) better than HRT determinations of rim area with mean visual field defect and retinal nerve fibre layer visibility. Conclusions-Measurements of absolute rim area and rim to disc area are significantly larger with the HRT compared with planimetry of disc photographs. Differences between both methods depend on disc area, cup size and glaucoma stage. The reason may be that the HRT measures the retinal vessel trunk as part of the neuroretinal rim. The differences between both methods, which should be taken into account if disc measurements performed by both methods are compared with each other, may not influence the main advantage of the HRT-that is, morphological follow up examination of patients with glaucoma.