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Optic Disc Measurements in Myopia with Optical Coherence Tomography and Confocal Scanning Laser Ophthalmoscopy

From the Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong, Hong Kong, Peoples Republic of China.

PURPOSE. To evaluate the relationships between optic disc measurements, obtained by an optical coherence tomograph and a confocal scanning laser ophthalmoscope, and myopia.

METHODS. One hundred thirty-three eyes from 133 healthy subjects with mean spherical equivalent –6.0 ± 4.2 D (range, –13.13 to +3.25 D) were analyzed. Optic disc measurements including disc area, rim area, cup area, cup-to-disc area, and vertical and horizontal ratios were obtained with an optical coherence tomograph (StratusOCT; Carl Zeiss Meditec Inc., Dublin, CA) and a confocal scanning laser ophthalmoscope (Heidelberg Retina Tomograph, HRT 3; Heidelberg Engineering, GmbH, Dossenheim, Germany). The modified axial length method derived from prior published work was used to correct the OCT measurements for ocular magnification. Bland-Altman plots were used to evaluate the agreement for each optic disc parameter. Associations between optic disc area and axial length/spherical equivalent were evaluated by linear regression analysis.

RESULTS. Disc area increased with the axial length/negative spherical equivalent in the HRT and the corrected OCT measurements although opposite directions of associations were found when the OCT measurements were not corrected for magnification. The difference of the corrected OCT and HRT disc area (corrected OCT disc area minus HRT disc area) was correlated with the axial length (r = 0.195, P = 0.025). When the ametropia was limited to –8.0 to +4.0 D, the correlations became insignificant in the HRT. Using the corrected OCT measurements, disc area, rim area, and cup area, cup-to-disc area, and cup-to-disc horizontal and vertical ratios were significantly larger than those measured by the HRT, with a span of 95% limits of agreement at 1.99, 1.33, and 1.86 mm² for the areas, 0.34, 0.53, and 0.58 for the ratios, respectively.

CONCLUSIONS. While optic disc area generally increased with the axial length and myopic refraction, the HRT measurements demonstrated that optic disc size was largely independent of axial length and refractive error between –8 and +4 D. OCT may overestimate optic disc size in myopic eyes and results in poor agreement between the two instruments.

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