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J. J. Rozema, M.-J. Tassignon; Project Gullstrand - Defining Emmetropia and Ametropia as a Function of Ocular Biometry. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1713.
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© ARVO (1962-2015); The Authors (2016-present)
To demonstrate that the concept of emmetropia hides a wide range of ocular biometric values, to delineate the range in which emmetropia is physiologically achievable and to define malignant myopia as a function of biometry.
In the period from March 2007 until March 2008 the ocular biometry and spherical equivalent (SE) refraction were determined for a set of 599 healthy eyes. These data were compared to the Gullstrand eye model, which was used as a set of reference values.
The SE refraction in our population ranged between -9 D and +5 D, the axial length L between 20 mm and 29 mm and the keratometry K between 39 D and 47 D. SE refraction was strongly correlated with axial length (R² = 0.678), but not with the keratometry K (R² = 0.046). Emmetropia between ±0.5D was found to occur in eyes with axial lengths L between 21.5 mm and 26 mm, while myopia of -6 ± 0.5 D was found to occur in axial lengths between 23.7 mm and 26.8 mm.In emmetropes it was seen that the balance between keratometry K and axial length L can be described by the following equation: K = -1.982•L + 89.467 (R² = 0.654). Similar models can be defined for different levels of ametropia, where it is seen that the directional coefficient for each refraction value remains around a value of -1.749 ± 0.261 D/mm.Only 2.2% of the emmetropic eyes were found to have a biometry similar to the Gullstrand eye model.
These results show that the ocular refractive balance between the system of cornea and crystalline lens on the one side and the axial length on the other side can be achieved in a wide range of biometric values. This is also true for malignant myopia, which is usually considered as eyes with a refraction higher than -6D. Since was observed that this refraction in some cases may correspond with normal axial lengths (23.7 - 24 mm), refraction alone does not appear to be an adequate discriminator for malignant myopia. As emmetropia does not occur in eyes longer than 26mm, we propose to use this value as a threshold to define malignant myopia.
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