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P. Hoenig, S.R. Bharadwaj, D. Simonian, K. Mau, S. Rastani, L. Breen, C.M. Schor; Variation of Vertical Fusional Vergence Amplitude with Horizontal Vergence . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3141.
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Clinicians often associate a deficiency in vertical fusional vergence amplitude (VFVA) with asthenopia. Prior studies have explored associations between horizontal and vertical vergence at different distances, and the VFVA has been shown to be larger at a ‘near’ convergence angle than at a ‘far’ convergence angle (Hara et al., 1998). In order to predict the VFVA at a given horizontal vergence angle, we measured a quantitative relationship between the horizontal and vertical vergence over a range of viewing distances.
The VFVA was measured in 10 young adults (age range: 24 – 30 years) at five different viewing distances (25 cm, 50 cm, 100 cm, 200 cm, and 300 cm). All subjects were visually normal, with normal horizontal and vertical phorias. Viewing distance was randomly varied across different sessions. At each viewing distance, subjects’ fused a pair of dichoptically presented red–green concentric circles that were projected on a computer monitor in an otherwise dark room. The horizontal vergence and size of the circles were kept constant. The vertical vergence stimulus was incremented in steps of 0.03° by vertically displacing the red–green concentric circles in opposite directions. The VFVA was defined as the maximum vertical vergence disparity that could be fused by the subject. Three measures of VFVA were obtained at each viewing distance and were averaged. Horizontal vergence stimulus at each viewing distance was calculated separately for each subject from interpupillary distance X (1 / viewing distance). Both horizontal vergence stimulus and vertical vergence were expressed in units of prism diopters (PD). VFVA was plotted as a function of horizontal vergence stimulus, and a linear equation was fit to the data.
In 9 of the 10 subjects, the VFVA increased linearly with the horizontal vergence angle. The combined linear regression equation obtained by averaging the data of 9 subjects was y = 0.09x + 1.88 with a correlation coefficient of 0.98. Linear regression equations fit separately to the data of each of the 9 subject’s yielded slopes ranging from 0.02 to 0.20 and y–intercepts ranging from 0.70 PD to 2.97 PD.
The VFVA increases linearly with the horizontal vergence angle in the range of viewing distances tested. These results confirm the findings of Hara et al (1998), and provide a predictive relationship between the VFVA and horizontal vergence angle or viewing distance in young adults.
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