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M. R. Stevenson, K. A. Muldrew, R. E. Hogg, U. Chakravarthy; When the Good Become Bad Do the Bad Become Good?. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4466.
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To fit a longitudinal model with objective of investigating correlation over time between better and worse eyes in individuals presenting at a retinal clinic for AMD.
702 patients were entered into a longitudinal study investigating trends in individual visual acuity of better and worse eyes (as defined at entry).234 patients completed only one visit. A minimum of 2 visits were required for the longitudinal model. Consequently 468 patients were eligible for longitudinal analysis.A general linear model was fitted, which involved entering patient as a categorical blocking variable and number of months in study and better eye distance visual acuity (logMAR scale) as continuously distributed covariates. The outcome variable was distance visual acuity (logMAR scale) in the worse eye. In order to provide the worse eye acuity an adequate opportunity to deteriorate (as well as an opportunity to improve) we restricted the investigation to patients with logMAR equal to or better than 1.3 in the worse eye. This reduced the number of eligible patients to 310.
Baseline visual acuity characteristics of better and worse eyes are shown in Table 1.Better eyes were found to deteriorate at a rate of 1 line per seven months. Interestingly, worse eyes were found to deteriorate at the same rate when better eyes did not change. If better eyes deteriorated more, worse eyes were found to deteriorate less, but this reduction in deterioration was not statistically significant.
We suspect competing issues here. Eyes often deteriorate because of common based factors (smoking, lifestyle, co-morbidities etc). This would lead to an expectation of some agreement in the extent of visual acuity deterioration in the two eyes. That we did not see this suggests that the jury is still out on the issue of one eye compensating for the other. This has been little aired in scientific journals and is highlighted in the hope that it might lead to further analyses of some of the larger longitudinal studies of visual acuity.
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