Night vision in the elderly: consequences for seeing through a "blue filtering" intraocular lens

Abstract

Relative scotopic spectral sensitivity depends only on the rhodopsin photopigment and ocular media absorption spectra. Rhodopsin is well characterised so the relative scotopic spectral sensitivity function can be calculated for intraocular lenses (IOLs) of known spectral density. In a recent perspective, Mainster and Sparrow concluded that an IOL with short wave absorbing chromophores would provide more retinal protection than conventional IOLs, but the practical consequences for scotopic vision are unclear. This paper uses published experiments to examine the implications for scotopic vision of the IOLs analysed by Mainster and Sparrow. A 14.6% reduction in scotopic sensitivity is expected for a SN60AT (AcrySof Natural) compared to a SA60AT (Conventional AcrySof) IOL under broadband illumination (equal quantum spectrum). This effect (0.07 log unit) is visually insignificant in relation to the approximately 4.0 log unit range of scotopic sensitivity. More importantly, it is expected that scotopic contrast sensitivity would be reduced by only approximately 0.01 log unit. It is thus improbable that a difference in scotopic vision between observers with the Natural and Conventional IOLs could be reliably detected using broadband stimuli.

Figure 1

Calculated log scotopic sensitivity plotted in terms of quanta delivered to the cornea as a function of wavelength. Calculations are based on a rhodopsin nomogram and 20 D IOLs. Open symbols, Conventional (Alcon SA60AT) and solid symbols, Natural (Alcon SN60AT).

Figure 2

Scotopic sensitivity plotted as a function of age. Symbols represent thresholds for test lights of 0.04 degree diameter, 10 ms flashes, 520 nm presented at 6 degrees nasal along the horizontal meridian. The solid line represents the linear regression fitted to these points. The broken line represents the linear regression obtained when the data are corrected for ocular media density and sensitivity is specified at the retina. The dotted line shows the loss in sensitivity expected from an equal quantum spectrum filtered by an AcrySof Natural IOL.

Figure 3

Upper panel shows mean scotopic contrast sensitivity functions for observers of three different age groups. The squares, circles, and triangles represent average sensitivities for subjects aged 20–40, 41–60, and 61–88 years, respectively. Error bars denote 1 SEM. The inset to the right shows a Gabor patch (sine wave with a Gaussian taper). Lower panel shows the high spatial frequency cut off (cpd) or resolution limit (Snellen equivalent) under scotopic conditions plotted as a function of age. (From Schefrin et al.)

Figure 4

Mean log contrast sensitivity (1.2 cpd) for older observers (mean age, 75 years) plotted as a function of log retinal illuminance. Error bars represent 1 SEM. (Data from Schefrin et al.)