NOV 14, 2012
This experimental study illustrates a number of factors that determine the presence of negative dysphotopsia and explains some of the clinical observations of this enigmatic phenomenon. For years we have been struggling with the mystery of why some patients and some eyes have negative dysphotopsia. The authors have done a fabulous job of studying, renaming and helping us to predict, understand and treat this vexing problem.
Negative dysphotopsia has been clinically reported with posterior chamber IOLs over the past 12 years with very specific symptoms of a black, temporal crescent in the extreme periphery that is more accentuated by pupil constriction, reduced by pupil dilation, and believed to be related to square-edged optics and higher index of refraction materials. However, no optical ray-tracing studies have validated these observations or proposed explanations.
The authors used ray tracing to simulate negative dysphotopsia in a model pseudophakic eye. In the first simulation they used a Lambertian light source, with each point on the surface treated as a point source. In the second simulation, only the horizontal section was considered.
Image analysis from the first simulation showed two ring-shaped shadows in the nominal parameters. The inner shadow was located from a retinal visual field angle of 86.0 to 100.0 degrees (width 14.0 degrees), and the outer annular shadow was located from 105.9 to 123.3 degrees (width 17.4 degrees). Superimposing the inner annulus on the human visual field showed that the shadow would be apparent only temporally, where it is within the limits of the visual field and functional retina. The patient would perceive this as a temporal dark crescent-shaped partial shadow (penumbra).
The authors conclude that the primary optical factors required for negative dysphotopsia are a small pupil; distance behind the pupil of 0.06 mm or more and 1.23 mm or less with an acrylic IOL; presence of an IOL with a sharp-edged design; and functional nasal retina that extends anterior to the shadow. Secondary factors include optic material with a high index of refraction, angle α, and nasal location of the pupil relative to the eye's optical axis.