"Many patients required cataract surgery for straylight even though visual acuity was relatively good," asserted Dr Tom van den Berg (Neuroscience Institute of the Royal Netherlands Academy of Arts and Sciences, Amsterdam, The Netherlands) when describing his recent presentation at the 2011 annual EVER congress in Crete, Greece.

In his presentation at EVER 2011, Dr van den Berg discussed the implications straylight domain has for the clinician as this can be a major factor to patient dissatisfaction. "Image formation in the eye is based on focusing, however, if this is not perfect a point is not projected onto the retina as a point but as a circle, called the 'blur circle," he explained. "Because the optical media of the eye are a bit turbid light scattering also occurs, like a mist. This causes part of the light to be scattered around on the retina, over much larger distances than the blur circle and can be most easily seen as light radiating from bright light sources against a dark background."

Straylight can be caused by multiple issues but two of the most common are cataract and ageing of the crystalline lens. Evaluating the importance straylight has over the clinical decisions of a physician, Dr van den Berg explained that it is firstofall necessary to quantify straylight.

Straylight logarithm

To comply with the nature of our visual system Dr van den Berg noted that a logarithmic expression of straylight would offer equal importance as logMAR for visual acuity, which would ease the comparison for clinical relevance.

Straylight can, therefore, be expressed as the logarithm of the straylight parameter, s, that can be directly related to the 'outer skirt' of the functional pointspread function (PSF): s = θ² PSF. "Expressed this way, the straylight value catches in one single figure the complete scatter of the eye," said Dr van den Berg.

A comparison could then be made between the importance of straylight and that of visual acuity. "Ophthalmologists are very well trained in using visual acuity as criterion for clinical decision-making. So, it would give them a good hold on the importance of straylight if they could compare it to an equivalent visual acuity problem," emphasized Dr van den Berg.

"We found that, when using for visual acuity the well established logMAR scale, straylight is about as important, using the log(s) scale," he continued. "Only, the log(s) scale is offset by approx 1.20. To give an example, if a clinician would find straylight log(s)=1.5, that would be equivalent to logMAR=0.3 or 6/12, and log(s)=1.2 would be equivalent to logMAR=0.0 or 6/6."

However, Dr van den Berg noted that visual acuity is a completely different problem for the patient. "If visual acuity gets worse, he can no longer read, if straylight gets worse, that has most often little consequence for reading. Straylight has consequence for other visual tasks like facial recognition, spatial orientation, driving," he said.