Purpose: To validate a novel compact optical instrument for the measurement of intraocular straylight in a clinical setting. The instrument, based on the principle of double-pass optical integration, was applied in a cohort of cataract patients.
Methods: The instrument (Sigma, Sinusmedii SL, Spain) projects onto the retina an extended source composed of a central disk (visual angle 3 degrees radius) and a concentric annulus (3-8 degrees) that are illuminated by high-brightness green LEDs (535nm). The central part of this source and the annulus are temporally modulated at different frequencies. Projection of this source onto the retinal fundus and analysis of the back-reflected signal in the Fourier domain allows the estimation of the straylight parameter (S) for an angle of 5.5 degrees (Ginis et al., Biomedical Optics Express 5 (9), 3036-3041). Optical straylight measurements were performed in 54 eyes of 32 cataract patients randomly recruited both prior and after pupil dilation. Four of the patients underwent 15 consecutive measurements to assess the variability of the measurement. Additional psychophysical measurement of straylight (c-Quant, Oculus, Germany), manifest refraction and cataract grading based on the LOCSIII system were also performed. As control, measurements of straylight were also performed in 4 eyes of 4 young subjects with no known ophthalmic pathology.
Results: The logarithm of the straylight parameter ranged from 0.90 to 2.21 in cataract patients and 0.79 to 1.20 in healthy eyes. The coefficient of variance for the optical measurements was 0.03. The distribution of the straylight parameter is well correlated with cataract grades and with the psychophysical measurements (Pearson’s r = 0.75). In subjects with small pupil diameters prior to dilation (<4mm) straylight measurements appeared elevated. Although the difference was not statistically significant (p=0.13), it showed that possible backscattering from the iris in eyes with small pupil diameters may be a source of measurement artefacts to be controlled.
Conclusions: The compact optical straylight meter (Sigma) measures the straylight parameter both in healthy and cataract eyes with high repeatability under standard clinical conditions. The optical values showed a good agreement with psychophysical estimates and clinical gradation of cataracts.