First Author: I.Van der Meulen THE NETHERLANDS
Co Author(s): E. Patryn C. Nieuwendaal R. Lapid Gortzak T. van den Berg
Purpose:
Intraocular lens (IOL) opacification can decrease quality of vision by diminishing visual acuity (VA) and increasing intraocular light scattering. Which effect predominates, is determined by the size of the disturbances. Large disturbances (> 100 µm) typically correspond to aberrations and disturb VA, while small particles (sizes comparable to the wavelength of visible light) cause increased straylight. These are separate effects, so subjective quality of vision can be diminished by increased straylight while VA remains good. Straylight denotes the visual effect of intraocular forward scattered light projected unto the patients retina. Clinically, straylight is assessed functionally with the Oculus C-Quant (Oculus GmbH, Wetzlar, Germany), based on the compensation comparison method. The isolated forward light scattering caused by the lens can be assessed optically in vitro with an optical set-up. The in vivo psychophysical straylight measurement is theoretically comparable to the in vitro measurement of scatter, but until now no clinical proof of this relation could be given. The present study reports on the unique opportunity to link the functional straylight measurement with the optical measurement of light scatter in a patient that had an opacified IOL explanted.
Setting:
Tertiary referral center for Corneal and External Diseases (department of Ophthalmology of the Academic Medical Center, Amsterdam, the Netherlands).
Methods:
Seven months after Descemet stripping endothelial keratoplasty for Fuchs endothelial dystrophy, a 70-year old woman presented with severe visual complaints due to an opacified IOL. Straylight measurements were performed with the C-Quant. Due to gravely decreased quality of vision, explantation of the opacified IOL was necessary, making it possible to isolate and quantify the amount of straylight caused by this IOL. In vitro measurements of the isolated forward light scattering caused by the explanted opacified IOL were performed with a validated optical set-up. In this set-up, the explanted IOL is placed in a holder filled with isotonic sodium chloride solution and illuminated by a halogen light source. A camera is moved in a horizontal plane around the sample to collect the light scattered in forward direction by the IOL, which in vivo would have fallen onto the retina. The scattered light is recorded for angles from 1.7 to 22 degrees.The straylight parameter s is calculated by dividing the amount of light registered by the camera at each angle by the total amount of light going through the IOL, apart from a calibration constant. Results are expressed as the logarithmic value of the straylight parameter s (log(s)).
Results:
VA had diminished from 20/40 to 20/50, while straylight measured with the C-quant with the intraocular opacified IOL was log(s) = 2.22 (s=166). A normal straylight value for a healthy, young eye is log(s) = 0.9 (s=8). Thus, the eye with the opacified IOL showed a twenty-fold increase in straylight compared to young, healthy eyes. One month after IOL explantation, VA had improved to 20/40 and straylight to log(s) = 1.57. The straylight value is 4x lower than pre-operatively and close to best normal values for age-matched phakic eyes. With the optical set-up, the straylight values of the explanted opacified IOL were found to be highest around 7°. This is also the angle used for in vivo straylight measurements with the Oculus C-Quant. At this angle, log(s) of the IOL was found to be 2.10, which corresponds well to the in vivo straylight measurement (see below). Around the 0° scatter angle, the in vitro straylight parameter dropped to lower values, corresponding to little effect on visual acuity.
Conclusions:
This is the first clinical comparison of the in vivo functional straylight measurement made by the C-Quant with an in vitro optical measurement of light scatter. Comparison of these measurements showed, that the amount of straylight caused by the explanted IOL was similar to the pre-operative in vivo straylight value. The pseudophakic eye with clear IOL had straylight value of log(s)=1.57 (s=37). With the intraocular opacified IOL, that same eye had an in vivo straylight value of log(s)=2.22 (s=166), so the increase in straylight parameter s was 166-37=129, making log(s)=log(129)=2.11. This is the same amount of straylight which the explanted opacified IOL caused in the in vitro set-up, proofing that both measurements are indeed clinically comparable, as was hitherto only shown theoretically.
Severe complaints of reduced visual function occur in patients with opacified IOLs, despite fairly good VA. VA is affected by the small-angle domain of the point spread function and straylight by the large-angle domain. Both domains affect quality of vision independently, so VA measurement alone is insufficient to appropriately assess visual quality, and straylight measurements have additional value. In our patient, VA was less affected by IOL opacification than straylight, because part of the light passed undisturbed through the uncovered part of the IOL to form a proper retinal projection. This projection was of decreased intensity, but sufficient for relatively good acuity. The design of the compensation comparison method used in the C-quant is such, that it cannot be biased by the patient, making it objective, free of learning effects, and repeatable. Measuring straylight is a useful independent indicator of visual function, which correlates better than VA to subjective complaints and the severity of IOL opacity as seen with the slit lamp. FINANCIAL DISCLOSURE?: No
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