SEP 2013

EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.

Issue link: https://digital.eyeworld.org/i/176967

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Page 20 of 98

18 EW NEWS & OPINION September 2013 Tools & techniques Why objective measurement of visual quality in surgical planning and decision making by Jay S. Pepose, MD, PhD T he clinical assessment of patients with symptomatic visual disturbances has become slightly more challenging as more and more individuals present with a history of corneal refractive surgery, dry eyes, and borderline cataracts. Although wavefront analysis and clinical examination can help pinpoint the cause for the optical symptoms, newer technology is helping give additional information and more "pieces of the puzzle" that will ultimately aid in recommending the best course of action in these patients. In this month's column, Jay Pepose, MD, PhD, reviews his experience with the Visiometrics HD Analyzer. This tool has aided him in the sometimes challenging art of decision making for the kerato- and lenticular refractive patient. As we see more of these patients in the future, and remove lenses at an earlier and earlier point in their cataractous development, we'll need all the help we can get to assess and treat these patients appropriately. Richard Hoffman, MD, Tools & techniques editor D ecision making and planning for cataract and refractive surgeries calls for a thorough preoperative assessment of retinal image quality. Until recently, obtaining a comprehensive, objective assessment of baseline and interblink optical quality was problematic in the clinical setting, particularly when attempting to gauge scattered light. But thanks to the recent developments, we now have the power to quickly and objectively measure the combined impact of optical scatter and higher-order aberrations on retinal image quality using a single device. The need to preoperatively assess retinal image quality in surgical planning and decision making With the convergence of lens and corneal-based refractive surgery, patients' expectations have increased markedly. Our case is a patient who had LASIK at age 40, and then presented at age 57 with complaints of decreased vision requesting a LASIK enhancement. The patient's complaints were mostly related to decreased vision at dusk and nighttime, increasing difficulty in night driving, and some fluctuation in vision throughout the day at work. Examination revealed well-healed LASIK flaps, trace nuclear sclerosis and uncorrected vision of 20/25 refracting to 20/20 with +0.5 D sphere. Slit lamp examination revealed anterior segment findings based on the back scatter of light. In contrast, the patient's complaints related predominantly to forward scatter of light (i.e., light moving in the direction of the retina), and there can be some discordance between these assessments. The scatter of light can be caused by an unstable tear film, from the LASIK flap interface and from the crystalline lens. Changes in scatter over time are generally caused by an unstable tear film, and it would be beneficial and more physiological to assess this temporal fluctuation in scatter without perturbing the tear film with fluorescein or other drops commonly employed in tear break-up time measurement. An objective measure of retinal image quality as well as its stability over time, along with comparison to age-matched normative values, would be an important tool in the ophthalmologist's armamentarium for surgical decision making, as illustrated by this case. A LASIK enhancement in someone with considerable intraocular scatter may not produce the desired results or alleviation of symptoms, which would likely worsen with further progression of lenticular opacity and increased aberrations and scatter. On the other hand, a patient with low aberrations and low optical scatter might do quite well with a laser vision enhancement for years to come. The impact of optical scatter and higher-order aberrations in degrading retinal image quality Relying on patients' subjective complaints, high and low contrast vision, and biomicroscopy alone may not give a full picture of the factors that may conspire to degrade the retinal image quality. HartmanSchack aberrometry may overestimate retinal image quality, especially in eyes that are more highly aberrated or have considerable optical scatter. The accurate interpolation of the eye's wavefront and its decomposition into specific aberrations are limited by several underlying causes, including the density of the lenslet array. This often leads to artifactual dampening of highly aberrated wavefronts as well as total insensitivity to the effects of ocular scatter. While the wavefront aberrometer has proven to be an important clinical tool to measure and identify higher-order and lower-order aberrations and the eye's optical performance in monochromatic light, it is encumbered by these limitations and especially by the inability to directly measure light scatter.1 Since both optical scatter and wavefront aberrations increase with age, reliance on wavefront aberrometry alone to assess the impact of these factors on retinal image quality can be suboptimal and at times misleading.2,3 Benefits of the HD Analyzer Fortunately, technological developments were able to overcome these limitations. The HD Analyzer (Visiometrics, Terrassa, Spain) is the next generation of the Optical Quality Analysis System (OQAS). It assesses optical quality objectively, with excellent repeatability,4 including light scatter measurement, among other metrics. The device is objective in that it does not rely on the description of suboptimal vision by patients or their subjective assessment of stray light. Rather, it measures and records the retinal point spread from the double pass images and from this the ocular modulation transfer function (MTF) is determined. The MTF incorporates all of the relevant effects of diffraction, higher-order aberrations and scatter in degrading the retinal image quality. The double pass images are the total summation of the disturbances in light's path through the ocular media, whether it is due to tear film instability, corneal disease causing higher-order aberrations, cataract, posterior capsular opacity, or a multifocal pseudophakos. It also enables multiple other assessments of visual quality, including the Strehl ratio, MTF, MTF cutoff, normalized MTF values of optical quality at 100%, 20% and 9% contrast and an optical scatter equivalent of a tear break-up time—all with a single instrument. With such technology, we can better understand what the patient is experiencing and its root cause. You get a total composite analysis of the retinal point spread, which is representative of what is projected onto the patient's retina (without the retinal and neural processing) and provides a direct measurement of the optical quality. Devices such as wavefront aberrometers are based on the spacing of the centroids compared with a perfect grid, and by Fourier transformation derive the shape of the wave, which can be decomposed into specific Zernike polynomials— this is an interpolation of data, rather than a direct measurement of the quality of the retinal image. Form The technology is based on the double-pass technique. A point source of near infrared light is imaged and recorded after reflection off of the retina and double pass through the ocular media, after which the device measures the size and shape of the light spot and the intensity of the light at the periphery versus the center of the retinal point spread image. These measurements objectively produce data on visual quality and opti-

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