An Objective Scatter Index Based on Double-Pass Retinal Images of a Point Source to Classify Cataracts

PURPOSE: To propose a new objective scatter index (OSI) based in the analysis of double-pass images of a point source to rank and classify cataract patients. This classification scheme is compared with a current subjective system. METHODS: We selected a population including a group of normal young eyes as control and patients diagnosed with cataract (grades NO2, NO3 and NO4) according to the Lens Opacities Classification System (LOCS III). For each eye, we recorded double-pass retinal images of a point source. In each patient, we determined an objective scatter index (OSI) as the ratio of the intensity at an eccentric location in the image and the central part. This index provides information on the relevant forward scatter affecting vision. Since the double-pass retinal images are affected by both ocular aberrations and intraocular scattering, an analysis was performed to show the ranges of contributions of aberrations to the OSI. RESULTS: We used the OSI values to classify each eye according to the degree of scatter. The young normal eyes of the control group had OSI values below 1, while the OSI for subjects in LOCS grade II were around 1 to 2. The use of the objective index showed some of the weakness of subjective classification schemes. In particular, several subjects initially classified independently as grade NO2 or NO3 had similar OSI values, and in some cases even higher than subjects classified as grade NO4. A new classification scheme based in OSI is proposed. CONCLUSIONS: We introduced an objective index based in the analysis of double-pass retinal images to classify cataract patients. The method is robust and fully based in objective measurements; i.e., not depending on subjective decisions. This procedure could be used in combination with standard current methods to improve cataract patient surgery scheduling

One-year follow-up of changes in refraction and aberrations induced by corneal incision.

PurposeTo evaluate the surgically induced changes in refraction (sphere and astigmatism) and higher order aberrations by corneal incision for one year.SettingUniversity Hospital "Virgen de la Arrixaca", Murcia, Spain.DesignRetrospective interventional case series.MethodsCorneal power, astigmatism and higher order aberrations (HOA) were calculated from corneal topography measured in 27 eyes prior to surgery and at 2 weeks, 1, 2, 3 and 6 months and 1 year following cataract surgery with 3.2-mm corneal incision. At every stage, optical changes were calculated as the difference between pre- and post-surgery data (in each follow-up) using the formulas of obliquely crossed cylinders for the refraction and Zernikes coefficients for HOA.ResultsAt 2 weeks after surgery the mean corneal values of induced sphere, cylinder and the root mean square (RMS) of HOA were +0.54±0.27 D, -0.77±0.32 D and 0.15 microns respectively. These parameters decreased significantly (p-values between 0 and 0.01) at 3 months to +0.33±0.27 D sphere, -0.50±0.24 D cylinder and 0.10±0.05 microns HOA and were stable at the next follow-ups. Induced spherical equivalent was around zero at all visits. The changes in HOA were mainly due to trefoil aberration.ConclusionsLinear corneal incisions do not change the spherical power but can induce significant values of astigmatism and trefoil aberration in the cornea. However, these changes revert fully or partially to preoperative values by the third month after surgery and remain stable with time

Example of DP and slit-laps images in four eyes as example of the four groups.

<p>The actual OSI value in each eye is noted.</p

Example of comparison of actual and synthetic DP images.

<p>The 1D section shows the range where scatter is significant and used to calculate the OSI parameter.</p

Example of the selection of the zone in the double-pass image used to define the objective scatter index (OSI).

<p>We compute the relation between the intensity in the peak of the image, the central area within 1 minute, with that in a ring comprised between 12 and 20 minutes. Two DP images with different levels of scatter (A less scatter, B more scatter) are represented to illustrate the procedure.</p

Schematic representation of the double-pass method.

<p>See text for further details.</p

Schematic representation of the procedure to compute the synthetic DP image from wavefront aberration data.

<p>We computed the PSF for a 2-mm (first-pass) and 4 mm (second-pass) pupil diameter. Convolution of both PSFs renders the synthetic DP image.</p

Subjects classified according to obtained OSI values.

<p>Increasing grey scale represents the gradation proposed by the quantification of cataracts, according to this objective parameter.</p

Estimated value of OSI as a function of induced defocus.

<p>Uncorrected refractive errors over 1 diopter induce artifacts ion the determination of OSI over the value of 1 (standard value in normal eyes). This indicates that refractive errors need to be well corrected during the DP data collection to provide accurate values of OSI. The y-axis is kept scaled to16 as in the figures where OSI is represented. This will permit a direct comparison on the impact of uncorrected defocus on OSI.</p

Example of DP images for the different range following the OSI classification scheme.

<p>Double-pass (DP) images from four eyes and corresponding OSI values. Below, proposed classification ranges of eyes according to the measured OSI values.</p