Stefan Schramm

Analysis of optical side-effects of fs-laser therapy in human presbyopic lens simulated with modified contact lenses

BackgroundIn a single-blinded study, optical side-effects of a potential femtosecond (fs)-laser therapy in presbyopic human lenses were tested. Simulation of this therapy was carried out by applying fs-laser patterns into standard contact lenses (CL).MethodsIn the first part of the study, the influence of the numerical aperture on optical side-effects was investigated by comparing a typical fs-LASIK configuration to a fs-presbyopia treatment (n = 11). The second part focused on a possible improvement of visual performance by comparing a regular grid pattern to a randomly chosen spacing of the laser spots (n = 16). Visual acuity was measured with ETDRS charts, contrast sensitivity with F.A.C.T. charts and mesopic vision with Mesotest II. Forward scattered light was measured with the C-Quant (both instruments: Oculus Optikgeräte GmbH, Germany). A questionnaire detected subjective quality of vision. Differences between laser-treated and untreated CL and among the modifications were analyzed.ResultsThe laser-treated and standard CL indicated no significant difference in visual acuity, contrast sensitivity and mesopic vision without glare. While wearing modified lenses with a regular grid, quality of vision decreased significantly by means of mesopic vision with glare and subjective straylight. These modifications also caused an impairment of subjective quality of vision. In contrast, there was no significant difference between the random pattern and standard CL.ConclusionThe increase of optical side-effects was reproducibly dependent on the geometry of the laser-structure. A randomized grid induced the least limitation. The study results are useful for planning possible laser-patterns in fs-laser therapy of the presbyopic lens.

Shack–Hartmann-based objective straylight assessment of the human eye in an increased scattering angle range

Abstract. Objective measurement of straylight in the human eye with a Shack–Hartmann (SH) wavefront aberrometer is limited in imaging angle. We propose a measurement principle and a point spread function (PSF) reconstruction algorithm to overcome this limitation. In our optical setup, a variable stop replaces the stop conventionally used to suppress reflections and scatter in SH aberrometers. We record images with 21 diameters of the stop. From each SH image, the average intensity of the pupil is computed and normalized. The intensities represent integral values of the PSF. We reconstruct the PSF, which is the derivative of the intensities with respect to the visual angle. A modified Stiles Holladay approximation is fitted to the reconstructed PSF, resulting in a straylight parameter. A proof-of-principle study was carried out on eight healthy young volunteers. Scatter filters were positioned in front of the volunteers’ eyes to simulate straylight. The straylight parameter was compared to the C-Quant measurements and the filter values. The PSF parameter shows strong correlation with the density of the filters and a linear relation to the C-Quant straylight parameter. Our measurement and reconstruction techniques allow for objective straylight analysis of visual angles up to 4 deg.

Effects of short term changes in the blood glucose level on the autofluorescence lifetime of the human retina in healthy volunteers

Purpose: Fluorescence lifetime imaging ophthalmoscopy (FLIO) provides in vivo metabolic mapping of the ocular fundus. Changes in FLIO have been found in e.g. diabetes patients. The influence of short term metabolic changes caused by blood glucose level changes on is unknown. Aim of this work is the detection of short-term changes in fundus autofluorescence lifetime during an oral glucose tolerance test. Methods: FLIO was performed in 10 healthy volunteers (29±4 years, fasting for 12h) using a scanning laser ophthalmoscope (30° fundus, 34μm resolution, excitation with 473nm diode laser with 70 ps pulses at 80 MHz repetition rate, detection in two spectral channels 500-560nm (ch1) and 560-720nm (ch2) using the timecorrelated single photon counting method). The blood glucose level (BGL) was measured by an Accu-Chek® Aviva self-monitoring device. Before and after a glucose drink (300ml solution, containing 75g of glucose (Accu-Chek® Dextrose O.G.T.), BGL and FLIO were measured every 15min. The FLIMX software package was applied to compute the average fluorescence lifetime τ on the inner ring of the ETDRS grid using a modified 3-exponential approach. Results: The results are given as mean ± standard deviation over all volunteers in ch1. Baseline measurement: BGL: 5.3±0.4 mmol/l, τ1: 49±6ps. A significant reduction (α=5%; Wilcoxon rank-sum test) in τ1 is detected after 15min (BGL: 8.4±1.1 mmol/l, τ1: 44±5ps) and after 90min (BGL: 6.3±1.4 mmol/l, τ1: 41±5ps). Results of ch2 show smaller reductions in the fluorescence lifetimes over time.

Objective straylight assessment of the human eye with a novel device

Forward scattered light from the anterior segment of the human eye can be measured by Shack-Hartmann (SH) wavefront aberrometers with limited visual angle. We propose a novel Point Spread Function (PSF) reconstruction algorithm based on SH measurements with a novel measurement devise to overcome these limitations. In our optical setup, we use a Digital Mirror Device as variable field stop, which is conventionally a pinhole suppressing scatter and reflections. Images with 21 different stop diameters were captured and from each image the average subaperture image intensity and the average intensity of the pupil were computed. The 21 intensities represent integral values of the PSF which is consequently reconstructed by derivation with respect to the visual angle. A generalized form of the Stiles-Holladay-approximation is fitted to the PSF resulting in a stray light parameter Log(IS). Additionaly the transmission loss of eye is computed. For the proof of principle, a study on 13 healthy young volunteers was carried out. Scatter filters were positioned in front of the volunteer’s eye during C-Quant and scatter measurements to generate straylight emulating scatter in the lens. The straylight parameter is compared to the C-Quant measurement parameter Log(ISC) and scatter density of the filters SDF with a partial correlation. Log(IS) shows significant correlation with the SDF and Log(ISC). The correlation is more prominent between Log(IS) combined with the transmission loss and the SDF and Log(ISC). Our novel measurement and reconstruction technique allow for objective stray light analysis of visual angles up to 4 degrees.