Merging PRK and Collagen Crosslinking: An Analysis of Literature and a Guide to Prevalent Protocols.

PURPOSE: The purpose of this review was to summarize the different surgical approaches combining photorefractive keratectomy (PRK) and corneal crosslinking (CXL), present each protocol template in a simple format, and provide an overview of the primary outcomes and adverse events. METHODS: A literature review was conducted as outlined by the Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines. Eight different databases were searched. Papers were included if PRK was immediately followed by CXL. RESULTS: Thirty-seven papers met the inclusion criteria of a total yield of 823. The latest research into simultaneous PRK and CXL has been shown to not only stabilize the cornea and prevent keratoconus progression but also improve the visual acuity of the patient. Improvements in uncorrected distance visual acuity and (spectacle) corrected distance visual acuity were found to be significant when considering all protocols. There were also significant reductions in K1, K2, mean K, Kmax, sphere, cylinder, and spherical equivalent. Random-effects analysis confirmed these trends. Corrected distance visual acuity was found to improve by an average of 0.18 ± 1.49 logMAR (Cohen's D [CD] 0.12; P <0.02). There was also a significant reduction of 2.57 ± 0.45 D (CD 5.74; P <0.001) in Kmax. Cylinder and spherical equivalent were also reduced by 1.36 ± 0.26 D (CD 5.25; P <0.001) and 2.61 ± 0.38 D (CD 6.73; P <0.001), respectively. CONCLUSIONS: Combining the 2 procedures appears to be of net benefit, showing stabilization and improvement of ectatic disease, while also providing modest gains in visual acuity. Since customized PRK and CXL approaches appear superior, a combination of these would likely be best for patients.

Influence of very-long-chain polyunsaturated fatty acids on membrane structure and dynamics.

The unique attributes of very-long-chain polyunsaturated fatty acids (VLC-PUFAs), their long carbon chains (n > 24) and high degree of unsaturation, impart unique chemical and physical properties to this class of fatty acids. The changes imparted by VLC-PUFA 32:6 n-3 on lipid packing and the compression moduli of model membranes were evaluated from π-A isotherms of VLC-PUFA in 1,2-distearoyl-sn-3-glycero-phosphocholine (DSPC) lipid monolayers. To compare the attractive or repulsive forces between VLC-PUFA and DSPC lipid monolayers, the measured mean molecular areas (MMAs) were compared with the calculated MMAs of an ideal mixture of VLC-PUFA and DSPC. The presence of 0.1, 1, and 10 mol % VLC-PUFA shifted the π-A isotherm to higher MMAs of the lipids comprising the membrane and the observed positive deviations from ideal behavior of the mixed VLC-PUFA:DSPC monolayers correspond to repulsive forces between VLC-PUFAs and DSPC. The MMA of the VLC-PUFA component was estimated using the measured MMAs of DSPC of 47.1 ± 0.7 Å2/molecule, to be 15,000, 1100, and 91 Å2/molecule at 0.1, 1, and 10 mol % VLC-PUFA:DSPC mixtures, respectively. The large MMAs of VLC-PUFA suggest that the docosahexaenoic acid tail reinserts into the membrane and adopts a nonlinear structure in the membrane, which is most pronounced at 0.1 mol % VLC-PUFA. The presence of 0.1 mol % VLC-PUFA:DSPC also significantly increased the compression modulus of the membrane by 28 mN/m compared with a pure DSPC membrane. The influence of VLC-PUFA on lipid "flip-flop" was investigated by sum-frequency vibrational spectroscopy. The incorporation of 0.1 mol % VLC-PUFA increased the DSPC flip-flop rate fourfold. The fact that VLC-PUFA promotes lipid translocation is noteworthy as retinal membranes require a high influx of retinoids which may be facilitated by lipid flip-flop.

HDL is the primary transporter for carotenoids from liver to retinal pigment epithelium in transgenic ApoA-I-/-/Bco2-/- mice.

Supplementation with antioxidant carotenoids is a therapeutic strategy to protect against age-related macular degeneration (AMD); however, the transport mechanism of carotenoids from the liver to the retina is still not fully understood. Here, we investigate if HDL serves as the primary transporter for the macular carotenoids. ApoA-I, the key apolipoprotein of HDL, was genetically deleted from BCO2 knockout (Bco2-/-) mice, a macular pigment mouse model capable of accumulating carotenoids in the retina. We then conducted a feeding experiment with a mixed carotenoid chow (lutein:zeaxanthin:ß-carotene = 1:1:1) for one month. HPLC data demonstrated that the total carotenoids were increased in the livers but decreased in the serum, retinal pigment epithelium (RPE)/choroids, and retinas of ApoA-I-/-/Bco2-/- mice compared to Bco2-/- mice. In detail, ApoA-I deficiency caused a significant increase of ß-carotene but not lutein and zeaxanthin in the liver, decreased all three carotenoids in the serum, blocked the majority of zeaxanthin and ß-carotene transport to the RPE/choroid, and dramatically reduced ß-carotene and zeaxanthin but not lutein in the retina. Furthermore, surface plasmon resonance spectroscopy (SPR) data showed that the binding affinity between ApoA-I and ß-carotene â‰« zeaxanthin > lutein. Our results show that carotenoids are transported from the liver to the eye mainly by HDL, and ApoA-I may be involved in the selective delivery of macular carotenoids to the RPE.

Retinal bioavailability and functional effects of a synthetic very-long-chain polyunsaturated fatty acid in mice.

Rare, nondietary very-long-chain polyunsaturated fatty acids (VLC-PUFAs) are uniquely found in the retina and a few other vertebrate tissues. These special fatty acids play a clinically significant role in retinal degeneration and development, but their physiological and interventional research has been hampered because pure VLC-PUFAs are scarce. We hypothesize that if Stargardt-3 or age-related macular degeneration patients were to consume an adequate amount of VLC-PUFAs that could be directly used in the retina, it may be possible to bypass the steps of lipid elongation mediated by the retina's ELOVL4 enzyme and to delay or prevent degeneration. We report the synthesis of a VLC-PUFA (32:6 n-3) in sufficient quantity to study its bioavailability and functional benefits in the mouse retina. We acutely and chronically gavage fed wild-type mice and Elovl4 rod-cone conditional knockout mice this synthetic VLC-PUFA to understand its bioavailability and its role in visual function. VLC-PUFA-fed wild-type and Elovl4 conditional knockout mice show a significant increase in retinal VLC-PUFA levels in comparison to controls. The VLC-PUFA-fed mice also had improvement in the animals' visual acuity and electroretinography measurements. Further studies with synthetic VLC-PUFAs will continue to expand our understanding of the physiological roles of these unique retinal lipids, particularly with respect to their potential utility for the treatment and prevention of retinal degenerative diseases.

Imaging lutein and zeaxanthin in the human retina with confocal resonance Raman microscopy.

Lutein and zeaxanthin are xanthophyll carotenoids that are highly concentrated in the human macula, where they protect the eye from oxidative damage and improve visual performance. Distinguishing lutein from zeaxanthin in images of the human retina in vivo or in donor eye tissues has been challenging because no available technology has been able to reliably differentiate between these two carotenoids, which differ only in the position of one C = C bond. Here, we report the differential distributions of lutein and zeaxanthin in human donor retinas mapped with confocal resonance Raman microscopy. Zeaxanthin is highly concentrated in the fovea, extending from the inner to the outer limiting membranes, with especially high concentrations in the outer plexiform layer, while lutein is much more diffuse at relatively lower concentration. Our results imply that zeaxanthin may play a more important role than lutein in human macular health and disease.

Supplementation with macular carotenoids improves visual performance of transgenic mice.

Carotenoid supplementation can improve human visual performance, but there is still no validated rodent model to test their effects on visual function in laboratory animals. We recently showed that mice deficient in ß-carotene oxygenase 2 (BCO2) and/or ß-carotene oxygenase 1 (BCO1) enzymes can accumulate carotenoids in their retinas, allowing us to investigate the effects of carotenoids on the visual performance of mice. Using OptoMotry, a device to measure visual function in rodents, we examined the effect of zeaxanthin, lutein, and ß-carotene on visual performance of various BCO knockout mice. We then transgenically expressed the human zeaxanthin-binding protein GSTP1 (hGSTP1) in the rods of bco2-/- mice to examine if delivering more zeaxanthin to retina will improve their visual function further. The visual performance of bco2-/- mice fed with zeaxanthin or lutein was significantly improved relative to control mice fed with placebo beadlets. ß-Carotene had no significant effect in bco2-/- mice but modestly improved cone visual function of bco1-/- mice. Expression of hGSTP1 in the rods of bco2-/-mice resulted in a 40% increase of retinal zeaxanthin and further improvement of visual performance. This work demonstrates that these "macular pigment mice" may serve as animal models to study carotenoid function in the retina.