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| May 2002 | Inside IOVS | Volume 43/5 |
| New Techniques and Tests for Use in Vision Research |
Lentivirus--A Novel Tool for Promoter Analyses
In a novel series of experiments, Coleman et al. (p. 1335) utilize lentivirus, a vector currently used in gene therapy applications, to study the expression characteristics of the guanylate cyclase activating protein-1 (GCAP1) gene promoter in developing retina. Lentiviruses carrying promoter-reporter transgenes were injected into the neural tubes of developing chicken embryos and reporter gene expression was examined at different stages of retinal development. The results of these experiments provide information about the organization of important cis-acting elements within the GCAP1 promoter and show that lentiviral vectors represent a powerful new tool for studying gene expression in vivo.
Cre/loxP Application to RPE
Recent advances in mouse genetics have enabled tissue-specific gene targeting using the Cre/loxP system. Mori et al. (p. 1384) generated a retinal pigment epithelium (RPE)-specific Cre transgenic mouse line using the promoter of tyrosinase-related protein 1 (TRP1) gene. Site-specific mutagenesis of a given gene can be performed selectively in the RPE by crossing this line with another in which the gene is loxP-flanked. This system will provide a new, powerful tool to study gene functions in the RPE in vivo.
Vernier Acuity in Glaucoma
A number of studies have identified abnormalities in foveal visual function in glaucoma. McKendrick et al. (p. 1393) show that foveal vernier acuity thresholds are elevated in glaucoma, and that this represents a genuine reduction in vernier performance rather than being secondary to decreased contrast sensitivity. Glaucoma patients also demonstrated elevated vernier thresholds for targets designed to assess performance in the short wavelength sensitive and magnocellular pathways. Vernier acuity, or other similar hyperacuity tasks that assess spatial sampling, may be useful in the detection of early glaucomatous damage prior to its detection using traditional perimetric tests.
Determining Visual Field Progression
Historically, many different methods have been proposed for the apparently intractable problem of determining whether glaucomatous eyes are deteriorating or not. Pointwise Linear Regression is one such method, seeking to identify individual progressing points within a visual field. Gardiner and Crabb (p. 1400) examine the benefits of performing further tests to confirm progression before making a clinical decision; such methods are shown to significantly improve the process. A new method of using such confirmation tests is proposed, and shown to provide improved specificity. Eventually these methods could form part of a universally accepted method for determining visual field progression.
Depth and Volume Assessment in Corneal Ablation
The estimation of the volume of the corneal tissue ablated by a laser refractive procedure may be useful to determine the influence of ablated volumes on corneal stability and procedure outcome. Using a simplified mathematical model, Gatinel et al. (p. 1445) have derived formulae that allow the calculation and the estimation of the amount of tissue removed by excimer laser photoablation to correct spherical refractive errors. The volume of photoablated tissue is a function of the magnitude of the treatment and the optical zone diameter to the fourth power, both of which may represent risk factors for keratectasia.
Pediatric Macula Topography Assessment
The topography of the macular pigment (MP), which is mainly in the axons of the foveal cones, is a proxy for macular structure. A child-friendly procedure for assessment of MP topography has been developed by Bour et al. (p. 1450). The difference in fundus reflectance of an aligned pair of macular photographs, one taken with blue (480nm; strongly absorbed by MP) and green (540nm; little absorbed by MP) in an 8° X 8° area about the fovea shows an identifiable central peak and circular symmetry in normal pediatric subjects (median age 10.5 years). The authors envision application of this procedure to studies of normal macular maturation and of pediatric macular disorders. It is likely that the basic procedure can be refined with the advent of digital fundus imaging equipment.
Corrections in Scanning Laser Polarimetry
Corneal birefringence is a major source of retardation in polarized light reflected from the fundus. As reported in Garway-Heath et al. (p. 1465), a fixed compensator, designed to neutralize corneal birefringence, results in clinically significant cornea/compensator-derived artifact in peripapillary nerve fiber layer thickness measurements. Imaging the macula enables the estimation of the magnitude of the cornea/compensator-derived artifact. Subtraction of retardation measurements made in the macula from those made in the peripapillary retina improves the ability of the scanning laser polarimeter to distinguish between normal and glaucomatous eyes. Macula imaging may in future allow a variable corneal compensator to neutralize corneal birefringence on an individual eye basis.
Drug Delivery to the Posterior Segment
Drug delivery to treat diseases of the posterior segment of the eye, such as choroidal neovascularization and its complications, is hampered by poor intraocular penetration and rapid elimination of the drug from the eye. Einmahl et al. (p. 1533) investigated the feasibility and tolerance of suprachoroidal injections in the rabbit of a bioerodible and biocompatible polymer, poly(ortho ester) (POE), as a potential new sustained drug delivery system for the posterior segment. The results of the study show that the injection of POE into the suprachoroidal space of the rabbit is an easy, controllable and reproducible procedure. POE was well tolerated in the suprachoroidal space where it stayed for at least 3 weeks. POE could therefore be a promising carrier of antiangiogenic drugs for the choroid.
DigiScope Screening for Vision Loss
At the present, more than half of diabetic patients do not undergo the recommended early eye exam necessary to reduce the risk of vision loss. Experts agree that this could be improved by screening at a site frequently visited by diabetic patients to identify those who need to be referred to an ophthalmologist. As reported in Zeimer et al. (p. 1581), the DigiScope has been developed to fulfill the requirements for a practical and cost effective instrument to screen in the offices of primary care physicians. Wide spread screening with the DigiScope is likely to reduce, the risk of vision loss, in an estimated 4 million individuals in the USA alone, who currently do not undergo an annual eye exam.
OCT Monitoring in Diabetes
OCT was has been reported as a useful technique for quantitative measurement of retinal thickness in diabetic patients. This study by Sánchez-Tocino et al. (p. 1588) fully supports previous suggestions that early changes in retinal thickness can be detected with OCT in spite of normal slit lamp biomicroscopy. The results suggest that abnormal macular thickening may be suspected if the foveal thickness measures more than 180 mm on OCT, which may indicate a candidate for more frequent and detailed follow-up. These findings lead therefore to the consideration of OCT as a potentially most useful tool for monitoring progression in diabetic patients.
Modified AV Vectors for Proliferative Retinopathies
Gene therapy using viral vectors provides a means for sustained intraocular delivery of therapeutic proteins. Using luciferase and b-galactosidase reporter genes, Mori et al. (p. 1610) demonstrated that intraocular injection of E1-deleted and partially E3-deleted type 5 adenoviral vectors results in much greater expression in eyes with proliferative retinopathy than in normal eyes due to differential transduction of neovascularization or avascular epiretinal membranes. This preferential transduction of cells participating in the disease processes is a desirable quality for acute treatment of proliferative retinopathies, such as diabetic retinopathy or proliferative vitreoretinopathy.
Understanding Multifocal ERG
The multifocal electroretinogram (mfERG) allows the simultaneous recording of many focal retinal responses, making it an important clinical tool both for detecting and diagnosing diseases of the retina. To enhance the utility of the mfERG, Hood et al. (p. 1673) propose a model of the retinal contributions to the human mfERG. This model, based upon pharmacological dissection of the monkey's mfERG, proposes that the waveform of the human mfERG can be understood as a combination of overlapping ON- and OFF-bipolar cell contributions combined with smaller contributions from the inner retina and the photoreceptors.
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