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Inside IOVS
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February 2002 |
Techniques and Technologies: Some New, Some Updated
FDT Perimetry
Turpin et al. (p. 322) have used computer simulation to develop two novel threshold estimation strategies for Frequency Doubling Technology (FDT) perimetry. One of the procedures is based on a rapid, efficient binary search (REBS) technique while the other uses Bayesian statistics and a maximum likelihood function (Zippy Estimation of Sequential Testing, or ZEST). Both REBS and ZEST were able to reduce testing time by 50% while maintaining the accuracy and reproducibility of the procedure used by the current commercial version of FDT. These new procedures should enhance the clinical utility and patient satisfaction for FDT perimetry.
Assessing Redness
The visual grading of redness (for example, on a scale of 1 to 20) is a common step in clinical examinations. Fieguth and Simpson (p. 340) had 72 clinicians grade the appearance of 30 images of redness on a 100-point sliding scale. The clinical judgements of the redness images were highly variable: the average grade range for each image was about 55, more than half the extent of the entire scale! The new median clinical grade was chosen as the most reliable measure of "truth," and was correlated against a Bayesian computer image-analysis estimate based on redness and edge features. The resulting accuracy of the computer estimates exceeded all but one of the 72 individual clinicians. This work opens the opportunity for other types of clinical analysis (other than redness) and for clinical telemedicine.
Measuring Corneal O2
A new technique for the direct non-invasive estimation of corneal oxygen consumption in humans is described. The technique relies on the ability to measure tear oxygen tension in the tears using an oxygen sensitive phosphorescent probe while wearing a hydrogel contact lens of known oxygen transmissibility. Bonanno et al. (p. 371) found that corneal oxygen consumption in young healthy subjects could be measured under conditions of high or low (i.e., hypoxia) oxygen tension and that oxygen consumption decreased with decreasing tear oxygen tension. This technique has applicability to studying: oxygen delivery through contact lenses, role of oxygen consumption in contact lens related complications, effects of disease, surgery and topical drugs on corneal oxygen consumption.
Scanning Laser Polarimetry
Scanning laser polarimetry (SLP), an assessment technology for the diagnosis and management of glaucoma, detects the birefringence of the retinal nerve fiber layer (RNFL). SLP has been used for several years without a detailed understanding of the measurement. Knighton et al. (p. 383) have now produced an analytical model of SLP that incorporates corneal birefringence, retinal birefringence, and the polarization optics of the instrument. The model explains 80-90% of the variance in SLP images of the macula obtained with different axes of corneal birefringence and provides a quantitative framework within which to interpret SLP images of the RNFL.
FDT Perimetry
Frequency doubling technology (FDT) perimetry uses coarse grating targets that flicker rapidly. However, it is unclear what mechanisms these targets isolate, and what influence detection criterion (simple detection versus pattern detection) has. Anderson and Johnson (p. 398) determined adaptation and spatio-temporal tuning functions using FDT targets, spatially uniform flicker, and non-flickering gratings, and found that FDT targets isolate similar mechanisms to uniform rapid flicker, both for simple detection and pattern detection criteria. This study suggests that simple detection strategies may be successfully used in FDT perimetry.
Macular Translocation and ERG
In macular translocation (MT) surgery for subfoveal neovascularization, the fovea is moved from the diseased retinal pigment epithelium (RPE) onto healthier RPE. This surgery, as recorded by Terasaki et al. (p. 452), has the potential of improving central visual acuity; however, the 360° retinotomy and temporary total retinal detachment necessary for the translocation has the potential of affecting retinal function over the entire retina. Full-field ERGs, recorded before and after surgery, demonstrated a functional alteration in both the rod and cone components of the ERGs after the MT surgery.
OCT and Retinal Thickness
Optical coherence tomography (OCT) is becoming an increasingly popular tool for the diagnosis and monitoring of retinal conditions. Muscat et al. (p. 490) have shown that measurements made from images of a test object are accurate and precise even in cases where the contrast at interfaces is deliberately degraded. Retinal thickness measurements from a control group of normal subjects were shown to be reproducible and repeatable according to the standards set by the British Standards Institution. These results indicate that OCT is a promising tool for quantitative monitoring of retinal changes.
Intraocular Drug Delivery
The rapid clearance of most drugs after intraocular injection makes injectable intraocular drug delivery impractical. Cheng et al. (p. 515) have developed a new intraocular drug delivery system using a crystalline lipid prodrug of ganciclovir and have shown that a single injection yields antiviral action for more than 20 weeks. This crystalline lipid prodrug was engineered to be slow releasing in the vitreous. It may be a prototype for other compounds to be modified and delivered in a similar way.
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