Lucinda J. Camras

A novel Schlemm's Canal scaffold increases outflow facility in a human anterior segment perfusion model.

PURPOSE An intracanalicular scaffold (Hydrus microstent) designed to reduce intraocular pressure as a glaucoma treatment was tested in human anterior segments to determine changes in outflow facility (C). METHODS Human eyes with no history of ocular disease or surgeries were perfused within 49 hours of death. The anterior segments were isolated and connected to a perfusion system. Flow rates were measured at pressures of 10, 20, 30, and 40 mm Hg. The scaffold was inserted into Schlemms canal of the experimental eye, while a control eye underwent a sham procedure. Flow rate measurements were repeated at the four pressure levels. Individual C values were computed by dividing the flow rate by its corresponding pressure, and by averaging the four individual C measurements. The change in C between control and experimental eyes was assessed by the ratio of the baseline and second C measurement. In two eyes, the placement of the scaffold was evaluated histologically. RESULTS After scaffold implantation in the experimental eyes, the average C increased significantly from baseline (n = 9, P < 0.05). Ratios of C at all pressure levels, except for 10 mm Hg, were significantly higher in experimental eyes (n = 9) than control eyes (P < 0.05, n = 7). Histologically, the scaffold dilated Schlemms canal with no visible damage to the trabecular meshwork. CONCLUSIONS The Hydrus Microstent provided an effective way to increase outflow facility in human eyes ex vivo.

Differential Effects of Trabecular Meshwork Stiffness on Outflow Facility in Normal Human and Porcine Eyes

PURPOSE The study was designed to determine trabecular meshwork (TM) stiffness and its relationship to outflow facility (C) in perfused normal human and porcine eyes. METHODS Human and porcine eyes were perfused at pressures of 10, 20, 30, and 40 mm Hg to determine C and how outflow resistance (R = 1/C) varied with the pressure. Following perfusions, TM tissue segments were dissected and stretched uniaxially to determine the circumferential bulk Youngs modulus (E). The statistical significance of difference between different groups was evaluated using a two-tailed Students t-test or Mann-Whitney U test. RESULTS A larger E correlated with a higher C measured at 10 and 20 mm Hg (P < 0.05), and a similar trend was observed at 30 and 40 mm Hg in human eyes (n = 7). Additionally, a higher C correlated to a lower variance of R, and a stiffer TM correlated to a lower variance of R in human eyes (P < 0.05). For porcine TM, E was inversely correlated to a cross-sectional area (P < 0.003, n = 11), and its value (24.9 and 1.5 kPa; geometric mean and geometric SE) was lower than E of human TM (515 ± 136 kPa; mean ± SE) (P < 0.01). C and variance of R were not significantly different between the species. CONCLUSIONS A higher circumferential stiffness of the TM correlated with a higher outflow facility and less IOP elevation-induced variation in outflow resistance in normal human eyes, but not in porcine eyes. For future studies, these correlations need to be evaluated in glaucomatous eyes to better understand normal and abnormal TM functions.

Aqueous humor dynamics during the day and night in healthy mature volunteers.

OBJECTIVES To investigate the daytime vs nighttime differences in intraocular pressure (IOP), aqueous humor dynamics, central cornea thickness, and blood pressure among a cohort of healthy volunteers. METHODS Thirty healthy volunteers (mean [SD] age, 57.0 [8.6] years) were enrolled in the study. Individuals underwent 1 daytime visit and 1 nighttime visit for the measurement of aqueous humor dynamics. Measurements included IOP by pneumatonometry, aqueous flow by fluorophotometry, outflow facility by fluorophotometry and tonography, uveoscleral outflow by mathematical calculation, central cornea thickness by pachymetry, and blood pressure by sphygmomanometry. Results between visits were compared by appropriate t test. Dependence of the pneumatonometer probe results on position was tested in enucleated rabbit eyes at set pressures and probe positions. RESULTS Compared with daytime seated IOP, nighttime seated IOP was reduced by 16%, whereas nighttime supine IOP was increased by 17% (P < .001 for both). The IOP changes were independent of the pneumatonometer probe position. Central cornea thickness was increased at nighttime from a mean (SD) of 560 (37) μm to a mean (SD) of 574 (37) μm (P < .001). Compared with daytime aqueous flow, nighttime aqueous flow was reduced by 49% (P < .001). During the night, fluorophotometric outflow facility was reduced by 45% (P = .05), and tonographic outflow facility was reduced by 17% (P < .01). Uveoscleral outflow at night was decreased when calculated using tonographic outflow facility but not fluorophotometric outflow facility in the Goldmann equation. All other measurements were unchanged. CONCLUSIONS Significant changes in aqueous humor dynamics at night in healthy mature humans include reductions in aqueous flow, outflow facility, and possibly uveoscleral outflow. Nocturnal changes in IOP are independent of the pneumatonometer probe position and are dependent on an individuals posture during the measurement.

Duration of anesthesia affects intraocular pressure, but not outflow facility in mice.

Purpose: The study of aqueous humor dynamics (AHD) in mice is becoming more prevalent as more strains with elevated intraocular pressure (IOP) are developed. High IOP is usually associated with reduced outflow facility making this one of the more important AHD parameters to evaluate. Ocular measurements in mice require anesthesia that has profound effects on IOP but unknown effects on outflow facility. This study evaluates the effects of anesthesia duration and latanoprost treatment on outflow facility and IOP in BALB/c mice. Methods: IOPs were measured in conscious and anesthetized mice by tonometry. Outflow facility was evaluated in 15-min intervals at three pressure levels over two 45-min periods. Comparisons were made between latanoprost-treated eyes and untreated contralateral eyes. To determine the effect of anesthesia duration on IOP, a microneedle method was used to follow IOP for 120 min in separate mice. Results: IOP was 9.7 ± 0.3 mmHg (mean ± SEM) in conscious mice and 7.1 ± 0.02 within 10 min of anesthesia initiation (p < 0.01). IOP changed significantly between but not within assessment periods. IOP at 75 min was significantly (p = 0.004) reduced compared to IOP at 15 min after initial anesthesia. In control eyes, outflow facility did not change between the two 45-min assessment periods during the 120 min test (p = 0.80). In latanoprost-treated eyes, outflow facility increased compared with control eyes during both assessment periods (p = 0.03). A test of filters in series with known resistance found that the method was sensitive enough to detect a change in outflow facility of 0.001 μl/min/mmHg. Conclusions: Administration of ketamine/xylazine anesthesia for 120 min did not alter outflow facility or lessen the effect of latanoprost on outflow facility in mice as determined by a new analysis system. Accurate IOP measurements must be made within minutes of anesthesia administration but outflow facility measurements can be made with less haste.

Circumferential tensile stiffness of glaucomatous trabecular meshwork.

PURPOSE Our previous work indicated that a larger circumferential Youngs modulus (E) of trabecular meshwork (TM) correlated with a higher outflow facility (C) in normal human donor eyes. The current study investigated the influence of glaucomatous TM stiffness and cellularity on C. METHODS Eight left eyes from glaucomatous human donors were perfused within 48 hours post mortem. Values of C were determined at pressures of 10, 20, 30, and 40 mm Hg. The TM was then dissected and imaged with optical coherence tomography to determine its cross-sectional area. Uniaxial tensile stress was applied longitudinally to TM segments to determine stress-strain curves. E was calculated at zero strain, representing the circumferential stiffness of the TM at a relaxed state. Confocal images of DAPI-stained TM segments were used to determine cellularity after mechanical stretching. RESULTS C (μL/min/mm Hg) of glaucomatous eyes was 0.18 ± 0.02 (mean ± SE) at 10 mm Hg and decreased to 0.11 ± 0.02 when the pressure was increased to 40 mm Hg. C measured at 30 and 40 mm Hg correlated with TM cellularity. E was 12.5 MPa and 1.4 (geometric mean and SE) and did not statistically correlate with postmortem time, age, C, or cellularity. CONCLUSIONS Compared with data of normal eyes observed in a previous study, C in glaucomatous eyes was reduced significantly, and the amount of reduction increased with increasing the pressure. E of glaucomatous TM was approximately one-fifth that of normal TM. Prospective studies are needed to further investigate the influence of TM tensile stiffness on outflow regulation.

Mathematical Modeling of Outflow Facility Increase With Trabecular Meshwork Bypass and Schlemm Canal Dilation.

Purpose:To mathematically model the conventional aqueous humor outflow system with trabecular meshwork (TM) bypass and Schlemm canal (SC) dilation. Methods:The SC was modeled as a rectangular channel with the TM modeled as a permeable membrane. The collector channels (CCs) were modeled as fluid sinks distributed along the outer wall of SC. Two different implants were investigated in this study. The Hydrus Microstent (scaffold) was modeled with a TM bypass and a dilated region in SC that was 7 or 15 mm long and approximately 5-fold larger than the normal height of SC (h0). The iStent trabecular microbypass was modeled with a similar structure except that the dilated region in SC was 1 mm long and 25% larger than h0. Results:Creation of a TM bypass structure would increase the pressure in the surrounding regions inside the SC and make it close to the intraocular pressure. SC dilation would increase the pressure more uniformly in the dilated region. The pressure increase led to higher flow rates in SC and CCs, and subsequently increased outflow facility (C). If CCs were uniformly distributed, the increase in C was the smallest after implantation of 1 microbypass, compared with that after implantation of 2 microbypasses or 1 scaffold. If CCs were nonuniformly distributed, the magnitude of increase in C was sensitive to the location of implant, and the sensitivity was higher for the microbypass than the scaffold. Conclusion:The study showed that creation of TM bypass and SC dilation significantly increased outflow facility, and the amount of increase correlated with the length of dilated regions in SC.

Aqueous Flow Measured by Fluorophotometry in the Mouse

Purpose A fluorophotometer designed to measure aqueous flow in murine eyes was tested with artificial fluorescein chambers and in live mice with different anesthesia regimens, aqueous flow suppressants, and an anterior chamber cannulation method. Methods Two hours following topical fluorescein application, one group of CD-1 mice was anesthetized with ketamine/xylazine, 2,2,2-tribromoethanol, or ketamine alone. Cornea and anterior chamber fluorescein concentrations were measured periodically for 60 to 90 minutes by fluorophotometric scans to calculate aqueous flow. Later, a subgroup of mice underwent aqueous flow measurement by anterior chamber cannulation. A third group was treated with timolol, dorzolamide, and vehicle in a crossover manner 1 hour prior to fluorophotometric scans. Results Aqueous flow with ketamine/xylazine anesthesia (0.09 ± 0.05 μL/min, mean ± SD, n = 24) was slower than with tribromoethanol or ketamine alone (P < 0.001). Timolol reduced aqueous flow from 0.20 ± 0.07 μL/min to 0.07 ± 0.03 μL/min (P = 0.001) under tribromoethanol anesthesia and from 0.14 ± 0.03 μL/min to 0.10 ± 0.02 μL/min (P = 0.004) under ketamine anesthesia but not under ketamine/xylazine anesthesia. Dorzolamide reduced aqueous flow from 0.09 ± 0.03 to 0.06 ± 0.03 μL/min (P = 0.04) under ketamine/xylazine anesthesia. Aqueous flow by anterior chamber cannulation (0.20 ± 0.13 μL/min) was greater (P = 0.05) than by fluorophotometry (0.09 ± 0.07 μL/min). Conclusions A new noninvasive fluorophotometric method detected effects of general anesthesia and known aqueous suppressants on aqueous flow in mice. Aqueous flow measured by fluorophotometry was slower than by cannulation, and was technically easier with less variability. The mouse fluorophotometer is useful for repeated measurements of aqueous flow in the murine eye making crossover and longitudinal studies possible.