Validation of rebound tonometry for intraocular pressure measurement in the rabbit.

Rabbits play a growing role in research into glaucoma surgical models and ocular drug delivery models. However, the lack of an accurate method for measuring intraocular pressure (IOP) in this animal has been a significant deficit. In this study we validated the use of the TonoVet rebound tonometer and provide conversion tables for its use in rabbits. Experiments were performed on 18 adult New Zealand White rabbits. The TonoVet measurements were obtained and compared to manometric readings by anterior chamber (AC) cannulation. The TonoVet position and 'd' (dog or cat) and 'p' (other species) modes were compared. The sensitivity of the TonoVet tonometer in assessing IOP changes was also tested. There was a strong linear correlation for both the 'd' mode (mean slope = 0.84 ± 0.03, r(2) = 0.99 ± 0.03) and the 'p' mode (mean slope = 0.64 ± 0.02, r2 = 0.97 ± 0.01) of the TonoVet with manometric IOP. However, the TonoVet had a tendency to underestimate IOP compared to manometry and conversion formulae were possible to calculate for both modes. The orientation of the TonoVet handle had no effect on IOP reading, as long as the groove was horizontal. No significant differences were observed when comparing right and left eyes (P > 0.05). IOP recovered four days after cannulation. Younger rabbits had lower IOP compared with older rabbits (P < 0.01). Timolol produced a 2.5 mmHg reduction in IOP 2 h later as detected by the TonoVet. Using the conversion table presented, the TonoVet is a reliable and precise tool for the measurement of IOP in rabbits.

Porosity of Bleb Capsule declines rapidly with Fluid Challenge.

INTRODUCTION: The porosity of the fibrous capsule around a glaucoma drainage device (GDD) may be the most important functional attribute. The factors that determine capsular porosity are not well understood. Failed GDD surgeries are usually associated with thick impervious capsules and components of aqueous have been implicated in this process. PURPOSE: In this study, we interrogated the effect of passage of nonaqueous fluid on capsular porosity in mature (but aqueous naïve) blebs in a previously reported GDD model (the "Center for Eye Research Australia Implant"). MATERIALS AND METHODS: The study was performed at two centers using 17 New Zealand White (NZW) rabbits. An experimental GDD was implanted into the subconjunctival space but without connection to the anterior chamber. After 28 days, balanced salt solution (BSS) was passed through the implant for 30 to 40 minutes at 12 mm Hg. Capsular porosity was measured as flow (µL/min) at a constant pressure. Porosity of the capsule was retested at 3 and 6 days. RESULTS: There was a marked reduction in capsular porosity within 3 days of exposure to BSS (fluid challenge). Even though the baseline porosity was significantly different in the two groups (3.00 ± 0.5 µL/min and 29.67 ± 12.12 µL/min, p < 0.001), the effect of passage of BSS was similar. Capsular porosity fell by approximately 80% in both groups from baseline after single BSS challenge. Capsular thickness was significantly less in Advanced Eye Center (AEC) rabbits at baseline. There was no change in the capsular thickness before and after single fluid challenge. CONCLUSION: Passage of BSS at physiological pressures for under 40 minutes caused marked reduction in the porosity of the fibrous capsule within 3 days. This was not associated with any significant thickening of the fibrous capsule within this time frame. HOW TO CITE THIS ARTICLE: Pandav SS, Ross CM, Thattaruthody F, Nada R, Singh N, Gautam N, Beirne S, Wallace GG, Sherwood MB, Crowston JG, Coote M. Porosity of Bleb Capsule declines rapidly with Fluid Challenge. J Curr Glaucoma Pract 2016;10(3):91-96.

A model to measure fluid outflow in rabbit capsules post glaucoma implant surgery.

PURPOSE: Prior models of glaucoma filtration surgery assess bleb morphology, which does not always reflect function. Our aim is to establish a model that directly measures tissue hydraulic conductivity of postsurgical outflow in rabbit bleb capsules following experimental glaucoma filtration surgery. METHODS: Nine rabbits underwent insertion of a single-plate pediatric Molteno implant into the anterior chamber of their left eye. Right eyes were used as controls. The rabbits were then allocated to one of two groups. Group one had outflow measurements performed at 1 week after surgery (n = 5), and group two had measurements performed at 4 weeks (n = 4). Measurements were performed by cannulating the drainage tube ostium in situ with a needle attached to a pressure transducer and a fluid column at 15 mm Hg. The drop in the fluid column was measured every minute for 5 minutes. For the control eyes (n = 6), the anterior chamber of the unoperated fellow eye was cannulated. Animals were euthanized with the implant and its surrounding capsule dissected and fixed in 4% paraformaldehyde, and embedded in paraffin before 6-µm sections were cut for histologic staining. RESULTS: By 7 days after surgery, tube outflow was 0.117 ± 0.036 µL/min/mm Hg at 15 mm Hg (mean ± SEM), whereas at 28 days, it was 0.009 ± 0.003 µL/min/mm Hg. Control eyes had an outflow of 0.136 ± 0.007 µL/min/mm Hg (P = 0.004, one-way ANOVA). Hematoxylin and eosin staining demonstrated a thinner and looser arrangement of collagenous tissue in the capsules at 1 week compared with that at 4 weeks, which had thicker and more densely arranged collagen. CONCLUSIONS: We describe a new model to directly measure hydraulic conductivity in a rabbit glaucoma surgery implant model. The principal physiologic endpoint of glaucoma surgery can be reliably quantified and consistently measured with this model. At 28 days post glaucoma filtration surgery, a rabbit bleb capsule has significantly reduced tissue hydraulic conductivity, in line with loss of implant outflow facility, and increased thickness and density of fibrous encapsulation.