The effect of anti-VEGF injections on tonographic aqueous outflow facility and a novel method to calculate facility using the post-injection IOP decay curve

Abstract

 

Purpose

 

Sustained IOP elevation has been reported following anti-VEGF injections in the treatment of neovascular age-related macular degeneration (NVAMD) by many observers, though the mechanism for this is poorly understood. It has been proposed that anti-VEGF therapy may damage or obstruct the trabecular outflow pathway producing a decrease in conventional outflow facility. The purpose of this study was to 1) assess the effect of anti-VEGF injections on tonographic outflow facility, and 2) evaluate a novel method to calculate facility based on the post-injection IOP decay curve and to benchmark this method against Schiøtz tonography.

 

Methods

 

Patients with unilateral NVAMD receiving anti-VEGF injections were recruited into 2 groups, those with ≤ 10 and those with ≥ 20 injections. Outflow facility was measured using Schiøtz tonography prior to injection in the treated and untreated fellow eye and compared. Post-injection IOP was assessed every 1-2 minutes using an iCare Rebound ® tonometer. The outflow facility was calculated utilizing the IOP decay curves that were fit using least-squared minimization to a mathematical model describing the pressure and volume decay as predicted by Goldmann's equation, Friedenwald's equation, and the conservation of aqueous humor mass.

 

Results

 

The mean outflow facility of all injected eyes (n=9) was 0.13±0.05 µl/min/mmHg (mean±SD) and 0.13±0.05 µl/min/mmHg in uninjected eyes (n=9), which was not significantly different (p=0.88). The mean outflow facilities of eyes with ≤ 10 or ≥ 20 injections were 0.13±0.04 and 0.14±0.06 µl/min/mmHg, respectively, which did not differ significantly (p=0.54). There was good correlation between tonographic and calculated outflow facility based on the IOP decay curves (R²=0.75, p=0.0001) (Figure).

 

Conclusions

 

We observed no difference in the mean outflow facility between eyes receiving multiple anti-VEGF injections and uninjected fellow eyes. The number of anti-VEGF injections did not appear to affect outflow facility. We validated a new method to calculate outflow facility based on the post-injection IOP decay curve that was highly correlated with conventional Schiøtz tonography, providing a novel method for estimating outflow facility in this population.

 

 

Correlation between tonographic and calculated outflow facility using the IOP decay curve.

 

Keywords: 633 outflow: trabecular meshwork • 748 vascular endothelial growth factor • 412 age-related macular degeneration