Using retinal function to define ischemic exclusion criteria for animal models of glaucoma

Abstract

Purpose : Most animal models of glaucoma rely on induction of ocular hypertension (OHT). A potential pitfall of such models is high and variable intraocular pressures (IOPs) which can lead to retinal ischemia. Thus, animals exceeding an IOP threshold (e.g. > 60mmHg) are often excluded to avoid confounding factors. However, due to the intermittent nature of IOP measurements, this approach can miss the presence of ischemia. Conversely, it can also inappropriately eliminate animals with brief IOP spikes that do not induce ischemic damage. Here, we explore the potential of electroretinography (ERG) to detect ischemic damage in OHT. It is known that acute ischemia selectively impairs inner retinal function (ON bipolar cells: ERG b-wave), but spares the outer retina (photoreceptors: ERG a-wave), resulting in reduced b-wave amplitude and b/a wave ratio < 1.

Methods : 73 male retired breeder Brown Norway rats received a unilateral injection of magnetic microbeads (25ul, 1:1 ratio of 2μm:6μm diameter) to induce OHT; contralateral eyes served as controls. IOP was measured every 2-3 days for 14 days after microbead injection. Retinal function was evaluated using dark-adapted bright flash ERG (2.1 log cd*s/m²) prior to, and at 7- and 14-days post, injection. We investigated two criteria for excluding animals: (IOP Criterion) a single IOP measurement > 60 mmHg; or (ERG Criterion) b-wave amplitude and b/a ratio below 99% confidence intervals (CIs) based on measurements from contralateral (non-microbead-injected) eyes.

Results : 99% CIs of untreated contralateral eyes were 361.8-983.4μV for b-wave amplitude and 1.70-2.54 for b/a ratio. 18 eyes were excluded using IOP Criterion, of which only 5 remained excluded using ERG Criterion. Conversely, 55 eyes were included with IOP Criterion, of which 42 remained included using ERG Criterion.

Conclusions : We suggest that ERG testing can detect unwelcome ischemic damage in animal models of OHT. Since brief IOP spikes do not necessarily lead to ischemic retinal damage, and because extended periods of elevated IOP can be missed, such ERG-based criteria may provide a more objective and robust exclusion criteria.

This is a 2020 ARVO Annual Meeting abstract.

 

ERG measurements in OHT eyes. The plotted quantities are each eye’s smallest recorded b-wave amplitude and b/a Ratio. The red box shows points falling below the lower 99% CI of control eyes for both b-wave and b/a ratio.