Functional characterization of retina and optic nerve after acute and chronic elevation of the intraocular pressure in rats

Purpose. To characterize the pupil light reflex (PLR), electroretinographic (ERG) and tonometric parameters in healthy, acute and chronic hypertensioe rat eyes. Methods. Brown Norway rats were used for experiments. The PLR was evaluated with a computerized pupillometer, ERGs were recorded simultaneously from both eyes and intraocular pressure (IOP) was measured with a Tonopen. Retinal ischemia was induced in rats by acutely increasing the IOP (110 mmHg/60 minutes), while chronic ocular hypertension was induced by cauterizing vortex and episcleral veins. Results. The analysis of the PLR parameters confirmed the consensual PLR was significantly smaller in amplitude (p = 0.03) and increased latency time (p = 0.001) compared to the direct PLR in healthy rats. Acute ocular ischemia caused significant decrease in retinal function. Preoperative values for the PLR_ratio in rats exposed to the acute ocular ischemia (ratio = consensual/direct PLR) were 76.7 ± 2.6 (mean ± SEM; %). 24h postoperatively the PLR _ratio was 15.2 ± 12.8, 10 days postoperatively 11.6 ± 9.8, 20 days postoperatively 26.5 ± 8.0 and 28 days postoperatively PLR_ratio was 33.27 ± 9.3. However, at day 35 the PLR was significantly recovered when compared to the 24h postoperative values (PLR _ratio = 41.1 ± 7.3%, p < 0.01, Repeated measures ANOVA). 42 days after surgery the PLR started to decrease once again in the operated eyes. Electroretinographic amplitudes followed a similar pattern. Seven days after surgery 5/14 rats, which received cauterization of the vortex and episcleral veins developed significant elevation of the IOP in operated eyes (p = 0.0004; Paired t-test). Elevation of the IOP was sustained at 3 (p = 0.002) and 5 (p = 0.007) weeks postoperatively. However, IOP values did not significantly differ between control and operated eyes 8 weeks postoperatively (p = 0.192, Paired t-test). Rats with elevated IOP had significant pupil defects at 7 (p < 0.05, Repeated measures ANOVA, n = 5) and 28 days postoperatively (p < 0.05), but not at 62 days postoperatively (p > 0.05) comparing to preoperative values. Rats with elevated IOP displayed a significant decrease in ERG amplitudes in operated eyes at 4 but not at 8 weeks postoperatively. Conclusions. Functional monitoring of the ERG and PLR is sensitive technique for the detection of retina and optic nerve deficits after acute and chronic elevation of the IOP.