Changes in the Anterior Chamber (AC) of Rat Eyes Induced by Injection of Hypertonic Saline in the Episcleral Venus Plexus (EVP)

Abstract

Abstract: : Purpose: To determine possible anatomic changes in the AC of rats after 4-6 weeks of elevated intraocular pressure (IOP). Methods: The EVP of one eye each of 12 Brown-Norway rats was injected with hypertonic saline to induce IOP elevation. 4-6 weeks later, both eyes were imaged with an Ultrasound Biomicroscope (UBM) and a modified eyecup. Measurements of the anterior chamber depth (ACD), trabecular-iris angle (TIA), iris thickness (IT), angle-opening distance (AOD-200) (distance between the posterior corneal surface and anterior iris surface 200 µm from the scleral spur), as well as cross sectional area of the ciliary body (CBA) were obtained. Experimental eyes were compared to contralateral control eyes using paired t-test. IOP was measured prior to imaging with a rebound tonometer and was correlated to parameters measured. Results: Mean IOP at the time of the UBM imaging was 10.6 mmHg for control eyes and 33.3 mmHg for experimental eyes. For control eyes mean (± SD) of the parameters measured were ACD: 0.780 (±0.044) mm, TIA: 12.8 (±4.2) degrees, AOD-200: 42,1 (±10.6) µm, IT: 255 (±39.6) µm, CBA: 406.3 (±91) pixels. The corresponding values for experimental eyes were ACD: 1.327 (±0.201) mm, TIA: 9.7 (±10.5) degrees, AOD-200: 34.9 (±35.8) µm, IT: 276.3 (±37.2) µm, CBA: 267.3 (±66) pixels. Mean AOD, TIA and IT did not differ significantly between control and experimental eyes. TIA was highly variable among various regions of the angle of each of the experimental eyes with some regions appearing closed. ACD was significantly deeper in experimental compared to control eyes (p<0.0001) and correlated well with IOP at the time of imaging (R²=0.88). CBA was significantly lower in experimental eyes compared to control (p<0.0003) but correlated less with IOP (R² =0.425). 2 out of 12 eyes had a significant inflammatory response in the anterior chamber. Experimental eyes appeared to have thickened zonules that were bowing posteriorly. Conclusions:The structural changes observed by UBM indicate that injection of hypertonic saline in the EVP of rats raises IOP through obstruction of aqueous humor outflow and variable angle closure. Inflammatory changes sometimes observable by UBM in the AC may be part of the mechanism that leads to angle closure. Hypertonic saline injection also causes an apparent reduction in the size of the ciliary body, which may be responsible for the initially low IOP. High IOP for a period of 4-6 weeks causes significant deepening of the AC that is proportional to IOP..

Keywords: imaging/image analysis: non-clinical • outflow: trabecular meshwork • injection