Why eye pressure rises after tropicamide-phenylephrine drops in dogs

Abstract

PURPOSE: To determine whether the differences in intraocular pressure (IOP) change after the topical application of 0.5% tropicamide and 0.5% phenylephrine (TP) are due to anatomical variations in the anterior chamber using ultrasound biomicroscopy (UBM). METHODS: Prospective clinical data from 27 eyes of 14 dogs with normal eye conditions were analyzed. IOP and UBM measurements were taken before and 30 min after administering a topical TP. Dogs were categorized into two groups based on an IOP increase criterion of 4 mmHg: the high group and the stable group. Parameters measured included the iridocorneal angle (ICA), angle-opening distance (AOD), ciliary cleft width (CCW), length (CCL), area (CCA), ciliary body axial length (CBAXL), and ciliary process-sclera angle (CPSA). RESULTS: Both groups showed a statistically significant decrease in ICA and an increase in AOD. However, in the stable group, CPSA increased, CBAXL decreased, and CCA increased, whereas in the high group, CPSA and CBAXL did not change significantly, and CCA decreased. CONCLUSION: In the stable group, although the pupil dilated, the ciliary body relaxed, leading to an increase in the CCA. Conversely, in the high group, the pupil dilated, and the ciliary body remained unrelaxed, resulting in a reduction in the CCA. These iridociliary changes with pupil dilation increase the risk of elevated IOP. Therefore, an increase in IOP following TP administration may serve as a prognostic indicator for possible glaucoma risk.