Effect of topical phenylephrine on aqueous flare levels and anterior scleral thickness in healthy adult eyes

Phenylephrine is widely used for pharmacological mydriasis in routine ophthalmic practice. While its vascular and pupillary effects are well established, its influence on anterior segment structures and intraocular inflammatory parameters remains unclear. This study aimed to evaluate the effects of topical phenylephrine on anterior scleral thickness (AST) and aqueous humor flare levels in healthy eyes. This prospective study included the right eyes of 20 healthy volunteers. Topical phenylephrine 2.5% was administered three times at 5-minute intervals. Measurements were obtained 45 min after the final instillation. Nasal and temporal AST were measured at the scleral spur (AST-0) and at 1000 μm (AST-1) and 2000 μm (AST-2) posterior to the scleral spur using anterior segment optical coherence tomography. Aqueous flare levels were assessed using laser flare photometry. Fellow untreated eyes served as controls. At the scleral spur level (AST-0), scleral thickness increased significantly in both nasal and temporal regions following phenylephrine administration ( p = 0.0003 and p = 0.01, respectively). At 1000 μm posterior to the scleral spur, temporal AST decreased significantly ( p = 0.005), while other changes in AST-1 and AST-2 were not statistically significant. Aqueous flare levels showed an increase after phenylephrine instillation; however, this change was not statistically significant ( p = 0.2). No significant changes were observed in the fellow control eyes. Topical phenylephrine was associated with localized changes in anterior scleral thickness, characterized by thickening at the scleral spur and relative thinning posteriorly. Although aqueous flare levels tended to increase, this effect was not statistically significant and likely reflects a minimal effect size. These findings suggest that phenylephrine may be associated with alterations in scleral configuration without a clinically meaningful impact on intraocular inflammatory status in healthy eyes.