Changes in the shape of the anterior and posterior corneal surfaces caused by mydriasis and miosis: Detailed analysis

Abstract

Purpose: To evaluate changes in the anterior and posterior corneal shape, corneal thickness, and anterior chamber depth (ACD) caused by mydriasis or miosis using scanning-slit corneal topography. Setting: Department of Ophthalmology, Iwate Medical University School of Medicine, Iwate, Morioka, Japan. Methods: Twenty-eight eyes of 28 healthy volunteers with refractive errors of −6.00 to +0.25 diopters were studied. One eye of each subject had instillation of tropicamide–phenylephrine hydrochloride (Mydrin P®) to obtain mydriasis and of pilocarpine hydrochloride 2% (Sanpilo®) to obtain miosis. To assess the corneal shape, the best-fit sphere (BFS), axial power, and tangential power were measured for the anterior and posterior corneal surfaces before and after mydriasis and before and after miosis using scanning-slit corneal topography (Orbscan version 3.0, Orbtek, Inc.). The pupil size, corneal thickness, and ACD were also examined before and after mydriasis and before and after miosis. Results: The mean age of the patients was 31.1 years ± 5.6 (SD) (range 20 to 46 years). The anterior BFS changed from a mean of 8.04 ± 0.3 mm at the time of mydriasis to a mean of 8.00 ± 0.3 mm at the time of miosis. The posterior BFS changed from 6.53 ± 0.3 mm to 6.46 ± 0.3 mm, respectively. Thus, the anterior and posterior cornea became significantly steeper after miosis ( P<.01). The ACD was significantly more shallow after miosis than after mydriasis. However, there was no significant difference in corneal thickness after mydriasis or miosis. Conclusions: The anterior and posterior corneal shapes changed as a result of mydriasis and miosis, and the refractive power of the cornea significantly increased after miosis. To date, changes in refractive power from changes in pupil size have been attributed to a change in the refractive power of the crystalline lens; however, it is now thought that changes in corneal refractive power also occur.