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Shan Fan, Vikas Gulati, Donna Neely, Nathan Harms, Carol Toris; Twenty-four hour Variations in Ocular Biometric Parameters in Patients with Ocular Hypertension. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2000.
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The overall refractive status of the eye is determined by the corneal power, anterior chamber depth, lens thickness and axial length. Intraocular pressure (IOP) has the potential to affect these parameters. Over a 24 hour period the changes in these parameters need to be complimentary to each other to keep vision clear. To determine the extent of the ocular physical changes and whether they correlate with IOP, this study evaluates biometric parameters throughout a day and night in patients with ocular hypertension (OHT) treated with brimonidine or vehicle.
Thirty patients with OHT (58.6±9.2 years of age) were enrolled in this randomized, double-masked, crossover study. Participants self-administered 0.2% brimonidine or vehicle three times daily for 6 weeks. At the end of each 6 week period, measurements of habitual (seated during the day and supine at night) intraocular pressure (IOP), central cornea thickness (CCT), anterior chamber depth (ACD), axial length (AXL) and lens thickness were made. The results were compared by two way ANOVA, one way ANOVA and post hoc testing. P values <0.05 were considered statistically significant.
Time of measurement had a significant effect on CCT, ACD and AXL. In vehicle-treated eyes, CCT was thicker at 3 AM than any other time (p<0.01), ACD and AXL were larger at 3 AM and 8 PM than 3 PM (p<0.01) and lens thickness did not change (p=0.40). Supine IOP at 3 AM was greater than seated IOPs during the day (p<0.01). Brimonidine, with a mean (±standard deviation) habitual IOP decrease of 1.09 ± 3.72 mmHg during the day and 0.33 ± 4.33 mmHg during the night, did not alter these patterns. Brimonidine lowered IOP during the day but not at 3 AM. The shortest AXL and ACD were temporally close to the lowest habitual IOP during the day.
The increase in axial length at night (approximately 75 µm) can be attributed to an increase in anterior chamber depth (approximately 90 µm). The increase in anterior chamber depth is independent of any change in lens thickness. These potentially IOP-mediated changes appear complimentary towards maintenance of refractive status of the eye, demonstrating inherent homeostatic mechanisms of the eye despite significant changes in the IOP. Brimonidine use does not alter the normal diurnal rhythm of ocular biometric parameters but a more potent ocular hypotensive drug may.
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