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Comment by Arthur Sit on:

46409 Aqueous humor dynamics during the day and night in volunteers with ocular hypertension, Fan S; Hejkal JJ; Gulati V et al., Archives of Ophthalmology, 2011; 129: 1162-1166

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Intraocular pressure (IOP) follows a circadian pattern with significant elevations at night when measured in the habitual positions of sitting during the day and supine at night.¹ The reasons why this occurs have been investigated for normal subjects, and appear to be related to a combination of changes in aqueous humor flow rate, outflow facility, episcleral venous pressure and uveoscleral outflow.^2,3 However, aqueous humor dynamics can be altered in ocular hypertension, and the circadian patterns may not be the same as in normal subjects. Fan et al. investigated the changes in aqueous humor dynamics that occur from day to night in a group of patients with ocular hypertension (OHT). They measured aqueous humor flow rate using fluorophotometry, outflow facility using tonography, episcleral venous pressure (EVP) using venomanometry, and calculated uveoscleral flow using the modified Goldmann equation. They reported a 48% decrease in aqueous humor flow at night, consistent with previous studies, but did not find a significant decrease in outflow facility. Uveoscleral flow decreased significantly at night by 0.61 µL/min. This study provides important insight into the role of aqueous humor dynamics in determining IOP patterns in OHT patients. Interestingly, the changes that occur at night appear to be different from the changes reported from the same laboratory for healthy mature subjects. In particular, there was no significant change in outflow facility in the OHT group, in contrast to the decrease reported for healthy volunteers. However, uveoscleral outflow was found to decrease at night, similar to the previously reported healthy subjects. There are some limitations in this study. In particular, the calculation of uveoscleral outflow is uncertain due to the limitations associated with the measurement of EVP using venomanometry, which involves estimating the pressure required to partially collapse an episcleral vein.⁴ This is evident from the negative values for uveoscleral flow rate reported using the measured EVP values. As well, the nocturnal uveoscleral flow rates are based on estimated EVP changes at night and not measured values. The studies used as a basis for calculating the magnitude of the nocturnal EVP change utilized a handheld version of a slit-lamp mounted device on healthy subjects.^5,6 Whether or not those results are reproducible or applicable to OHT patients is unknown. Another limitation is that some measurements at night are necessarily performed with the subject awake, including tonometry and tonography, and may not reflect true sleeping values. This study helps to advance our understanding of aqueous humor dynamics at night in OHT patients, but further work is required to fully understand the circadian changes in aqueous humor dynamics.

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