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Editors Selection IGR 9-4

Examination methods: Aqueous humor

Comment by Arthur Sit on:

15137 Concordance of aqueous humor flow in the morning and at night in normal humans, Radenbaugh PA; Goyal A; McLaren NC et al., Investigative Ophthalmology and Visual Science, 2006; 47: 4860-4864

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Aqueous flow rates decrease by 50-60% from the diurnal period to the nocturnal period.¹ Radenbaugh et al. (1001) investigated the concordance of diurnal and nocturnal aqueous humor flow rates - i.e., do individuals with relatively low flow in the day remain low at night, and similarly for those with medium or high flow? This is significant, because previous studies have shown changes in sample populations, but the concordance of diurnal to nocturnal flow changes in individuals has not previously been reported and may provide insight into the mechanisms of circadian IOP variations.

The authors performed fluorophotometry scans in 28 healthy normal subjects between 8 AM to 12 PM (daytime), and between 12 AM to 6 AM (nighttime). They evaluated concordance using three methods. In the first method, flows were categorized into tertiles representing low, medium and high flow categories for daytime and nighttime. The authors found that 68% of flow rates for morning and night were concordant, which was higher than the 33% expected if the flows changed randomly. In the second method, pairs of daytime and nighttime flow measurements were recorded on a dot plot with the pairs rank ordered. Using this plot, a trend towards concordance was suggested. In the third method, the correlation between daytime and nighttime flows was examined using a scatter plot. The authors indicated a good Loess fit with r² = 0.62, and modelled the data using a two-piece linear least squares regression.

While the apparent concordance of aqueous flow changes is intriguing, the paper does have some limitations. Most importantly, only descriptive statistics are reported with no significance levels, and sample size calculations were not given. For the scatter plot, it is unclear why a two-piece regression was required to fit the data. Finally, the authors speculate on a relationship between concordance of flow and IOP, but do not report a correlation (or lack of one) between IOP and flow. Based on this report, there appears to be a trend towards concordance of aqueous humor flow rates from daytime to nighttime. However, further work is required to elucidate the factors which influence IOP and result in circadian and random changes.

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