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Editors Selection IGR 9-4

Clinical Examination Methods: Water-drinking test and diurnal IOP fluctuations

Comment by Remo Susanna Jr on:

95801 Comparison of intraocular pressure peak and fluctuations among Filipino patients with non-glaucomatous eyes and glaucoma suspects using water drinking test and diurnal intraocular pressure, Koh A; Verzosa C, International Journal of Ophthalmology, 2021; 14: 1729-1734

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The authors of this study compared IOP peaks and fluctuations using water-drinking tests and mean diurnal IOP in normal patients and glaucoma suspects. The water-drinking test has been used as a surrogate marker for outflow reserve and to detect IOP instability. The peak IOP elicited during the WDT correlates with the IOP peak that occurs during the day, is highly reproducible, and is associated with the risk of the visual field (VF) progression and severity of glaucoma. Therefore, it is expected that in eyes with worse outflow facility, IOP elevation is higher and remains higher for a longer time than normal eyes.¹ That explains why the IOP peaks occurred at 15' in this study compared with studies in glaucomatous patients in which the highest IOP usually occurs at 30 or 45 minutes. There was no difference in results between the two study groups. However, in this study, patients were normal or glaucoma suspects (including normal and pre perimetric glaucoma patients). Therefore, the more normal patients are in the glaucomatous suspect group, the lesser the difference between both groups.

The WDT and DTC should not be considered diagnostic tests but risk assessment tests. Glaucoma is an optic nerve neuropathy, and IOP elevation above a pre-determined level is not a diagnostic criterion

While seemingly straightforward, there were several other issues with the study. First, the reproducibility of fluctuation in phasing (or diurnal tension curves) and WDT is fair, while IOP peaks are excellent in both tests.^2-6 This fact should be considered when performing both tests on separate visits.

Second, higher IOP fluctuation and peak in the WDT compared with phasing was expected as WDT is a stress test (even though the WDT peak tends to underestimate the 24-h peak IOP), but is strongly correlated to the peak IOP obtained during the 24-h period.⁷

Third, the author state that WDT usually takes two hours of examination, and in this study the test duration was 60 minutes, but in most published studies the test duration is 45 minutes.

Fourth, most studies use 800 ml or 10ml/ kg of water ingestion instead of a fixed 1L, and the IOP measurement 5 minutes after water ingestion is not performed.

Fifth, the authors stated that fistulizing glaucoma surgery might impair WDT interpretation due to increased outflow facility. In fact, this test can evaluate efficacy and detect an early failure of surgical procedures (laser or incisional surgeries), as already shown in previous studies.

Finally, the WDT and DTC should not be considered diagnostic tests but risk assessment tests. Glaucoma is an optic nerve neuropathy, and IOP elevation above a pre-determined level is not a diagnostic criterion.

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