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Editors Selection IGR 17-3

Normal Pressure Glaucoma: Provocative tests

Comment by Kouros Nouri-Mahdavi on:

54376 Intraocular pressure change over a habitual 24-hour period after changing posture or drinking water and related factors in normal tension glaucoma, Sakata R; Aihara M; Murata H et al., Investigative Ophthalmology and Visual Science, 2013; 54: 5313-5320

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The authors report the results of an interesting study exploring the predictive factors for peak IOP under three different conditions in 33 Japanese untreated NTG patients: IOP measured with the subject in their habitual position (sitting during the day and lying down at night), IOP after a positional change from sitting to supine position (PCT-IOP), and after a water-drinking test (WDT). The habitual position IOP peak (peak_H24h-IOP) was defined as the highest-measured IOP in the sitting or supine position. They also investigated whether the peak IOP in supine position (PCT IOP) or after WDT predicted the peak habitual IOP.

Almost a third of the eyes had a peak IOP > 21 mmHg during at some point during the day. Most eyes had their peak IOP at some time during the night in the supine position (55 out of 66 eyes). Higher peak_H24h-IOP was only weakly correlated with less myopia and worse visual field mean deviation on multivariate analyses. Higher baseline IOP, more positive refraction, older age, higher BMI, and higher mean BP were the strongest predictors for peak_PCT-IOP or a change in PCT-IOP from baseline. Thicker CCT, and higher baseline IOP, higher BMI, and older age were positively related to higher peak_WDT-IOP or change of IOP from baseline during the WDT. Overall the correlation of either peak_PCT-IOP or peak_WDT-IOP with peak_H24h-IOP was only moderate. This means that the former measurements cannot be reliably used to predict peak_H24h-IOP in eyes with NTG. Interestingly IOP fluctuation was inversely related to baseline IOP, i.e. eyes with lower IOP had higher fluctuations.

Because all the correlation data are expressed as regression coefficients, it is hard for the reader to gauge the magnitude of the correlations, but given the multitude of comparisons performed, some of the seemingly significant p values are likely not relevant. However, three general conclusions can be drawn from this study: (1) PCT-IOP or peak IOP after WDT does not adequately pre - dict peak diurnal IOP to be clinically useful. I would have expected that the PCT-IOP would demonstrate a higher correlation with H24h-IOP compared with the WDT-IOP since the mechanisms of IOP increase seem to be more closely related for those two measurements; (2) Myopic eyes tend to have lower IOP peaks and higher peak diurnal IOP is predictive of worse glaucoma severity; both findings are consistent with the available data in the literature; and (3) Systemic factors such as age and possibly BMI can potentially affect IOP homeostasis under various conditions such as supine position or after a water-drinking load.

The authors are to be commended for shedding further light on a challenging but very important clinical question. The results of this study need to be interpreted taking into account that other potentially important variables, such as the CSF pressure, could affect ocular perfusion pressure.

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