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Editors Selection IGR 17-3

Clinical Forms of Glaucoma: Circadian IOP fluctuation in NTG

Comment by Kouros Nouri-Mahdavi on:

70741 Circadian Patterns of Intraocular Pressure Fluctuation among Normal-Tension Glaucoma Optic Disc Phenotypes, Moon Y; Kwon J; Jeong DW et al., PLoS ONE, 2016; 11: e0168030

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Moon and colleagues reported on the prevalence and patterns of 24-hour IOP peaks in a group of 164 eyes with untreated focal ischemic (FI) and myopic normal tension glaucoma (NTG). Eighty-two eyes were enrolled in each group, matched for age and disease severity. Intraocular pressure was measured with Tonopen in the habitual position during the day and at night. They found that: (1) no evident acrophase (IOP peak) was seen in the myopic group as a whole; (2) the FI group had more nocturnal IOP peaks whereas IOP peaks occurred more commonly in the morning in the myopic group; and (3) on multivariate analyses, FI phenotype and less myopic refractive error were the only predictors of nocturnal IOP elevation defined as nocturnal average supine IOP minus diurnal average IOP in sitting position.

The lower rigidity of the sclera could have also resulted in altered strain/stress relationships in these eyes

This is the first report in the literature comparing 24-hour IOP curves in specific phenotypes of NTG and the results provide important information regarding diurnal/nocturnal IOP changes in these specific subtypes of NTG eyes. It would have been interesting to know the relative changes of blood pressure and ocular perfusion pressure (OPP) in these two NTG groups although it is possible that the authors plan to report this in an upcoming study. This team of investigators has previously reported on the higher fluctuation of OPP as a risk factor for NTG.¹ The higher IOP fluctuation in the FI subtype is a very interesting finding and consistent with some of the available data in the literature. The investigators duly mention various reasons why myopic eyes did not have an IOP peak in the supine position (decreased choroidal volume and possibly increased uveoscleral outflow) to which I would like to add that the lower rigidity of the sclera could have also resulted in altered strain/stress relationships in these eyes, i.e., the IOP increase in response to increased choroidal volume in the supine position could be less prominent due to lower scleral rigidity.

Although multiple comparisons were made and inflation of p values is an issue, overall the trends point to the conclusions drawn by the investigators. The authors have previously reported a negative correlation between nocturnal habitual-position IOP elevation and axial length in a group of younger myopic glaucoma subjects and this study confirms this finding although the refractive error rather than axial length measurements were used for this purpose.²

I would like to commend the investigators for having provided another piece of the puzzle on the role of IOP in glaucoma eyes with normal pressures. We will have to wait for future studies based on continuous IOP monitoring by the Triggerfish 'Smart' Contact Lens and possibly intraocular IOP sensing devices for more in-depth understanding of the complex relationships between the IOP and various types of glaucoma.

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