Time of day, perfusion pressure are important when evaluating optic nerve head topography

This study investigated whether optic nerve head (ONH) topography changes throughout the day in patients with untreated early primary open-angle glaucoma (POAG) and if any such changes are significantly associated with diurnal variation in IOP and mean ocular perfusion pressure (MOPP). The results demonstrate that ONH topography significantly changed during the day, with this variation associated with change in reference height, which in turn was associated with MOPP. The authors recommend repeated measures and consideration of time of day and level of ocular perfusion pressure when examining ONH topography with the Heidelberg Retina Tomograph (HRT).

The authors measured IOP, blood pressure and ONH topography between 7 a.m. and 10 p.m. in 14 untreated POAG patients and 14 age-matched normal controls. Variance component analysis indicated that the greatest variability was due to differences among patients who had the highest variance component, whereas time made up the smallest contributed percentage of variance component.

The authors found significant differences between the groups in cup volume, rim volume and cup shape measure in the temporal and temporal-inferior sectors (P< 0.05). Cup volume (temporal and temporal-inferior) and reference height (RH) showed a significant (P < 0.001) diurnal change in the untreated POAG group, whereas rim volume (temporal and temporal-inferior) showed a significant diurnal change in the control group (P ≤ 0.01). There was no significant (P > 0.05) association between the change in IOP, MOPP and ONH topography in either group. In both groups, there was a significant association between cup volume and RH (P < 0.001, global and temporal) and between MOPP and RH (P < 0.001).

This study is important because it points out that time of day and MOPP are particularly important factors when performing HRT optic nerve topographic scans since their variation can lead to relatively large variations in optic nerve cup volume measurements. These variations are higher in patients with early glaucoma compared with individuals without glaucoma. Although the study's sample size is relatively small, the findings were fairly consistent between patients and are also consistent with the results of previous studies.

The study's results imply that if systemic blood pressure, IOP and the resultant MOPP, which is related to the time of day the scans are taken, are not taken into account, incorrect conclusions about implied changes due to glaucoma are likely. This, in turn, may result in incorrect clinical decision making.