A total of 149 eyes of 149 patients with NTG meeting the inclusion criteria was analyzed in this prospective study. Of these, 70 NTG patients were enrolled in the low IOP group. Of the 70 low IOP subjects, 34 were men, 36 were women, and all were native Koreans. The remaining 79 NTG patients (47 men and 32 women) comprised the high IOP group. The demographic and clinical characteristics of both groups are summarized in
Table 1.
Regarding nocturnal IOP changes in the overall group, the mean habitual position IOP during sleeping hours was significantly higher than that of waking hours (15.27 vs. 14.57 mm Hg,
P < 0.001). In subgroup analysis, the mean habitual position IOP during sleeping hours also was significantly higher than that of waking hours in the low IOP group (
P < 0.001,
Table 2), whereas no such difference was found in the high IOP group (
P = 0.706,
Table 2). When different IOP cutoff values (14 and 16 mm Hg, respectively) were used (data not shown), similar findings also were found in that the mean habitual position IOP during sleeping hours (mean IOP = 14.21 and 14.70 mm Hg, respectively) was significantly higher than that of waking hours (12.53 and 13.72 mm Hg, respectively) in the low IOP group (
P < 0.001). However, in the high IOP group, the mean habitual position IOP during sleeping hours (mean IOP = 16.33 and 17.50 mm Hg, respectively) showed similar IOP level compared to that of waking hours (16.07 and 17.65 mm Hg, respectively;
P = 0.155 and 0.641, respectively).
In intergroup comparisons, the mean habitual position IOP in the high IOP group was significantly higher than that of the low IOP group during waking and sleeping hours (
P < 0.01,
Table 2). However, short- and long-term nocturnal IOP changes were greater in the low than in the high IOP group (
P = 0.072 and
P < 0.001, respectively,
Table 3).
The overall group showed an IOP peak during sleeping hours (3–6 AM) in the habitual body position, whereas it showed an IOP peak during waking hours (6–10 AM) in the sitting position (
Fig. 1A). Regarding subgroup patterns, the low IOP group showed an IOP peak during sleeping hours (3–6 AM), whereas the high IOP group showed an IOP peak during waking hours (6–10 AM) in the habitual body position (
Fig. 1B). In the sitting position, however, the low and high IOP groups showed an IOP peak during waking hours (6–10 AM,
Fig. 1B).
An analysis of the overall group based on the cosinor model showed no evident peak (acrophase) in habitual position IOP measurements during the 24-hour period (
Fig. 2A). However, further analysis of individual patients indicated that 39 (26.2%) had a diurnal acrophase (
Fig. 2B), 68 (45.6%) had no evident acrophase (
Fig. 2C), and 42 (28.2%) had a nocturnal acrophase (
Fig. 2D). When analyzing the low IOP group, a nocturnal peak (acrophase) of habitual position IOP was found in the overall patient group at approximately 3 to 6 AM (
Fig. 3A), whereas in an analysis of individual patients, 11 (15.7%) had a diurnal acrophase (
Fig. 3B), 29 (41.4%) had no evident acrophase (
Fig. 3C), and 30 (42.8%) had a nocturnal acrophase (
Fig. 3D). In contrast, no evident peak (acrophase) was found in the overall high IOP group (
Fig. 4A), whereas in an analysis of individual patients, 28 patients (35.4%) had a diurnal acrophase (
Fig. 4B), 39 (49.4%) had no evident acrophase (
Fig. 4C), and 12 (15.2%) had a nocturnal acrophase (
Fig. 4D). There were significant differences in the distribution of the diurnal and nocturnal acrophases between the two groups (
P < 0.05, χ
2 test,
Table 4).
The results of univariate and multivariate modeling of the ability of various clinical variables to predict nocturnal habitual IOP elevation (nocturnal supine average IOP minus diurnal sitting average IOP) are presented in
Table 5. In univariate modeling, among various clinical parameters, the mean daytime IOP level and AL significantly correlated with nocturnal habitual IOP elevation. In multivariate modeling, the mean daytime IOP level and AL remained as significant predictors of nocturnal habitual IOP elevation. A statistically significant correlation was found between nocturnal habitual IOP changes and daytime IOP level in the same eye using Pearson's correlation analysis (
P < 0.001,
Fig. 5).