The aim of the present study was to comprehensively observe the changes in IOP and OPP in patients with OHTN over a 24-h period. The maximum, minimum and mean IOP were significantly higher, and the maximum, minimum and mean MOPP were significantly lower in patients with OHTN than in healthy control subjects (P < 0.05). The minimum and mean SOPP and DOPP values in the OHTN group were lower than those in the healthy controls (P < 0.05). The IOP, MOPP, SOPP and DOPP diurnal and nocturnal fluctuation values were significantly larger in the OHTN group than in the healthy control group (P < 0.05). Furthermore, the peak and trough IOP timepoints were similar in both groups, with a trend towards low values during the day and higher values at night. The peak and trough MOPP and SOPP times for the two groups also overlapped, with higher daytime values which declined at night. The 24-h DOPP in the healthy control group tended to be high during the day and low at night, with peak values between 19:00 and 23:00 h and trough values between 3:00 and 7:00 h. No obvious changes were observed in the OHTN group from day to night. Over the 24-h timeframe, the top three times with the greatest frequency of peak IOP counts were 1:00, 5:00 and 11:00 h in the OHTN group, while those in the healthy control group were 7:00, 3:00 and 5:00 h. The top three times at which the highest frequency distribution of MOPP trough values occurred were 1:00, 3:00 and 5:00 h in the OHTN group, and 3:00, 7:00 and 5:00 in the healthy control group.
There are various methods for conducting tonometry, including the use of, for example, the Schiotz tonometer, the Goldmann applanation tonometer, the Ton-Pen tonometer and the non-contact tonometer. Each tonometer has its own advantages and disadvantages. The Goldmann applanation tonometer method is the internationally recognized gold standard for IOP measurement, but its clinical application is more cumbersome. The non-contact tonometer (NCT) method is non-invasive, requires no anesthetic and is easy to perform. It is widely used in the clinic and is more suitable for repeated IOP measurements within a 24-h period, the accuracy of which has been confirmed in previous studies. Additional methods were adopted in the present study, which included sitting in the daytime and sitting immediately after waking up at night; these factors minimize the influence of higher suprascleral vein pressure and BP on the IOP results during the nocturnal position (lying flat). Therefore, the sitting position was uniformly adopted and the comparison standard was unified.
In the current study, the overall IOP of the OHTN group was increased, the mean IOP was higher, and the fluctuation amplitude of the diurnal and nocturnal IOP (8.91 ± 3.07 mmHg) was significantly higher than that of the healthy control group (5.89 ± 0.86 mmHg). Xu et al [11] studied the repeatability of 24-h IOP monitoring in OHTN, and found that the two 24-h IOP fluctuation values were 8.94 ± 3.03 mmHg and 9.06 ± 3.19 mmHg, which is similar to the results of the current study (8.91 ± 3.07 mmHg). At the same time, Xu et al reported that the peak IOP occurred at 6:00 h, and that the trough IOP was observed at 20:00 h, which is similar to the results of the current study. Grippo et al [12] revealed that in habitual positions (diurnal sitting and nocturnal supine), the peak IOP occurred at 11:30 − 5:30 h, and the trough at 5:30 − 21:30 h; while in the supine position, the peak IOP was primarily observed at 7:30 − 3:30 h, and the trough occurred at 17:30 − 23:30 h. In the present study, a sitting position was adopted. Although the IOP peak time was at 9:00 h in patients with OHTN, the peak IOP occurred most frequently at 1:00 and 5:00 h, which is not consistent with the results of the aforementioned studies. The IOP value can be influenced by various random factors, such as light, activity or fluid intake, in addition to the habitual activities of the subject during the day, which may increase IOP variability. A large number of studies have confirmed that elevated IOP and higher IOP diurnal and nocturnal fluctuations are important risk factors for POAG[13,14], but that OHTN does not result in glaucoma-associated optic nerve damage. In recent years, scholars have discovered that the pressure difference across the sieve plate, which is the difference between IOP and intracranial cerebrospinal fluid pressure, is an important factors for the pathogenesis of glaucoma. It is speculated that patients with OHTN may experience high intracranial pressure resulting in low-pressure differences across the sieve plate, such that glaucoma-associated optic nerve injury does not occur [15].
A large number of studies have confirmed that the vascular mechanism is an influencing factor for the pathogenesis of glaucomatous optic nerve injury. Insufficient or unstable blood supply to the eye can cause optic nerve and axon ischemia and/or reperfusion injury. OPP is an important factor in determining blood flow in the eye. Using the OPP calculation formula, it can be seen that an increase in IOP or a decrease in BP results in a decrease in OPP, which may cause hypoxia and ischemia in the optic nerve, initiating or aggravating visual field defects in patients with POAG.
A Handan Eye Study revealed that the SOPP, DOPP and MOPP of patients with POAG were consistently lower than those of suspected POAG patients, indicating that OPP plays an important role in the development of glaucoma [3]. An epidemiological study in Singapore indicated that lower MOPP, SOPP and DOPP were independent risk factors for POAG [9]. Early research in Barbados demonstrated that DOPP was decreased by 20% and that the incidence of glaucoma was increased by 3.3 times in patients with POAG. After a 9-year follow-up study, low SOPP, low DOPP and low MOPP were confirmed to be risk factors for glaucoma [16]. Additionally, Topouzis et al [17] suggested that DOPP was more highly correlated with the progression of POAG than SOPP.
Low perfusion pressure and vascular autoregulatory dysfunction are important factors in the pathogenesis of glaucoma. Choi et al [8] assessed patients with normal tension glaucoma for > 6 years, and found that the progression of visual field defects was closely associated with the fluctuation amplitude of the 24-h OPP. Moreover, Sung et al [14] followed 101 patients with normal-tension glaucoma for > 4 years and found that the 24-h MOPP fluctuations of those with progressive visual field damage were significantly greater than those without progression.
Sehi et al [18] revealed that the daily percentage reduction in MOPP of untreated POAG patients was significantly higher than that of normal subjects, indicating that relative diurnal changes in MOPP may be a risk factor for POAG. These findings suggest that the alterations in OPP are closely associated with the occurrence and development of glaucoma. Although countless individuals experience low OPP, they do not develop glaucoma due to a normal self-adjusting ability which compensates for the low blood supply caused by low OPP. In the capillary bed of these individuals, and in the range of automatic adjustment ability, a change in perfusion pressure will not cause a change in blood flow [19,20]. When the posture changes, IOP increases or blood pressure decreases, and the vascular autoregulation mechanism is required to maintain OPP stability. OHTN may be due to the normal automatic adjustment ability, which can compensate for the low blood supply resulting from low OPP, thus glaucoma does not develop.
In the present study, the MOPP trough times were between 1:00 and 3:00 h, which are considered to be non-working hours. This suggests that it is best to monitor changes in the 24-h OPP when observing the OHTN perfusion pressure, and to pay particular attention to the occurrence of low OPP at night. The average and fluctuating DOPP values in OHTN subjects were significantly higher than those in the healthy control group. Furthermore, compared with MOPP and SOPP, no significant change in trend was observed, suggesting that changes in DOPP are more sensitive indicators of OHTN than those in MOPP and SOPP. These findings are consistent with the results of Topouzis et al [17], suggesting that DOPP is more highly correlated with POAG progression than SOPP. Therefore, for the future diagnosis and treatment of OHTN, more attention should be paid to changes in DOPP.
There are some shortcomings to the present study. Firstly, the study was conducted over a 1-year period; seasonal changes in temperature may influence IOP and BP, which require further research in the future. Secondly, the traditional posture was adopted, and the IOP at night was measured after waking. At this time, the subject’s physiological status, such as hormone levels and the effect of the eyelids, may change, and may cause differences from the IOP measured in the lying position. Thirdly, taking measurements every 2 h cannot fully reflect these physiological changes, and measurements at night will inevitably be affected by other factors, such as exposure to light. In future studies, measurements may be taken using a 24-h tonometer [21], or a simpler tool might be developed to allow measurements to be taken in the patients’ home. Ultimately, carrying out day and night IOP and OPP monitoring will help to improve our understanding of IOP and OPP fluctuations to guide the treatment of those with OHTN.