Several limitations should be taken into account when interpreting the findings of this study. First of all, the sample size was small, although the study’s crossover design had sufficient statistical power to assess any differences between the two regimens. Nevertheless, the comparison concerning DOPP may have been underpowered. Second, the population was highly selective because systemic hypertension is very frequent in the age group of patients with POAG, and our results cannot be generalized to the overall POAG population. However, systemic hypertension and its treatment were considered exclusion criteria because of their potential effect on BP due to the greater variability in BP in patients treated with systemic antihypertensive medications,
19 and the possibly negative impact of antihypertensive medications (such as systemic β-blockers) on the hypotensive effect of timolol.
20 Third, the study was only observer masked and should have been performed in a double-masked fashion to minimize any potential bias. However, this was an investigator-initiated trial, and appropriate double masking was not feasible. Fourth, perfusion pressure has been calculated by subtracting the IOP from the BP. This formula is not fully compatible with the widely used formula for the calculation of OPP [mean BP ∗ (2/3) − IOP], which better considers the physiological pressure difference between the brachial BP and the BP close to the eyeball. Nevertheless, the DOPP, as calculated in the present study, has been found to be associated with glaucoma in several epidemiologic investigations
6 7 8 9 10 and is probably a reasonable surrogate for the OPP when the patient is awake and in a vertical body position. Although It is well known that the relationship between OPP and systemic BP in a recumbent body position (nighttime) is different from that in a vertical body position (daytime),
44 it is difficult to discuss any physiological implication that could be related to the different impact of OPP during day- and nighttime. Finally, the use of a handheld tonometer (TonoPen; Bio-Rad) rather than GAT for measurement of nocturnal IOP may be a concern, as there is no close agreement between the two procedures.
45 However, it did allow us to make the nighttime IOP and BP measurements with the patients in bed and supine (which may better reflect real-life conditions), thus minimally disturbing the patients and avoiding the physiological changes in BP and IOP that occur when they rise from a supine to a standing position.
46 Furthermore, the fair agreement between GAT and tonometer IOP measurements should not be considered a major problem as the study design allowed the comparison of paired measurements always made at the same time: This may require a certain caution in interpreting the accuracy of the nocturnal IOP measurements, but should not have any impact on comparisons of the nighttime measurements in individual patients.