We thank Drs Sodhi and Chawla for their interest in our recent article.
First, we agree with them in that a study such as ours should include nocturnal variability as well as diurnal variability on blood pressure and ocular perfusion pressure by performing 24-hour study on intraocular pressure (IOP) and blood pressure. In the current study, we have done so and provided data for the nighttime as well as daytime period. Our group previously showed that increased 24-hour (long-term) variability of mean arterial pressure or mean ocular perfusion pressure was a consistent risk factor for normal-tension glaucoma (NTG) development and progression. In the current study, however, we were interested in knowing an independent effect of short-term (daytime or nighttime) variability of mean arterial pressure or mean ocular perfusion pressure on visual field progression in untreated NTG patients during follow-up using 24-hour blood pressure and IOP data in the habitual body position. Our results showed that increased daytime mean arterial pressure or mean ocular perfusion pressure standard deviation was a significant predictor of subsequent visual field progression using the Cox proportional hazards models ( P = .023 and P < .001, respectively). Therefore, we believe that the title of our article appropriately reflects the outcome of the current study and importance of increased daytime blood pressure or ocular perfusion pressure variability as a risk factor for future VF progression in untreated NTG patients.
Drs Sodhi and Chawla recommended the description of difference between daytime and nighttime variability of blood pressure and ocular perfusion pressure in the Results section in order to emphasize that daytime variability is as significant as nocturnal dip in our study. In the current study, we presented both daytime and nighttime variability data of blood pressure and ocular perfusion pressure in Table 1. Upon comparison, their values were similar, without statistical significance, between daytime and nighttime ( P > .05, paired t test), indicating that indeed daytime variabilities of blood pressure and ocular perfusion pressure were as large and significant as nighttime variabilities.
Regarding the question of whether or not study subjects went to sleep during daytime while undergoing 24-hour IOP and blood pressure measurements, our study design required that admitted NTG patients stayed alert and continued normal indoor activities without going into sleep throughout the daytime period. During the daytime period, IOP was measured while patients were in the sitting position only. Therefore, we believe that daytime variability data of blood pressure or ocular perfusion pressure was not affected by the act of sleep, which can not only reduce blood pressure by way of a variety of changes in autonomic and endocrine functions but also raise IOP by way of posture change.