Recently Kim et al studied ONSD responsiveness to two levels of EtCO2 ,40 and 50mmHg, each measured at 1 and 5min and they observed significant changes in the diameter of ONSD. Thus their results are at variance with our study. Various possibly reasons for differences in findings are Kim et al studied a small number of patients as opposed to a relatively larger sample size in our study. Also they used total intravenous anesthesia combining propofol and remifentanil infusion. This combination is more likely to predispose patients to systemic hypotension which in turn would increase ICP by causing cerebral vasodilation. Other possibility for different results may be that sonographic measurements of ONSD may vary by the observer's skills or even type of ultrasound device.