The Effect of Speed of Normal Saline Injection on Optic Nerve Sheath Diameter in Thoracic Epidural Anesthesia

Background: Intracranial pressure (ICP) is affected after epidural saline solution or local anesthetic injection. Both ICP and epidural pressures have been shown to reach peak pressure just after epidural injection and begin decline thereafter. Measuring the optic nerve sheath diameter (ONSD) through ultrasonography is one of the noninvasive methods used for ICP assessment. Objectives: The purpose of this study was to investigate the effect of the speed of epidural saline injection on the ONSD under awake conditions. Study Design: Prospective randomized trial. Setting: An interventional pain management practice in South Korea. Methods: This study included 40 patients receiving thoracic epidural catheterization for pain management after upper abdominal or thoracic surgery. Following successful epidural space confirmation, patients were randomized to receive epidural saline infusion with a speed of either 1 mL/second (slow speed, A group) or 3 mL/second (rapid speed, B group), respectively. For the measurement of ONSD, transorbital sonography was performed and ONSD was measured at 3 mm posterior to the optic nerve head. Results: The A and B groups showed significant increases in ONSD according to time. Post hoc analysis of this result revealed that ONSD at T10 and T30 were significantly increased from baseline values (T0) (*P < 0.05 vs. T0; +P < 0.001 vs. T0). The mean values at any of the time points and degree of changes (T1-T0, T10-T0, and T30-T0) in ONSD between groups A and B did not show any significance. Limitations: We could not confirm the time of normalization of ONSD after the end of epidural injection of normal saline. Conclusions: Thoracic epidural injection of 10 mL of normal saline solution resulted in a significant increase of ONSD compared to baseline, however, the speed of injection did not affect the increase of ONSD. Key words: Epidural, saline, optic nerve, diameter