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Ultrasound Measurement of the Optic Nerve Sheath Diameter in the Parturient
Abstract Number: T2C-149
Abstract Type: Original Research
Introduction: The optic nerve sheath (ONS) is a flexible tube contiguous with the sclera and dura mater. ONS diameter (ONSD) correlates with intracranial pressure (ICP) due to continuity with the subarachnoid space (1). ONSD can be measured on ultrasound (US) and correlates with direct measurements of ICP by invasive monitoring (2) and with head computer tomography findings of increased ICP (3). Thus, ONS US is a noninvasive tool for identifying patients with elevated ICP. Multiple studies have identified an ONSD of >5 mm as a sensitive threshold value for an ICP>20 mmHg. Changes in ICP may occur in pregnancy, and ONS US may be useful for detecting and monitoring these changes. However, normal values in pregnancy are unknown. In this study, we aimed to establish normal values for both antepartum and postpartum ONSD on US in healthy term parturients.
Methods: 20 women undergoing scheduled cesarean delivery with neuraxial anesthesia enrolled. A small 12 MHz linear transducer was placed over the closed eyelid of each eye, the ONS was identified, and the best possible images were stored for measurement (Fig 1a). For each participant, a total of 12 antepartum ONSD measurements were taken (3 of each eye in both the seated and supine positions). These measurements were repeated at least 24 hours postpartum. The Shapiro-Wilk test was used to assess the normality. Independent-samples t-test and Mann-Whitney U test were used to compare outcomes, as appropriate.
Results: Chronbach’s alpha (0.93) showed a high level of intrapatient consistency within each set of 3 measurements. No significant difference in means between left and right ONSD was seen in any data set. The median ONSD in the antepartum supine position was 5.29 mm [0.485-0.565]. All median values were above 5.23 mm, and no significant difference was found when comparing Supine to Sitting (p=0.821), Supine to Post-Partum Supine (p=0.794) and Supine to Post-Partum Sitting (p=0.965) (Fig 1b).
Conclusions: Measurement of ONSD by US can be used with high levels of intrapatient reliability. There is no statistically significant difference in ONSD from pre- to post-partum in our interim analysis, which suggests there may be a single range of normal for the entire peripartum period. Median values for healthy parturients appear to exceed the previously established threshold for elevated ICP.
1. Surg Radiol Anat 1996; 18(4):323-8.
2. Acad Emerg Med 2008; 15:201-204.
3. Ann Emerg Med 2007; 49:508-14.