Optic Nerve Sheath Diameter Measurement by Ultrasound: Evaluation of a Standardized Protocol

BACKGROUND AND PURPOSE: Treatment of elevated intracranial pressure (ICP) is central to neurocritical care, but not all patients are eligible for invasive ICP-monitoring. A promising noninvasive option is ultrasound measurement of the optic nerve sheath diameter (ONSD). However, meta-analyses of ONSD for elevated ICP show wide confidence intervals. This might be due to baseline variations, inter-rater variability, and varying measurement methods. No standardized protocol has been validated. Corrections for eyeball diameter (ED) and optic nerve diameter (OND) may compensate for baseline variations. We evaluated a protocol and compared two different measurement methods for ONSD ultrasound. METHODS: Two operators, blinded to each other’s measurements, measured ONSD, ED, and OND twice in 20 patients. ONSD was measured with two different methods in use: internal (ONSDint) or external (ONSDext) of the dura mater. Intra-class correlation (ICC) was calculated for inter-rater and intra-rater reliability. RESULTS: ICCs for inter-rater reliability of ONSDext and ONSDint (95% confidence interval) were 0.96 (0.93, 0.98) and 0.88 (0.79, 0.94), respectively. ICCs for intra-rater reliability of ONSDext and ONSDint were 0.97 (0.94, 0.99) and 0.93 (0.87, 0.96), respectively. There was no significant bias or difference in intra-rater reliability between operators. CONCLUSIONS: ONSD can be measured with an excellent inter- and intra-rater reliability and low risk of inter-rater bias, when using this protocol. ONSDext yields a higher inter- and intra-rater reliability than ONSDint. Corrections for ED and OND can be performed reliably.