Background: Ultrasound (US) measurement of optic nerve sheath diameter (ONSD) has been widely used as a diagnostic predictor of an increased intracranial pressure. Measuring the cross-sectional area of the ONS is subject to a substantial variability depends on the location of ONSD that measured, anatomical variations, ultrasound scanning method and presence of artifacts.
The purpose of this study was to measure different ultrasound methods for quantification of ONSD and compare with the orbital MRI to analyze ONS dimensions.
Methods: In patients who needed an emergent MRI, an orbital US was performed by an EM faculty, resident or fellows trained on ocular US protocol. US measurements were included ONSD in axial and coronal views in straight gaze for both eyes.
Brain MRI images with an additional orbital coronal view (T2 -FS sequence) were obtained and analyzed twice by two different neuro-radiologists. Neuro-radiologists quantified ONSD in 5 coronal and axial ONS dimensions. The measurements were placed at 1-2.9 and 3-5.9 mm behind the globe. The median of three measurements was used to compare with two US measurement of ONSD. Correlation and agreement between different readings was assessed using Pearson’s correlation.
Methods: 132 ONSD measurements by US and MRI were compared in 66 patients with a mean age of 53 years. T2-FS orbital views with coronal images was available in 86 exams (43 patients). MRI yielded mean axial ONSD of 6.2 mm at a position 1-2.9 mm behind the eye and declined to 5.6 mm at a position 3-5.9 mm. A mean coronal measurement of ONSD was 5.7mm at 1-2.9 mm behind the globe. The US finding includes a mean ONSD of 4.9 mm in axial view (at 3 mm behind the globe) and 3.7mm in the coronal view.
Compared with MRI readings, US showed a weak correlation (r= -0.058) in 132 axial measurements and consistently yield smaller diameter (US/MRI ratio=0.8 in axial and 0.65 in coronal). Comparing coronal ONSD in US and MRI showed a weak correlation in both right and left eyes. (r=0.104).
Conclusion: The study shows a high variability in US measurements of ONSD in axial and coronal views. US persistently underestimate the actual measurement of ONSD. Comparing US measurement to MRI as standard criterion failed to show a strong correlation.