The CLOSED protocol to assess optic nerve sheath diameter using color-Doppler: a comparison study in a cohort of idiopathic normal pressure hydrocephalus patients

Abstract

Background: Sonographic assessment of the optic nerve sheath diameter represents a promising non-invasive technique for estimation of the intracranial pressure. A wide inter-observer variability, along with a lack of a standardized protocol for the optic nerve sheath diameter measurements, could lead to over- or under-estimation. The present study was aimed at evaluating feasibility of color-Doppler for better delineating optic nerve sheath borders, comparing it to B-mode imaging, using the magnetic resonance measurements as a comparison.

Methods: Optic nerve sheath diameters were evaluated using magnetic resonance by an expert radiologist in a cohort of patients with suspected idiopathic normal pressure hydrocephalus. Magnetic resonance findings were evaluated twice. In the first half of this cohort, optic nerve sheath diameters were measured using B-mode only, in the second half applying color-Doppler. Measurements obtained using these two techniques were compared to magnetic resonance imaging measurements. The Bland-Altman analysis and concordance correlation coefficient were computed to quantify the strength of agreement between the two magnetic resonance assessments. Box plots and average (± SD) were used to compare assessments by sonographic and magnetic resonance methods.

Results: Fifty patients were included. MRI assessment showed a moderate concordance correlation coefficient. Optic nerve sheath diameters measured applying color-Doppler were lower (p < 0.001) and less scattered compared to B-mode assessment, which approached more to magnetic resonance measurements.

Conclusions: In this cohort of patients, magnetic resonance showed high intra-rater variability in optic nerve sheath diameter assessments. Optic nerve sheath diameter assessments using color-Doppler yielded lower and less scattered diameters compared to B-mode only.

Keywords: Hydrocephalus; Intracranial pressure; Neurocritical care; Optic nerve sheath diameter; Point-of-care ultrasound.

Fig.1

Three ultrasound images of the eyeball and optic nerve with sheath, enriched by the parameters/landmarks highlighted in CLOSED protocol. a B-mode image in gray-scale: dashed white arrow points to the optic disc, in yellow window 4.3 mm ONSD measurement, in red window acoustic power output = 20%; b same image but with color-mode: white arrow points to the lens, in yellow window 3.9 mm ONSD measurement, in red window PRF 940 Hz (almost 1 Kh), yellow arrow points to central retinal artery; c same image but with color-mode plus Doppler wave (at the bottom): in red window Gain 50%, yellow arrow points to ophthalmic artery, in the enlarged panel the typical nock of the ophthalmic artery flow

Fig. 2

Bland–Altman plots displaying the intra-rater agreement for neuroradiologist with two sets of measurement in magnetic resonance imaging. The values in the reported axes are in millimeters. a Average difference (violet line): − 0.165; 95% limits of agreement (red lines): − 1.216, 0.886. b Average difference (violet line): − 0.112; 95% limits of agreement (red lines): − 0.944, 0.721. c Average difference (violet line): − 0.190; 95% limits of agreement (red lines): − 1.022, 0.642. d Average difference (violet line): 0.154; 95% limits of agreement (red lines): − 1.080, 0.772

Fig. 3

The values in the reported axes are in millimeters. a Color-Doppler measurements are less scattered and significantly lower than those in B-mode. b MRI vs B-mode (without Doppler) in 27 patients; c MRI vs color-Doppler mode in 23 patients; box plots show a clear difference in comparison between the MRI and the two US modalities, highlighting an evident difference in all the analyzed diameters. SDL sagittal diameter left, SDR sagittal diameter right, TDL transverse diameter left, TDR transverse diameter right

Fig. 4

Cadaver endoscopic anatomical exemplificative view. It is possible to appreciate how in the distal segment of the optic nerve (ON), contiguous to the posterior portion of the eyeball, the perineural vessels surround the sheath together with the fat. Instead, in the proximal segment, the optic nerve is contiguous to the ophthalmic artery (OA), which runs parallel to the internal carotid artery (ICA)