Table 1.

Proposed sonographic quality criteria for optic nerve sheath diameter (ONSD) measurements (adapted from Sargsyan et al. [21]).

Sonographic quality criteria for optimization of ONSD measurements
(i) ONSD measurement should not be made through the lens (even the edge of the lens may not be visible on the image).
(ii) Sonographic differentiation (contrast) between the nerve proper and the arachnoid (cerebrospinal fluid space) must be obvious; measuring a “dark stripe” behind the globe without nerve and arachnoid differentiation is not acceptable.
(iii) The outer border of the arachnoid must be identifiable for actual ONSD measurement; clear, well-focused images must thus allow confident measurement of the inner diameter of the dural sheath.
(iv) Ideal views of the optic nerve demonstrate the point of its penetration into the globe, that is, “dark meets dark” (nerve meets vitreous without interposition of thick echogenic layer of the posterior sclera).
(v) Good views offer opportunities for additional information potentially useful with growing experience, such as tortuosity of the nerve, hypoechogenicity of the arachnoid, and its irregularity; this also allows seeing the optic disk area protrusion into the globe and flattening of the posterior globe in chronic ICP elevations (premorbid) that may mimic acute states in ICU.
(vi) Correct standardized measurements: since the most distensible portion of the sheath is at the 3-4 mm distance from the vitreoretinal interface, measurements are performed at this level in a direction perpendicular to the axis of the nerve.
(vii) It is highly recommended to measure ONSD bilaterally and in more than one image frame. This is an important quality assurance mechanism.
(viii) For ONSD trend monitoring, the previous record with images must be reviewed to ensure similar views and measurement technique. Prior images should be available at bedside (from the machine or in printout) for reference. ONSD measured in sagittal planes should not be compared with ONSD from axial planes.