Written by Chris Thom
Spoon Feed
In this retrospective study on optic nerve sheath diameter (ONSD), the sagittal measurement was statistically disparate from the transverse measurement, with the authors recommending the transverse approach.
Demystifying the ONSD
This was a retrospective post-hoc analysis of 213 ICU patients who received ONSD measurements with ocular ultrasound. The objective was to assess agreement between transverse and sagittal approaches to ONSD measurement, as well as agreement between measurement approaches both external and internal to the dura mater (see below GIF). In addition, diagnostic accuracy of the various approaches for ICP > 20 mmHg was calculated using the 139 patients with ICP monitoring in place.
The sagittal ONSD measurement was approximately 0.2 mm larger than the transverse ONSD measurement in this patient cohort (p<0.001), including both the external and internal dura measurements. Agreement between transverse and sagittal measurements within the same patient was poor. The optimal accuracy cutoff for external dura transverse ONSD measurements was 6.9 mm in men and 6.5 mm in women.
How will this change my practice?
This study highlighted many of the common misconceptions about ONSD measurements, pointing out the resultant discrepancies in the literature. It appears that the sagittal and transverse approaches are not interchangeable, and I agree with the authors that it makes sense to use the transverse approach. Furthermore, we should attempt to be more consistent with the “external to the dura” versus “internal to the dura” measurement approach. The dark band that is separate but adjacent to the optic nerve is the dura. I would recommend including this in your measurement, and it would be helpful if future studies specify this detail. However, the old adage of 5 mm or even 5.5 mm cutoffs may not be ideal, as the inclusion of the dura mater likely will push the ideal cutoff higher, as it did in this study.
POCUS pro tips and clips
Ocular ultrasound gives us high-quality images without much difficulty, considering the superficial nature of the scan and the lack of overlying gas or bone. While there is some evidence of slight image degradation with a Tegaderm, I find this helps with cleanliness, so I still tend to use one. Use plenty of gel, then balance your hand on the nasal bridge so you can keep the image stabilized and use the correct amount of pressure. Focus first on the anterior chamber and the lens, followed by the posterior chamber, and then the ONSD posterior to this. If you are not seeing the ONSD, sweep the probe cephalad and caudal. You can also have the patient look slowly to the left and the right to “uncover” the ONSD if it was tucked off to the side of the imaging field due to lateral gaze.
Source
Optic Nerve Sheath Diameter: Which Axis Should Be Measured? J Neuroimaging. 2025 Jul-Aug;35(4):e70076. doi: 10.1111/jon.70076. PMID: 40754891; PMCID: PMC12319288.
Curious if and how people are utilizing ONSD to influence clinical decisions in the ED?
The IIH use case seems like one of the most useful ones. Perhaps someone told them they had migraines and now they’re bouncing in again for another HA evaluation. The presence of optic disc elevation or significant ONSD dilatation can pretty quickly confirm that there might be more at hand than just a “migraine”. And the ocular US findings in IIH are usually not subtle, which is helpful
Very cool. Thank you!