Sound Rounds

Sound Rounds 1 – Optic Nerve Sheath Diameter

In this first published edition of the Duke EUS Sound Rounds, our very own chief resident Dr. Brendan Flanagan gave us a thorough overview of the literature behind using POCUS to evaluate optic nerve sheath diameter (ONSD).

Maker:L,Date:2017-8-14,Ver:5,Lens:Kan03,Act:Kan02,E:Y
Dr. Brendan Flanagan making POCUS look almost too easy

What We’re All Taught

We’re classically taught that because the coverings of the brain and optic nerve are contiguous, elevated pressure in the brain should be reflected in changes in the optic nerve, which is visible by way of ophthalmoscopy – hence the practice of looking for papilledema as a secondary sign of increased intracranial pressure:

Mild papilledema on the left; severe papilledema on the right. Images linked from EyeWiki via the American Academy of Ophthalmology.

However, if you find yourself inadequately practiced at visualizing the retina (not that any of us would know anything about that…), you might find yourself desperately searching for an alternative method of non-invasively diagnosing elevated intracranial pressure. Enter POCUS, stage right…

Ultrasound Method

fig2-optic-nerve-sheath-diameter-onsd-in-a-healthy-subject-the-measurement-is-taken-3
Wilson, Stephen & Mackay, Andrew. (2012). Ultrasound in critical care. Continuing Education in Anaesthesia, Critical Care & Pain. 12. 190-194.

As it turns out, the eye is essentially nothing more than a naturally-formed water bath designed specifically to allow for ultrasonographic visualization of the retina (it’s true – look it up). Again, traditional teaching will tell us that using POCUS to measure the width of the optic nerve sheath is a reliable way of determining if someone has elevated intracranial pressure or not. This is typically done by using a linear high-frequency probe to obtain an anterior-posterior view through the center of the retinal nerve, which appears as a relatively hypoechoic structure projecting posteriorly from the retina. The width is measured parallel to the retina, and the standard measurement is performed 3 mm back from that point. The most common upper limit of normal is quoted as 5 mm across, with anything greater than that being considered abnormal. “But how do we know that’s the normal measurement?” you might ask at this point. Well, that’s an excellent question…

The Literature

After parsing through a substantial portion of the world’s literature, Dr. Flanagan was able to provide us with the following tables, summarizing the actual data behind ultrasound of the ONSD:

table1.png
Table 1: Normal values derived from healthy control group in each study.
table2.png
Table 2: Average ONSD for both controls and patients with known elevated ICP, as well as calculated test characteristics for authors’ recommended cutoff.

Conclusion

The take-away from the above tables seems to be that there is a much greater variability in ONSD, even among normal healthy controls, than most practitioners of POCUS probably realize; and a better method of interpreting the width might be:

< 5 mm: Normal

> 6 mm: Abnormal

5-6 mm: Indeterminate

And with that, we conclude the first edition of Sound Rounds. A huge thanks to Dr. Flanagan for his hard work on this presentation.

Thoughts? Opinions? Disagreements? Join the #FOAMus conversation in the comments below!

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