Optic nerve sheath diameter & intra-cranial pressure

Measurement of the optic nerve sheath diameter by ultrasound is a promising non-invasive technique that may help detect intra-cranial pressure. Many studies demonstrate that this measurement appears to demonstrate a high degree of diagnostic accuracy including sensitivity and specificity in the detection of elevated intra-cranial pressure (1-7). There are caveats, however, and we encourage you to peruse the material and watch the tutorial to get a more complete sense of this important tool.

This page and video tutorial will explore this measurement, how it is performed, and the current controversies.

WHy the optic nerve sheath?

The optic nerve emerges from the posterior orbit and is wrapped by a layer that derives from the meninges called the optic nerve sheath. Between the optic nerve and the sheath itself is a small circumferential compartment of cerebrospinal fluid that is contiguous with the intra-cranial subarachnoid space. As intra-cranial pressure increases, so does the optic nerve sheath, giving us potential insight into the brain.

How to perform

A high-frequency linear probe is used for this exam. Ensure the mechanical index (MI) and thermal index (TIS) are as low as reasonably achievable (ALARA). This exam should be done quickly to minimize the amount of time the probe is spent on the eye. Copious amounts of gel should be used with/without a tegaderm. While the gel is not injurious to the eye, it can sting. The hand should be braced to minimize pressure on the eye itself. Measurements should be done in duplicate in both transverse and sagittal.

Measurement

The following image is a classic clip of a globe with the optic nerve and sheath emerging posteriorly. An electronic caliper is first placed 3 mm behind the papilla (red). A second caliper (White) is placed directly across, at 3 mm, from the surrounding edges of the nerve sheath. The cut-off threshold is ~5 mm for the upper-limit of normal. A 5.5 mm cut-off is less sensitive, but likely offers greater specificity. Again, measurements should be done in two axis in both eyes.

Potential false positives

Here is a list of potential false positives that may lead to an increased optic nerve sheath diameter, potentially without corresponding elevations in intra-cranial pressure. We would encourage you consider underlying medical and/or neurological illnesses in your approach to this measurement. Many of these possibilities will require neurological and/or opthalmological consulation.

ONSD & ICP tutorial

In this tutorial we will breakdown measurement of the optic nerve sheath. We will also explore pitfalls and controversies as it applies to measurement of the optic nerve sheath including the measurement itself.

References

  1. Kimberly HH, Shah S, Marill K, Noble V. Correlation of optic nerve sheath diameter with direct measurement of intracranial pressure. Acad Emerg Med. 2008;15(2):201-204.
  2. Koziarz A, Sne N, Kegel F, Nath S, Badhiwala JH, Nassiri F, et al. Bedside Optic Nerve Ultrasonography for Diagnosing Increased Intracranial Pressure. Ann Intern Med [Internet]. 2019 Nov 19
  3. Rajajee V, Vanaman M, Fletcher JJ, Jacobs TL. Optic nerve ultrasound for the detection of raised intracranial pressure. Neurocrit Care. 2011;15(3):506-515.
  4. Fernando SM, Tran A, Cheng W, Rochwerg B, Taljaard M, Kyeremanteng K, et al. Diagnosis of elevated intracranial pressure in critically ill adults: Systematic review and meta-analysis. BMJ. 2019;366:1–24.
  5. Bäuerle J, Schuchardt F, Schroeder L, Egger K, Weigel M, Harloff A. Reproducibility and accuracy of optic nerve sheath diameter assessment using ultrasound compared to magnetic resonance imaging. BMC Neurol. 2013;13. 
  6. Ohle R, McIsaac SM, Woo MY, Perry JJ. Sonography of the optic nerve sheath diameter for detection of raised intracranial pressure compared to computed tomography: A systematic review and meta-analysis. J Ultrasound Med. 2015;34(7):1285–94.​
  7. Wang L, Chen L, Chen Y, et al. Ultrasonography Assessments of Optic Nerve Sheath Diameter as a Noninvasive and Dynamic Method of Detecting Changes in Intracranial Pressure. JAMA Ophthalmol. 2018:1-7.