Written by Chris Thom
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Ocular ultrasound on healthy volunteers found a small decrease in optic nerve sheath diameter (ONSD) in females vs males and in the sitting position vs. supine.
ONSD remains an imperfect measure of intracranial pressure
This study enrolled 25 healthy volunteers who had ocular ultrasound performed by two separate investigators, for a total of 50 ocular examinations. Ultrasound evaluations included the standard ONSD measurement occurring 3 mm posterior to the retina in the transverse plane, as well as doppler evaluations of the ophthalmic, retinal, and posterior ciliary arteries. Interrater reliability for these measurements was assessed via the intraclass correlation coefficient (ICC). A subset of 20 ocular examinations were also performed in the seated upright position to compare measurements between supine and sitting positions.
The mean ONSD measurement was 5.93 mm (SD 0.58) and 5.85 mm (SD 0.54) for the two investigators, which gave an excellent ICC of 0.87. Female gender was noted to be associated with a 0.4 mm decrease in ONSD diameter (p=0.015). ONSD measurement in the sitting position was 0.2 mm lower compared to the supine position (95%CI 0.1-0.3, p < 0.001). ICC for ophthalmic and retinal artery indices was high, with lower ICC noted for posterior ciliary artery indices.
How will this change my practice?
Ocular ultrasound ONSD measurement for ICP remains an inexact science. The correct cutoff has been a moving target in the literature and a standard technique for reproducible measurements is not always employed. This can lead to lower than needed interrater reliability (1). The current study lacks generalizability given the two investigators, but it does prove the point that high interrater reliability for ONSD is indeed possible. It also adds to the literature that suggests a higher cutoff closer to 6 mm might lead to improved accuracy over the often discussed 5 mm. Interestingly, a small decrease in average ONSD was seen in the sitting position as compared to supine. The absolute difference there is small, but does provide evidence that we should perhaps consider patient position and gender when establishing the correct cutoffs for normal versus abnormal.
Pro-tips and clips
Ocular ultrasound is generally easy and affords high quality ultrasound imaging. One can use a bioocclusive dressing over the eyelid to keep the eye free from gel, but the gel is not harmful to the eye, and there is some evidence that imaging quality might be slightly reduced due to the dressing (2). The ONS is identified just posterior to the globe and one can ask the patient to look gently left and right to maximize visualization and center the ONS. Keep in mind that the optic nerve (ON) is surrounded by a sheath of CSF, which can appear as hypoechoic lines just adjacent to the ON. Measure the entirety of the ONS 3 mm posterior to the retina by including this hypoechoic area in the measurement.
Source
The impact of demographics and positioning on the imaging features of the optic nerve sheath and ophthalmic vessels. Ultrasound J. 2025 Jan 16;17(1):6. doi: 10.1186/s13089-025-00403-x. PMID: 39821587
Additional References
- Oberfoell S, Murphy D, French A, Trent S, Richards D. Inter-rater Reliability of Sonographic Optic Nerve Sheath Diameter Measurements by Emergency Medicine Physicians. J Ultrasound Med. 2017;36(8):1579-1584.
- Marks A, Patel D, Chottiner M, Kayarian F, Peksa GD, Gottlieb M. Covered or uncovered: A randomized control trial of Tegaderm versus no Tegaderm for ocular ultrasound. Am J Emerg Med. 2022;61:87-89.
