Bedside optic nerve ultrasonography may be useful for measuring intracranial pressure

The test involves applying a layer of gel to the closed eyelid with the patient in supine position and measuring the optic nerve sheath.

Optic nerve ultrasonography at the bedside may be able to quickly and accurately diagnose increased intracranial pressure, according to a recent study.

Researchers reviewed studies published through May 2019 to determine the accuracy of optic nerve ultrasonography for diagnosing increased intracranial pressure in both adults and children. Optic nerve ultrasonography, also called optic nerve sheath diameter sonography, involves applying a layer of gel to the closed eyelid with the patient in supine position and measuring the optic nerve sheath using a linear array transducer 3 mm posterior to the orbit, usually in the coronal plane. Prospective studies of the diagnostic accuracy of this test were selected from 13 databases, as well as reference lists and meeting proceedings. The results of the systematic review and meta-analysis were published Nov. 19 by Annals of Internal Medicine.

Seventy-one studies involving 4,551 patients were eligible for inclusion. Of these, 61 included adults and 35 were considered to have a low risk for bias. Eighteen studies included patients with traumatic brain injury, 26 included patients with nontraumatic brain injury, and 27 included both. In patients with traumatic brain injury, the pooled sensitivity, specificity, positive likelihood ratio, and negative likelihood ratio of optic nerve ultrasonography were 97% (95% CI, 92% to 99%), 86% (95% CI, 74% to 93%), 6.93 (95% CI, 3.55 to 13.54), and 0.04 (95% CI, 0.02 to 0.10), respectively. For patients with nontraumatic brain injury, these values were 92% (95% CI, 86% to 96%), 86% (95% CI, 77% to 92%), 6.39 (95% CI, 3.77 to 10.84), and 0.09 (95% CI, 0.05 to 0.17). Estimates of accuracy were similar when stratified by patient age, operator specialty and training, reference standard, sonographer blinding, and cutoff value. The researchers found that 5.0 mm was the optimal cutoff.

Most of the included studies were small and had imprecise summary estimates, publication bias was possible, and effect of optic nerve ultrasonography on outcomes was not evaluated. However, despite these limitations, the authors concluded that optic nerve ultrasonography at the bedside is a noninvasive, quick, easy-to-use test. “A normal reading on optic nerve sheath ultrasonography (<5.0 mm) with high sensitivity and low negative likelihood ratio may rule out increased intracranial pressure, whereas an elevated reading (≥5.0 mm) with high specificity and positive likelihood ratio may indicate increased intracranial pressure and require confirmatory diagnostic tests,” they wrote.

The authors of an accompanying editorial stressed that the 5-mm cutoff described in the study should be interpreted and applied with caution, since the clinical significance of one bedside measurement is limited and cutoffs varied markedly in the 71 included studies. They agreed that bedside ultrasonography of the optic nerve sheath can decrease time to definitive management of brain injury but recommended that future studies should focus on developing a standardized, multidisciplinary approach to this test, as well as a cutoff that considers sex and other characteristics. “Once an agreed cutoff and protocol are determined, novice sonographer-clinicians could be supported by ultrasound experts for remote integration of bedside ultrasonography,” the editorialists wrote.