Assessment of Elevated ICP Using Optic Nerve Sheath Diameter
You’re in the middle of a TCC shift when you receive a patient with profound altered mental status from EMS. AFter intubating the patient (with succinylcholine) for airway protection, you whisk her away to the CT scanner. CT reveals a large intracranial hemorrhage with midline shift. Neurosurgery comes to see the patient but reports they cannot get a “reliable exam” because the patient received succinylcholine one hour ago. They request opthalmology evaluation of the optic nerve to assess for increased intracranial pressure prior to placing a bolt. Not wanting to wait for another consult, you wonder if there is a better way to assess increased ICP. How accurate and reliable is bedside ultrasound ocular nerve sheath diameter to assess increased ICP?
Population: Adult or pediatric patients with suspected increased intracranial pressure
Intervention: Ultrasonographic measurement of optic nerve sheath diameter
Comparison: Invasive monitoring of intracranial pressure, CT signs of elevated intracranial pressure, CT measurement of optic nerve sheath diameter
Outcome: Diagnostic accuracy (sensitivity, specificity, likelihood ratios)
Pubmed was searched using the terms ultrasound, optic nerve sheath diameter; and intracranial pressure, resulting in 170 articles. The reference sections of these articles were searched for additional citations, and the four most relevant articles were chosen.
Article 1: Dubourg J, Javouhey E, Geeraerts T, Messerer M, Kassai B. Ultrasonography of optic nerve sheath diameter for detection of raised intraranial pressure: a systematic review and meta-analysis. Intensive Care Med. 2011 Jul;37(7):1059-68
Article 2: 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 Jul;34(7): 1285-94
Article 3: Komut E, Kozaci N, Sonmez BM, Yilmaz F. Komut S, Yildirim ZN, Beydilli I, Yel C. Bedside sonographic measurement of optic nerve sheath diameter as a predictor of intracranial pressure in ED. AM J Emerg Med. 2016 Jun;34(6): 963-7
Article 4: Hassen GW, Bruck I, Donahue J, Mason B, Sweeney B, Saab W, Weedon J, Patel N, Perry K, Matari H, Jaiswal R, Kalantari H. Accuracy of optic nerve sheath diameter measurement by emergency physicians using bedside ultrasound. J Emerg Med. 2015 Apr;48(4):450-7
In cases of suspected life-threatening elevated intracranial pressure (ICP), the current gold standard for monitoring ICP is via ventricular catheter placement. This method, while often necessary, requires neurosurgical expertise and is invasive, carrying with it the risk of hemorrhage and infection (Wilberger 1997). Beside ultrasonographic measurement of optic nerve sheath diameter (OSND) has been proposed as a noninvasive surrogate for evaluation for increased ICP. Such a method would be easily accessible to the emergency physician and would eliminate the risks of invasive measurement.
Several studies have looked at the accuracy of ONSD measurement by bedside ultrasound. We reviewed on study that compared ONSD measurement by ultrasound to measurement by CT scan (Hassen 2015). While this study demonstrated high correlation (within 0.5 mm in 58 of 61 cases) it primarily consisted of patients with normal ICP undergoing CT scan for other reasons. Additionally, the authors do not make it clear whether ultrasound measurement was performed by the bedside operator; or if it was determined later by a more highly skilled physician with ultrasound certification.
We reviewed an additional observational study comparing ONSD measurement to CT findings (Komut 2016). In this study, the authors enrolled adult patients with suspected non-traumatic intracranial event. Using a cutoff of 4.7 mm, bedside ONSD measurement had sensitivity and specificity of 70% and 86%, respectively (LR+5.0. LR-0.35) for the presence of CT abnormality. Unfortunately, rather than looking for signs of elevated ICP, the authors considered any CT abnormality as a positive CT. The diagnostic accuracy reported, as evidence by the likelihood ratios, was not very helpful, likely in part due to this unusual outcome measure.
We also reviewed two systematic reviewed on this topic. One of these looked at studies comparing ONSD to signs of increased ICP on CT scan (Ohle 2015), defined somewhat more appropriately as evidence of shift, collapse of ventricles, or intracranial bleeding of >2 mm. Using CT as the reference standard ONSD had a positive LR of 12.5 (95% CI 4.2-37.5), and negative LR of 0.05 (95% CI 0.016-0.14), indicating a high degree of accuracy for both ruling in and ruling out elevated ICP. The study was limited by the use of CT findings as a surrogate marker for elevated ICP and by a great deal of methodological and clinical heterogeneity between included studies.
Perhaps the best evidence to date on this topic comes from a systematic review and meta-analysis comparing ONSD with direct intraparenchymal or intraventricular ICP monitoring (Dubourg 2011). Pooling the results of the 6 inclded studies, the authors report a positive LR of 6.11 (95% CI 3.28-11.37) and a negative LR of 0.12 (95% CI 0.06-0.23), suggesting the ultrasonographic ONSD measurement only moderately predicted elevated ICP, but was mroe accurate at ruling it out. Unfortunatley, this meta-analysis only consisted of a total 231 patients, making it somewhat poorly powered.
The evidence presented does seem to suggest that ONSD measurement by bedside ultrasound is fairly accurate, with some limitations to the included studies, and may be helpful at predicting signs of increased ICP on CT scan. Whether this information would be clinically useful, and whether these CT findings accurately predict life-threatening elevated ICP is still quite uncertain. Most importantly, the final meta-analysis suggests that bedside ONSD can be used to rule out elevated ICP with high accuracy, though again, the clinical utility of this has yet to be shown.