Detecting Elevated ICP with Optic Sheath Nerve Diameter
Written by Dr. Emily Harkins. Faculty reviewer Dr. Phil Chan.
Case
A 33-year-old woman with history of migraines presents to the emergency department with a headache for 72 hours. She has associated photophobia and nausea. She has a benign neurological exam and no focal neurological findings. She said that a physician she saw months ago said something about testing her for pseudotumor, but she wasn't able to follow up due to insurance issues. You begin a fundoscopic exam, but she becomes tearful, stating that the light makes her headache much worse, and then vomits. You need a plan B.
Clinical Question
How can you assess for elevated intracranial pressure in an extremely photophobic patient?
Background
Optic nerve sheath diameter (ONSD) can be accurately measured via point of care ultrasonography by trained emergency physicians. Elevations in the size optic nerve sheath are strong indicators of increased intracranial pressure. To measure the ONSD, grab the linear probe, lay the patient down, have them close their eyes, and apply ultrasound lube to the eyelids. I prefer to place a Tegaderm over each eye to prevent the jelly from causing irritation, but this is not strictly required as ultrasound gel is safe even in the eyes. Being careful not to press down on the eye, have the patient look straight ahead. Scan all the way through both eyes in sagittal and transverse planes. The optic nerve will be visible at the back of the eye as a long, linear shadow extending away from the probe.
To calculate the ONSD, place a marker on the ultrasound screen 3mm behind the posterior edge of the globe, approximately at the center of the optic nerve shadow. This is where changes due to increased ICP are the most notable and easiest to measure reliably. Then use the calipers to measure across the sheath at the level of your marker, perpendicular to the nerve's course. Performing this measurement three times and then taking the average will improve accuracy. Normal ONSD values are </=5.0mm in adults, </=4.5mm in children, and </= 4mm in infants.
A group at the University of Michigan evaluated ONSD in their Neuro-ICU for detecting elevated ICP. They correlated ONSD with intracerebral pressure as monitored by invasive devices to determine if ultrasound was a reliable and accurate modality. In this study, ONSD greater than 0.48cm had 96% sensitivity for detecting ICP >20mmHg as measured at the same time by an invasive device, and 94% specificity. The 65 patients included in this study suffered from subarachnoid hemorrhage, traumatic brain injury, intracerebral hemorrhage, stroke and brain tumor. ONSD measurements are reliable for detecting increased cerebral pressure because the subarachnoid space is in continuity with the sheath, so increased ICP causes a dilation in the optic nerve sheath before any changes in the optic disc occur. Papilledema takes minutes to days to develop in the setting of elevated ICP, while ONSD distension will occur within seconds—so ultrasound is not just reliable, it measures change faster!
Normal OSND of 3.6 mm. Image courtesy of Dr. Deborah Shipley Kane
Large OSND of 5.6 mm, suggesting elevated ICP. Image courtesy of Dr. Deborah Shipley Kane
A meta-analysis by Doubourg et al compiled six studies investigating the accuracy of ONSD ultrasounds to detect elevated ICP. Doubourg et al corroborated the reliability of ONSD measurements for detecting elevated ICP. Pooling the sensitivity among these studies, the authors found that it was more likely 90%, as compared to the 96% found by the folks at UM. The cut-offs of normal ONSD ranged among these studies from 5.0mm to 5.9mm, and each was corroborated with an invasive ICP measuring device[RA2] . The total pooled study population was 231 patients, all greater than 18 years of age. Of note, all of the patients enrolled in this meta-analysis had experienced TBI, intracranial hemorrhage, or stroke.
The next question, after verifying that it works, is whether I can do it or if only the specially trained neuro-intensivists get reliable results. A handful of physicians at SUNY Downstate evaluated whether EM physicians would be able to accurately measure ONSD, and their results were promising. They had 7 participants, ranging from PGY1 to attending, train [RA3] with a registered diagnostic medical sonographer, and all scans were performed with the 7.5-10 MHz linear array. Staffing prevented them from being able to repeat ONSD measurements between researchers, so no interrater reliability was calculated. The authors then compared the ultrasound measurement to the CT scan measurement. The agreement was outstanding, but they only enrolled 61 patients, and all but 4 of them had normal ONSD.
The biggest take-away points are that ultrasound works to accurately measure optic nerve sheath diameter, which is a better indicator of increased ICP than papilledema, and that a well-trained EM physician can accurately measure ONSD at bedside. This is truly great news for the funduscopically impaired, and for photophobic patients.
Case Conclusion
Bedside ultrasound for ONSD found that our patient's sheath diameter was within normal limits. She was treated with a special migraine cocktail, after which she reported complete relief of her symptoms. She was discharged home with re-scheduled neurology follow up.
References
1. Dubourg J, Javouhey E, Geeraerts T, Messerer M, Kassai B. Ultrasonography of optic nerve sheath diameter for detection of raised intracranial pressure: A systematic review and meta-analysis. Intensive Care Med. 2011;37(7):1059-1068.
2. Hassen GW, Bruck I, Donahue J, et al. Accuracy of optic nerve sheath diameter measurement by emergency physicians using bedside ultrasound. J Emerg Med. 2015;48(4):450-457.
3. Walker, Richard A., and Srikar Adhikari.. "Eye Emergencies." Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 8e Eds. Judith E. Tintinalli, et al. New York, NY: McGraw-Hill, 2016, http://accessmedicine.mhmedical.com.beckerproxy.wustl.edu/content.aspx?bookid=1658§ionid=109444274.
4. Rajajee, V., Vanaman, M., Fletcher, J.J. et al. Neurocrit Care (2011) 15: 506. https://doi-org.beckerproxy.wustl.edu/10.1007/s12028-011-9606-8