The optic nerve sheath, contiguous with brain dura and containing CSF communicating with cerebral subarachnoid components, can be used as a means of indirectly detecting increased ICP.
Optic nerve is ontogenetically a part of the CNS. It is surrounded by CSF and dura mater (called the optic nerve sheath or ONS). The diameter of the optic nerve sheath (or ONSD) changes with variations in intracranial pressure.
Ultrasound measurement of the ONSD is a reliable means of detecting elevated ICP in patients with spontaneous intracerebral hemorrhage. However, ONSD measurements are still not considered as a substitute for invasive ICP monitoring in critical care.
By measuring the anterior part of the optic nerve, specifically 3 mm behind the globe, ONSD can be measured via ultrasound with 5-mm ONSD roughly translating to an ICP of 20.
- use a linear ultrasound probe (7.5-MHz) over the upper closed eyelid
- keep HOB 30-45 degrees
- measure the ONSD 3mm behind the globe (take 2 measurements for each optic nerve sheath, sagittal and transverse)
ONSD cut off for elevated ICP:
- 5.7 mm [Sn 93%, Sp 96%]
- 5-5.7 mm [Sp 83%]
- Inter/intraobserver not a limiting factor
- ONSD variation dependent on individual factors such as age and underlying pathology
- difficult to create an absolute ONSD cutoff value for ICP crises
Girisgin, A. S. et al. ‘The Role Of Optic Nerve Ultrasonography In The Diagnosis Of Elevated Intracranial Pressure’. Emergency Medicine Journal 24.4 (2007): 251-254. Web. 23 Oct. 2015.
Moretti, Riccardo et al. ‘Reliability Of Optic Nerve Ultrasound For The Evaluation Of Patients With Spontaneous Intracranial Hemorrhage’. Neurocritical Care 11.3 (2009): 406-410. Web. 23 Oct. 2015.
Roh, David and Soojin Park. “Brain Multimodality Monitoring: Updated Perspectives”. Current Neurology and Neuroscience Reports 16.6 (2016): n. pag. Web.