Optic Nerve Sheath ultrasound

David Goldemund M.D.
Updated on 11/03/2024, published on 25/01/2024

the optic nerve sheath (ONS) is an anatomical extension of the dura mater, and the subarachnoid space around the optic nerve is continuous with the intracranial subarachnoid space
any pressure rise within the intracranial compartment impacts the optic nerve head as swelling of the optic disc and papilledema
however, papilledema evolves and may be a delayed manifestation, besides requiring a skilled observer for precise identification
dilatation of the optic nerve sheath is a much earlier manifestation of ICP rise
the optic nerve sheath is fairly easy to visualize by insonation via the orbit

optic nerve sheath diameter (ONSD) is evaluated via transorbital window using a 9-12 MHz probe [Dubourgh, 2011]
measurement provides a dynamic value because ONS responds rapidly to changes in ICP (unlike papilledema)
it is measured ~ 3 mm posterior to the globe
normal ONSD: typically 4.0-5.4 mm with an upper limit ranging from 5.0 to 5.7 mm [Chacko, 2014]
- there are considerable differences across studies on the upper cut-off values
- ONSD < 5 mm means normal finding, ONSD > 6 mm is definitely pathological
a normal ONSD does not exclude serious brain pathology, and the patient still may need further investigation (such as CT, MRI, or lumbar puncture)

the cut-off value indicating intracranial hypertension > 5.7 mm (various values are reported)
- 6.4 ± 0.6 mm [ Bäuerle, 2012]
- 6.3 ± 0.6 mm on MRI [Geeraerts, 2008]
cut-off value indicating brain death 7.2 ± 0.4 mm [Lovrencic-Huzjan, 2012]

papilledema was traditionally diagnosed with an ophthalmoscope, which is rarely performed in critical care practice
optic disc elevation may also be detected using ultrasonography using the same view used to determine optic nerve sheath diameter
optic disc height on ultrasonography correlated well with optical coherence tomography among 14 patients evaluated in an ophthalmology clinic (Teismann, 2013)
ultrasound thus confirms papilledema (edema due to raised ICP) in 2 ways (disc elevation + increased ONSD)

	normal ONSD	enlarged ONSD
papilledema –	normal	acutely elevated ICP (not enough time to form disk swelling)
papilledema +	pseudo-papilledema	ICP elevation

calcium deposits within the optic disc create a very bright signal with a shadow
the finding may prove pseudo-papilledema (if ONSD is normal)

inflammation of the nerve may occur (optic neuritis, sarcoidosis, syphilis, lupus, etc.)
typically, unilateral optic nerve edema may be observed