NEUROIMAGING

Optic Nerve Sheath ultrasound

Updated on 25/01/2024, published on 25/01/2024

Optic nerve sheath (ONS)

  • 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)

Optic Nerve Sheath UltraSound (ONSUS)

  • 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)
Optic Nerve Sheath Diameter (ONSD)
Enlarged Optic Nerve Sheath Diameter (ONSD) due to the intracranial hypertension

Pitfalls

  • the problem is that a lot of patients fall into the ~5-6 mm grey zone
  • additional features may help sort out these patients

Papilledema

  • 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

Optic disc drusen

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

Neuritis

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

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Optic Nerve Sheath ultrasound
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