NEUROIMAGING

• 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]
  • limit examination time to minimize eye exposure, set output power to 10-20%
• 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 associated with brain death 7.2 ± 0.4  mm  [Lovrencic-Huzjan, 2012]

• a lot of patients fall into the ~5-6 mm gray zone
• additional features can help to sort them out