Tag Archives: ICP

Beating a Dead Horse: Why Ocular Ultrasound Beats Papilledema for Detecting Increased Intracranial Pressure

Our earlier article on ocular ultrasound for measuring intracranial pressure was met with a common question. Primarily, why not just use papilledema as your initial assessment for intracranial pressure? This is what we are taught in medical school. Optic nerve disc swelling equals increased intracranial pressure (ICP). End of Story. Right? Not exactly…  While optic disc swelling can indicate increased ICP, it is an inferior measure of acutely elevated ICP. And here is why…

  • It’s an indirect measure.
  • It’s a late sign of increased ICP.
  • It’s more subjective.
  • It’s not a dynamic measurement.
  • It’s not always practical.

pap vs oc us 1

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Keeping an Eye on Intracranial Pressure: Measuring ICP Using Ocular Ultrasound

Measuring Intracranial Pressure Using Ocular Ultrasound

Measuring ICP

  • There is a caliper function on ultrasound machines that gives you precise measurements of selected structures.
  • Take your optic nerve sheath diameter (ONSD) measurement 3 mm posterior to the globe.
    • This area has the greatest contrast with surrounding tissue (more contrast = more accurate measurements).
    • Use the calipers to determine 3 mm.
  • Measure across the optic nerve sheath (not just the optic nerve).
  • Measure ONSD for both eyes and average the two measurements.

Interpreting Your Results:

  • Increased ONSD correlates with increased ICP
  • Upper limit of normal ONSD vary with age…
    • Adults: < 5 mm
    • Children > 1 yo: < 4.5 mm
    • Children < 1 yo: < 4 mm
  • ONSD 5 – 5.7 mm: may indicate ICP > 20 mmHg, especially if symptomatic
    • > 5 mm is 100% sensitive for elevated ICP
    • All patients with elevated ICP have ONSD > 5mm
  • ONSD > 5.7 mm: indicates ICP > 20 mmHg
    • > 5.7 mm is 100% specific for elevated ICP
    • Only patients with elevated ICP have ONSD > 5.7 mm
  • ONSD measurements increase with increasing ICP.
  • ONSD measurements plateau around 7.5 mm even with significantly increased ICP.
  • In severe cases, an echoluscent circle called a crescent sign may be present.
    • Crescent sign is formed by the separation of the optic nerve sheath from the optic nerve by high ICP.

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