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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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