Measuring Intracranial Pressure Using Ocular Ultrasound
- 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.