The Optic Nerve Sheath Diameter (ONSD) is an important measurement that can be used to detect elevated intracranial pressure (ICP).
- In the transverse view, rock the probe about 10-15° laterally to visualize where the hypoechoic (darkly colored) optic nerve radiates away from the base of the globe (see figure below).
- Tip: If you do not see the ocular nerve immediately, tilt the probe up and down until it comes into view.
- Once you have a good view, freeze the image.
- Use the caliper function to measure 3 mm (.3cm) posterior to where the optic nerve sheath attaches to the retina. This is the location where you will use to measure your optic nerve sheath diameter.
- Next, use the calipers again to measure the outermost lateral borders of the optic nerve sheath (anechoic border). The figure below measures the ONSD diameter to be 4.68mm (.468cm).
- To obtain better accuracy, you can obtain a few measurements and take the average of the ONSD values.
Interpretation of Optic Nerve Sheath Diameter (ONSD)
- In adults, an ONSD < 5 mm indicates that the patient has a normal optic nerve width and a normal intracranial pressure (ICP) value of <20 cm H2O. However, if the ONSD > 5 mm, you will not be able to correlate this with an exact ICP value; all this tells you is that the ICP is elevated.
- Below is a quick table regarding normal ONSD measurements for adults vs children:
Note: To remember how to measure ONSD, think of a 3×5 index card. You need to measure 3 mm posterior to the globe and >5 mm indicates a high ICP.
|Age Category (Years)||Normal ONSD Measurement|
|Adults (16+)||< 5 mm|
|Children (1-15)||< 4.5 mm|
|Children (<1)||< 4 mm|
Normal ONSD by Age