Category: 3. Ocular Ultrasound Protocol

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

• Next, turn the probe 90° clockwise so the indicator points superiorly towards the patient’s head to obtain a sagittal view. Identify the same structures you found in the transverse view.

• Then, have the patient look up and down and increase the gain slowly to assess for symmetric extra-ocular movements and to rule out intraocular pathology.
  • In the sagittal view, you are looking for Superior and Inferior (or Up and Down) movements of the eye.

• Place the probe lightly on the gel covering the patient’s eye with the probe indicator pointed towards the patient’s right to obtain a transverse view.

Identify the following ocular ultrasound anatomy from anterior to posterior:

• Eyelid
• Anterior Chamber
• Lens
• Iris
• Vitreous Body
• Retina
• Optic Nerve
• *Make sure to tilt/fan through the entire eye

Assess for Extraocular Movements

• Next, ask the patient to look left and right to evaluate for extraocular movements. This is important when patients have severe periorbital edema from facial trauma.
  • In the transverse view, you are looking for MEDIAL and LATERAL (or Left and Right) movements of the eye.
  • Increase the gain slowly to better detect intraocular pathologies such as mobile findings of retinal detachment, posterior vitreous detachment, or vitreous hemorrhage.

In this section, you will learn a step-by-step approach on how to use ultrasound of the eye to detect normal ocular structures in the transverse and sagittal views, assess for extra-ocular movements, and measure the optic nerve sheath diameter to estimate intracranial pressure.

Step 1: Anchor the Probe

In addition to a generous amount of gel, it is important to anchor your probe to decrease the amount of pressure applied to the patient’s eyes.

• Grasp the linear probe and anchor your fingers on a bony surface of the patient’s face.
• The example below assumes using the right hand to scan the patient.
• For the Right eye, anchor your right pinky finger on the patient’s nose.
• For the Left eye, anchor your right pinky finger or palm on the zygomatic arch.