3. Ocular Ultrasound Protocol

Step 4: Measure Optic Nerve Sheath Diameter (ONSD)

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.
Ocular Ultrasound Rocking Probe Laterally
Optic Nerve Sheath Diameter for ONSD Ultrasound
  • 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.
ONSD Measurement 3mm behnd retina
  • 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.
ONSD Measurement - NORMAL
Normal ONSD Measurement

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

3 x 5 ONSD Measurement
Think of a 3×5 card for ONSD!
3. Ocular Ultrasound Protocol

Step 3: Obtain Sagittal View

  • 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.
Ocular Ultrasound Sagittal View with Labels
Ocular Ultrasound Sagittal 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.
3. Ocular Ultrasound Protocol

Step 2: Obtain Transverse View

  • 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.
Ocular Ultrasound Transverse View Hand Position
Ocular Ultrasound Probe Position – 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
Ocular Ultrasound Illustration
Ocular Ultrasound Transverse View with Labels
Ocular Ultrasound Transverse View

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.
Ocular Ultrasound Extraocular Eye Movements
Extraocular Ocular Movement on Ultrasound
3. Ocular Ultrasound Protocol

Ocular Ultrasound Protocol

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.
Ocular Ultrasound Right Eye Anchor Nose
Right Eye – Anchor on Nose
Ocular Ultrasound Left Eye Anchor Zygomatic Arch
Left Eye – Anchor on Zygomatic Arch