Emergency Ocular Ultrasonography to Measure Optic Nerve Sheath Diameter

By ooman Hossein-Nejad, MD; Maryam Bahreini, MD; and Fatemeh Rasooli, MD | on June 20, 2017 | 0 Comment

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ACEP Now: Vol 36 – No 06 – June 2017

Figure 4: Optic nerve ultrasound technique. The probe is directed slightly inferiorly and medially with the eyes closed. For better visualization, place the probe on the superior and lateral margin of the orbit.

Purpose

The evaluation of ONSD via bedside ultrasonography is beneficial in suspected cases of increased intracranial pressure transmission through the optic nerve, which leads to papilledema.

Applicability

There are common situations in which intracranial pressure assessment is necessary, but it is not possible to perform fundoscopy. Some examples include ocular media opacities such as cataract formation, hemorrhage, periorbital swelling (eg, palpebral ecchymosis in trauma patients), fixed gaze, and some infectious respiratory cases that may impose the risk of respiratory transmission.

Indications

Bedside ultrasonography can be performed in cases of suspected increased intracranial pressure (eg, traumatic brain injuries, various types of intracranial hemorrhage, space-occupying lesions, etc.).

Figure 5: The optic nerve sheath diameter should be measured 3 mm behind the retina.

Scanning Procedure

To measure the ONSD:

1. Place the patient in the supine position.
2. Apply sufficient amount of sterile (preferably) gel on the closed eyelid.
3. Place a 6–12 MHz linear probe on the superior and lateral aspect of the upper eyelid (see Figure 4).
4. Angle the probe slightly medially and caudally until the oblique hypoechoic tract of the optic nerve can be visualized with clear margins posterior to the globe. Doppler capabilities are helpful in assessing the vascularity of the optic nerve (see Figure 3).
5. The diameter should be measured 3 mm behind the retina (see Figure 5).⁵
6. For a more accurate measurement, calculate a mean of three measurements.

Pitfalls

Some reported measurements indicating an anechoic image behind the retina were probably artifacts of lamina cribrosa, the mesh-like bony structure for optic nerve fibers’ passage through the sclera, mimicking the optic nerve route, which does not follow the vascular pattern of the central retinal artery on color Doppler.¹

Another pitfall is measuring the ill-defined non-clear shadow, probably the previously discussed shadow superimposed on the real ONSD. This can be prevented by carefully angulating the transducer medially and caudally to visualize the oblique ONSD with clear margins.

ONSD measurement with this method has yet to be determined in settings such as various races, different disease acuities, eye angulations, etc. Some researchers defined various measures for the ONSD. The median normal and elevated ONSD was measured at 0.42 cm (IQR 0.37–0.48 cm) in accordance with the median invasive intracranial pressure measurement that was 12 mm Hg (IQR 9–19 mmHg) and ≥ 0.48 cm for the intracranial pressures > 20 mmHg, respectively.⁵ The median intracranial pressure was measured by intraparenchymal external ventricular catheters. However, getting the view of the oblique hypoechoic tract of the optic nerve provides lower normal measures, around 0.35 cm, that need to be validated.