Optic Nerve Sheath Point of Care Ultrasound: Image Acquisition

Point of Care Ultrasound of the Optic Neuro Sheath Diameter is a useful technique in identifying patients with elevated intracranial pressure. It can be used outside of an ICU because it does not require any invasive techniques, so it can be used in the emergency room or potentially even in the pre-hospital setting. This technique can be used immediately upon the patient's arrival and is easily repeatable.

However, the lack of standardization has hampered its widespread use. Here, we present a standardized acquisition protocol. This technique is targeted to patients with severe traumatic brain injury.

It can help determine if there is elevated intracranial pressures, which may require specific therapies. To begin, press the select scanner tab on an ultrasound machine and select a linear high frequency probe. Allow the probe to connect wirelessly.

For setting up the ultrasound, go to applications and choose ocular or small parts. Check the frequency of the scanner. Set the depth to four centimeters for adult patients.

Adjust the depth by scrolling up or down on the touch screen. Set the focal zone to 2.5 centimeters. Check the mechanical index and thermal index.

Set the video capture length to at least four seconds. Then select the mode as B mode, and select two dimensional gray scale ultrasound. Patients that are candidates for this procedure are those with concern for intracranial hypertension, such as those with severe traumatic brain injury.

Exclude patients with ocular trauma. Position the patient in the default position with the head of the bed at 30 degrees. To perform the scan, apply the ultrasound gel to cover the length of the transducer.

Close the patient's eyelid and use a transparent film dressing to hold it closed to prevent corneal abrasion or eye damage. Position the probe for the transverse view over the pupil. Center the probe over the pupil and sweep or fan it over the eye.

The globe will be hypoechoic with a distinct transition visible at the back of the eye. Identify the optic nerve as a linear hypoechoic structure posterior to the globe. The sheath will be visible as a hypoechoic boundary of the nerve.

Attempt to obtain a view with the nerve headed directly posterior from the back of the eye. Ensure the nerve is visible three millimeters posterior to the globe. Once the adequate view is acquired, take a video of the optic nerve sheath.

Then scroll back on the video until the maximum diameter of the optic nerve sheath is seen. Alternatively, press the freeze button when at the maximum ONSD, and use the review slider to ensure that the maximum ONSD has been captured. To measure the nerve, press the annotation button, select the distance caliper.

Using the caliper function, measure three millimeters posterior from where the optic nerve attaches to the retina. Press the distance caliper again and measure the outermost lateral borders of the optic nerve sheath. Save the annotated image by pressing the save button on the bottom right of the screen.

To obtain a sagittal view, center the probe on the eye. Follow the same steps. As for the transverse view, obtain an adequate image and measure the ONSD.

When taking measurements, when the eye is in a neutral position, the nerve typically heads back at an angle. Patients can be requested to move the eye so that the nerve is headed straight back. In cases where commands cannot be followed, for example, in patients with traumatic brain injury, the nerve will head back at an angle.

It is recommended to measure along the axis of the nerve if it does not head straight back. It is also essential to be aware of potential pitfalls such as mistaking the optic nerve for its sheath, or not using settings or probes with sufficient resolution to distinguish the sheath and nerve. Also, acoustic artifacts like blooming can create interference and misreading.

A blooming artifact from structures on the posterior eye shows that there is no rounded nerve head, no sheath, and the shadow appears very rectangular. Whereas when the optic nerve and sheath are visible, the rounded head of the nerve and the sheath are clearly seen. A common struggle in Point Of Care Ultrasound for the Optic Nerve Sheath Diameter is finding the nerve on a sweep.

This can be mitigated by gently rocking the probe back and forth until it comes into view, and by angling the probe slightly towards the midline. Additional to this technique, you can also use color flow doppler to help identify the optic nerve. This can help differentiate it from other artifacts that may be present in the image.