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Ask patient to look straight ahead; stabilize hand on patient's nose, malar bone, or forehead (if scanning from head of bed) to avoid pressure on globe.
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Measurement of optic-nerve sheath diameter (ONSD)
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Measure the ONSD 3 mm posterior to the globe; normal diameter, <5.0 mm.
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Indications for exam: ocular trauma, sudden vision changes, concern for increased intracranial pressure or foreign-body.
Probe selection: high-frequency linear probe.
Use the ocular setting if available. In addition to image optimization, this preset reduces power to 50% to minimize the risk of retinal damage.
When prepping the patient for the exam, a clear plastic adhesive dressing may be used. Gel placed on the outside of the dressing decreases the likelihood of gel entering the eye. We do not recommend this technique because air bubbles may be trapped under the dressing, there is a greater likelihood of applying pressure to the globe, and there may be eyelid or eyelash trauma with removal of the adhesive dressing.
Apply gel generously so that images are obtained without actual contact with the skin. The footprint of the probe and curved surface of the globe can make maintaining contact without applying excessive pressure challenging.
Stabilize hand on a boney structure: nose, forehead, or malar bone of cheek.
Image in both transverse and sagittal planes.
Ask patient to look right and left as well as up and down if concerned for hemorrhage or foreign body.
The optic nerve is a dark, well-demarcated tubular structure that arises ...