Page 96 - ATP-P 11th Ed

Ultrasonic Assessment of Optic Nerve Sheath Diameter
If a patient is unconscious (i.e. does not follow commands or open eyes spontaneously),
SECTION 1 they may have elevated ICP. There is no reliable test for elevated ICP available outside of a
hospital; however, optic nerve sheath diameter (ONSD) measurement is a rapid, safe, and
easy-to-perform ultrasonographic assessment that may help identify elevated ICP when
more definitive monitoring devices are not available.
a. The optic nerve sheath directly communicates with the intracranial subarachnoid space.
Increased ICP, therefore, displaces cerebrospinal fluid along this pathway. Normal
ONSD is 4.1–5.9mm. 30
b. A 10–5-MHz linear ultrasound probe can be used to obtain ONSDs. ONSD is measured
from one side of the optic nerve sheath to the other at a distance of 3mm behind the eye
immediately below the sclera. 31
c. In general, ONSDs >5.2mm should raise concern for clinically significant elevations
in ICP in unconscious TBI patients. 5,32 The ONSD can vary significantly in normal in-
dividuals, so one single measurement may not be helpful; however, repeated measure-
ments that detect gradual increases in ONSD over time may be more useful than a single
measurement.
33
d. ONSD changes rapidly when the ICP changes, so it can be measured frequently. If
ONSD is used, it is best to check hourly along with the neurologic examination.
Technique
1. Check to make sure there is no eye injury. A penetrating injury to the eyeball is an abso-
lute contraindication to ultrasound because it puts pressure on the eye.
2. Ensure the head and neck are in a midline position. Gentle sedation and/or analgesia
may be necessary to obtain accurate measurements.
3. Ensure the eyelids are closed.
4. If available, place a thin, transparent film (e.g., Tegaderm; 3M, http://www.3m.com)
over the closed eyelids.
5. Apply a small amount of ultrasound gel to closed eyelid.
6. Place the 10(–5) MHz linear probe over the eyelid. The probe should be applied in a
horizontal orientation (Figure 1) with as little pressure as possible applied to the globe.
7. Manipulate the probe until the nerve and nerve sheath are visible at the bottom of
screen. An example of a proper ultrasonagraphic image of the optic nerve sheath can
be seen in Figure 2.
8. Once the optic nerve sheath is visualized, freeze the image on the screen.
9. Using the device’s measuring tool, measure 3mm back from the optic disc and then ob-
tain a second measurement perpendicular to the first. The second measurement should
cover the horizontal width of the optic nerve sheath (Figure 2). An abnormal ONSD is
shown in Figure 3.

86 SECTION 1 TACTICAL TRAUMA PROTOCOLS (TTPs) ATP-P Handbook 11th Edition 87