How to perform bedside ocular ultrasound for raised ICP

1. Optic Nerve Sheath
Diameter
(ONSD)Measurement
Dr Shweta Gidwani MBBS FCEM

2. • The purpose of ultrasound measurement
of ONSD in the ED is to provide a
safe, effective, accurate and reproducible
bedside test for measuring intracranial
pressure (ICP).
• This is a demonstration of the technique
used to measure ONSD in the ED.

4. Anatomy
The optic nerve is part of the central nervous system (CNS). It
is surrounded by a dural sheath within which is contained
subarachnoid space with cerebrospinal fluid.

5. The anterior part of the optic nerve, is loosely attached to the
dural sheath and is only surrounded by orbital fat, thus
making it distensible.
• Due to the direct communication with the brain, the
intraorbital subarachnoid space around the optic nerve is
subject to the same pressure changes as the intracranial
compartment.
• This means it can inflate in response to raised ICP.

6. Patient Position

7. Probe
• To do this procedure you need a High
Frequency Linear probe.
• Most EDs commonly have this probe as
it is used for vascular and
musculoskeletal ultrasound in the ED)

9. • A thick layer of ultrasound gel is placed
on the closed upper eyelid of the patient.
• Some studies demonstrate the use of a
barrier such as a clear dressing
(egTegaderm) or an eyepad, to make the
exam more comfortable for the patient.

11. The probe should then be gently placed over the
eyelid paying careful attention not to exert too
much pressure.

12. • The probe is placed in two planes:
• The horizonal or CROSS-SECTIONAL axis
and then rotated 90 degrees to lie in
along the VISUAL axis

13. • In each plane, the probe is adjusted from the
temporal to the nasal end to find a suitable angle
for displaying the entry of the optic nerve into the
globe.

14. The optic nerve must then be placed in the centre of the
image before making the measurements.
The ONSD is measured 3mm behind the globe by
convention.

15. Ref: Tayal VS, Neulander M, Norton HJ, et al. Emergency depart- mentsonographic
measurement of optic nerve sheath diameter to detect findings of increased
intracranial pressure in adult head injury patients. Ann Emerg Med 2007;49: 508—14.

16. Measurement
The probe is placed in two planes: The horizonal or
CROSS-SECTIONAL axis and then rotated 90 degrees
to lie in along the VISUAL axis
2 measurements in 2 planes (axial and visual are
made for each eye and the average is recorded
as the final measurement.