Optic Nerve Sheath Diameter

(ONSD)MeasurementDr Shweta Gidwani MBBS FCEM

•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.

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.

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.

Patient Position

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)

•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 (eg Tegaderm) or an eyepad, to make the exam more comfortable for the patient.

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

•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

• 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.

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.

Ref: Tayal VS, Neulander M, Norton HJ, et al. Emergency depart- ment sonographic 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.

MeasurementThe 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.