Geeraerts_Layout 1 28/10/2009 10:01 Page 51
Measurement of Optic Nerve Sheath Diameter for the Assessment of Raised Intracranial Pressure
Figure 1: Typical Ocular Sonography Scan Figure 2: Magnetic Resonance Imaging of the
Optic Nerve Sheath
T2-weighted sequence. On T2-weighted imaging, the optic nerve sheath appears as
high-signal. As described for ocular sonography, the optic nerve sheath diameter can be
measured 3mm behind the ocular globe.
of the optic nerve sheath on ocular ultrasound scans.
19
Mean ONSD
was 6.3mm in patients with radiological signs of elevated ICP and
4.4mm in patients with no such signs. The authors considered 5mm to
be the upper limit of normal value. The specificity of this cut-off value
for detecting tomographic elevated ICP was 93% and its negative
predictive value was 100%. These results were confirmed in another
study performed by the same team.
21
Very recently, four clinical studies
22–25
have compared sonographic
The optic nerve sheath appears as an hypoechogenic structure surrounded by the
ONSD with invasive intracranial ICP, which remains the gold standard.hyperechogenic periorbital fat. The optic nerve sheath diameter (ONSD) is measured 3mm
behind the ocular globe, perpendicularly to the optic nerve axis. The measurement can be
Measuring ONSD and ICP simultaneously, a good relationship between
repeated in several directions.
ONSD and ICP has been described (r=0.71 and r=0.68, respectively).
22,24
placed on the gel in the temporal area of the eyelid (not on the eye itself) Interestingly, these studies, which were performed in the intensive care
to prevent pressure being exerted on the eye. The placement of the unit (ICU) or emergency setting in acute neurocritical care patients,
probe must be adjusted to allow a suitable angle for displaying the entry found a very similar best cut-off value of ONSD to diagnose raised
of the optic nerve into the globe.
17
The 2D mode is generally used, and ICP: the best cut-off value was 5.7–5.8mm for predicting elevated ICP
ONSD must be measured in its retrobulbar segment, 3mm behind the (>20mmHg). As this test is likely to be used for detection, its sensitivity
globe and perpendicularly to the optic nerve axis (see Figure 1). Recent (probability of positive tests for sick patients) and negative predictive
advances in sonographic technology allow promising 3D measurements. value (probability of the patient being healthy if the test is negative)
must be excellent. Both of these values were above 90% for an ONSD
There is a growing body of evidence in the clinical setting suggesting cut-off of 5.7mm. The probability of having high ICP was very low (less
that millimetric increases in sonographic ONSD are related to raised than 5%) when ONSD was less than 5.8mm. When comparing
ICP. Clinical studies have suggested that sonographic measurements sonographic ONSD and ICP measured with ventricular drain, it has been
of ONSD correlate with clinical signs of high ICP in children with shown that the best cut-off value for detecting ICP >15mmHg
hydrocephalus or liver failure.
17–20
In adults with moderate traumatic (20cmH
2
O) was 5mm, with a sensitivity and specificity of 88 and 93%,
brain injury (TBI) ONSD is also correlated with signs of high ICP on respectively.
23
Interestingly, changes in ONSD are also strongly related
computed tomography (CT).
19,21
In 35 adult patients with TBI assessed to ICP variations (r=0.73).
24
Changes in ICP could therefore be detected
in the emergency department, Blaivas et al. found that signs of by changes in sonographic ONSD. However, optic nerve diameter (OND),
elevated ICP on the brain CT scan were closely related to dilation i.e. not taking into consideration the sheath but only the nerve itself, has
EUROPEAN NEUROLOGICAL REVIEW 51