Optic nerve sheath diameter
The optic nerve sheath demonstrates changes in diameter with CSF pressure changes as there is a layer of subarachnoid space between the nerve and its sheath, which expands due to raised intracranial pressure. These changes are appreciated more along the anterior aspect of the nerve.
Measurements of the optic nerve sheath diameter (ONSD) are most often taken at a distance of 3 mm from the posterior globe margin as this is believed to be the site of maximum pressure changes along the long axis of the optic nerve .
It should be noted, however, that measuring at this location is not universally accepted, and other authors suggest measuring 8 mm from the globe .
These measurements can be performed on ultrasound using a linear array probe, on a T2 weighted sequence on MRI or on CT. The intermodality difference in measurements is minimal and correlates well with CSF pressure changes .
Intensive care and emergency point of care ultrasonography literature typically uses the following cutoffs, as measured inner-edge to inner-edge, for the upper limit of normal of the ONSD :
- up to 4 mm in infants
- up to 4.5 mm in children
- up to 5 mm in adults
- measurements above 5 mm (bilaterally) correspond with elevations in intracranial pressure above 20 mmHg
Further degrees of ICP elevation result in a linear increase in ONSD up to 7.5 mm, at which the diameter appears to plateau. Confirmation that the measured diameter represents a fluid shift due to elevated ICP can be confirmed with a "30 degree test;" measurements are taken in primary gaze and 30 degrees from primary gaze.
A decrease in ONSD by >15% width on 30 degree eccentric gaze is moderately specific for increased intracranial pressure as the etiology of the elevated ONSD, while a negative test (no change in nerve sheath diameter on eccentric gaze) would suggest alternative etiologies e.g. infiltrative disease.
Differentiation between acute and chronic elevations in ICP may be aided by the crescent sign (the sonographic correlate of papilledema ), the presence of which infers chronicity.
Normal maximal diameter values vary somewhat from publication to publication and similarly, the threshold above which raised ICP should be considered also varies depending on definitions and study technique.
When measuring 3 mm from the globe, the upper-limit maximum optic nerve sheath diameter threshold has ranged between 4.8 to 6.2 mm
When measuring 8 mm from the globe on CT normal maximum optic nerve sheath diameter is 5.8 m .
It should be also noted that there is probably a relationship between globe diameter and normal optic nerve diameter and some have advocated that a ratio of the two may be more appropriate .
Irrespective of technique or value is chosen it is important to recognize that this measurement is at best imperfect and does not replace the need for pressure monitoring via other means if clinically indicated.