Optic nerve sheath diameter

Optic nerve sheath diameter has significance in the assessment of papilloedema in cases of elevated intracranial pressure.

Pathology

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 nerve¹.

Radiographic features

The measurements are most often taken at a distance of 3 mm from the globe (theas this is believed to be the site of maximum pressure changes) along the long axis of the optic nerve^1,2,4.

It should be noted, however, that measuring at this location is not universally accepted, and other authors suggest measuring 8 mm from the globe ³. 

These measurements can be performed on ultrasound using a linear array probe and, on a T2WT2 weighted sequence on MRI or on CT ^1-4. The intermodality difference in measurements is minimal and correlate well with CSF pressure changes ^ref.

Values

• normal

  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 rangethreshold has ranged between 4.8 to 6.2 mm ⁴

  When measuring 8 mm from the globe on CT normal maximum optic nerve sheath diameter is 4.8-55.8 mm

• m ³.

  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 ⁴. 

  Irrespective of technique or

  value of >5.8 mm corresponds well with a raised CSFis chosen it is important to recognise that this measurement is at best imperfect and does not replace the need for pressure of 25 mmHg indicating papilloedema due to intracranial hypertension