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 1.
The measurements are taken at a distance of 3 mm from the globe (the site of maximum pressure changes) along the long axis of the optic nerve. These measurements can be performed on ultrasound using a linear array probe and on a T2W sequence on MRI. The intermodality difference in measurements is minimal and correlate well with CSF pressure changes ref.
- normal optic nerve sheath diameter range is 4.8-5.8 mm
- a value of >5.8 mm corresponds well with a raised CSF pressure of 25 mmHg indicating papilloedema due to intracranial hypertension
- 1. Ehrhart IC, Parker PE, Weidner WJ, Dabney JM, Scott JB, Haddy FJ. Coronary vascular and myocardial responses to carotid body stimulation in the dog. (1975) The American journal of physiology. 229 (3): 754-60. doi:10.1152/ajplegacy.19126.96.36.1994 - Pubmed
- 2. Wang LJ, Chen LM, Chen Y, Bao LY, Zheng NN, Wang YZ, Xing YQ. Ultrasonography Assessments of Optic Nerve Sheath Diameter as a Noninvasive and Dynamic Method of Detecting Changes in Intracranial Pressure. (2018) JAMA ophthalmology. 136 (3): 250-256. doi:10.1001/jamaophthalmol.2017.6560 - Pubmed