Brain Trauma Measurement of Optic Nerve Sheath Diameter for the Assessment of Risk of Raised Intracranial Pressure

Raised intracranial pressure (ICP) is associated with poor outcome after brain injury, but is difficult to detect without invasive devices. As a part of the central nervous system, the optic nerve is surrounded by a dural sheath, and the subarachnoid space surrounding the optic nerve is subject to the same pressure changes as the intracranial compartment. Distension of the optic nerve sheath reflects an increase in cerebrospinal fluid (CSF) pressure and can be used to estimate the risk of raised ICP. Ocular sonography or brain magnetic resonance imaging (MRI) enables valid measurement of the distension of the dural sheath surrounding the optic nerve. An optic nerve sheath diameter greater than 5.8mm is likely to be associated with raised ICP. This non-invasive estimate of ICP may detect patients at risk, help make decisions regarding the placement of invasive ICP devices and allow the selection of patients for transfer to specialist centres.