Pulsatile Dynamics of the Optic Nerve Sheath and Intracranial Pressure: An Exploratory In Vivo Investigation.

Assessment of intracranial pressure (ICP) remains a fundamental tool in the care of patients with certain neurological disorders. Invasive monitoring of ICP, using microsensor devices placed within the brain parenchyma or transduced external ventricular drains remain the gold standard. These techniques provide valuable diagnostic information, but have specific limitations, with the most significant of these being the risk of infection and hemorrhage.1-3 The indications for ICP monitoring beyond some of the guidelines for severe traumatic brain injury, however, still remain unclear.4,5 This results in unnecessary invasive procedures being performed and highlights the need for a reliable noninvasive technique to estimate ICP. Numerous noninvasive surrogate markers of ICP have been described,4-7 but none of these have yet been able to replace invasive monitoring as the criterion standard technique. The optic nerve sheath (ONS) is a continuation of the intracranial meninges, and the perineural subarachnoid space surrounding the optic nerve is a septated, trabeculated region filled with cerebrospinal fluid (CSF).8 The ONS compartment represents a cul-de-sac space, which is in communication with the intracranial subarachnoid space, and an increase in CSF transmission to the ONS occurs when ICP is increased.9-11 Consequently, the accumulation of CSF within the perineural space due to increased ICP can result in distension of the ONS and the optic nerve protrusion or flattening of the posterior globe.12 The relationship between ICP and the optic nerve sheath diameter (ONSD) has been described by several authors with varying accuracy.13 The most distinct limitation of these studies is the difficulty in defining the stiffness of the ONS, a parameter that would be extremely helpful in interpreting the ONSD.