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Measurement of Optic Nerve Sheath Diameter for the
Assessment of Risk of Raised Intracranial Pressure
David K Menon
and Dan Benhamou
1. AP-HP, University of Paris – South, Department of Anaesthesia and Critical Care, University Hospital Bicêtre;
2. Department of Anaesthesia, Addenbrooke’s Hospital and University of Cambridge
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.
Intracranial pressure, neurocritical care, ocular sonography, optic nerve sheath diameter, traumatic brain injury
Disclosure: Thomas Geeraerts is supported by grants from the Société Française d’Anesthésie et de Réanimation (SFAR) and Journées d’Enseignement Post-Universitaire
d’Anesthésie-Réanimation (JEPU)-Novo Nordisk. David K Menon is supported by grants from the Medical Research Council, UK, Royal College of Anaesthetists, Wellcome Trust,
the Evelyn Trust and Queens’ College Cambridge. Dan Benhamou has no conflicts of interest to declare.
Received: 22 October 2008 Accepted: 16 February 2009
Correspondence: Thomas Geeraerts, University Department of Anaesthesia, Addenbrooke’s Hospital and University of Cambridge, Cambridge CB2 2QQ, UK.
Raised intracranial pressure (ICP) is frequent and is associated with poor of the optic disc (papilloedema) requires a few days to develop and
outcome after brain injury, and also after liver failure, acute ischaemic resolve, making it less useful when acute changes in ICP are
stroke, cerebral venous thrombosis, meningitis and encephalitis.
Papilloedema thus appears more as a delayed
However, the early detection of raised ICP can be difficult when invasive consequence of chronic CSF accumulation in the retrobulbar optic
devices are not available. Clinical signs of raised ICP are not specific and nerve dural sheath due to raised CSF pressure in the cranial cavity.
are often difficult to interpret. In sedated patients, clinical signs of raised Direct assessment of such CSF accumulation by measuring optic nerve
ICP frequently appear late, when ischaemic brain injury is already sheath diameter (ONSD) may provide an earlier and more reactive
The gold standard method for ICP measurement is based measure of intracranial hypertension. In cadavers, the ONSD displays
on the use of invasive devices such as intraventricular drain or predominantly anterior enlargement following injection into the orbital
intraparenchymal probes. Nevertheless, invasive ICP monitoring is not perineural subarachnoid space.
In humans, following an intrathecal
routinely used in many centres – principally because of the lack lumbar infusion of Ringer’s solution, ONSD dilation reaches a maximum
of availability of neurosurgeons – or may be contraindicated in cases of at peak CSF pressure, strongly suggesting a close relationship between
coagulation disorders. The worst attitude would be to ignore the danger CSF pressure and dilation of the orbital perineural subarachnoid space.
of raised ICP in these patients and to run an unacceptable risk of
cerebral ischaemia. In these situations, a non-invasive estimation of the Optic Nerve Sheath Diameter Measurement
risk of raised ICP may be clinically valuable. Using Ocular Sonography
Ocular sonography has been extensively and safely used for ophthalmic
Anatomical Background evaluation for more than 20 years.
Ocular ultrasound is regarded as
The optic nerve, as part of the central nervous system, is surrounded by safe as long as Doppler-frequency analysis is not used for a prolonged
a dural sheath. The intraorbital subarachnoid space surrounding the duration.
B-mode ultrasound equipment, which is commonly used
optic nerve is subject to the same pressure changes as the intracranial today, is not capable of producing a harmful temperature rise.
The retrobulbar part of the perioptic subarachnoid Nevertheless, prolonged exposition to Doppler frequency may be
space is distensible and can therefore inflate if pressure increases. harmful due to heating of the retina. Patients must be placed in a supine
Hayreh showed in monkeys and humans that the subarachnoid spaces position at 20° to the horizontal. Multipurpose ultrasound units with
surrounding the optic nerve communicate with the intracranial cavity, high-frequency transducers (>7.5MHz), now available in most
and that changes in cerebrospinal fluid (CSF) pressure can be ultrasound systems, have high lateral and axial precision.
A thick layer
transmitted along the optic nerve sheath.
He also showed that oedema of gel must be applied over the closed upper eyelid. The probe must be
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