Nerve Sheath: The Intracranial Subarachnoid Temperature
Optic nerve sheath diameter (ONSD)
The anatomy of the optic nerve sheath : The intraorbital section of the optic nerve extends from the globe, where it inserts medially, to the optic canal located in the lesser wing of the sphenoid bone. It is encased by a meningeal sheath consisting of dura mater, arachnoid mater and pia mater. Cerebrospinal fluid is contained in the trabeculated subarachnoid space and is continuously and slowly filtered. As a result the optic nerve sheath is in direct communication with the intracranial subarachnoid space. It is this relationship that forms the physiological basis for using the optic nerve sheath as a surrogate for intracranial pressure measurement. The anatomical relationships underpinning the …show more content…
The anterior chamber is anechoic, as generally is the lens, while the iris appears bright and echogenic. The choroid and retina may be seen as a thin grey layer at the posterior aspect of the globe. The optic nerve is the ‘black stripe’ running away from the posterior aspect of the globe and optic disc, and should ideally be positioned in the centre of the ultrasound screen. The nerve sheath, as seen on ultrasound examination, has a high reflectivity compared to the homeogenous appearance of the nerve, and should be relatively easy to distinguish. If the optic nerve sheath is markedly dilated, it may be possible to diagnose this from visual estimation alone. In general, however, the software calipers should be used to ensure accurate measurement and recording. In severely raised intracranial pressure, it may be possible to visualisa a ‘crescent sign’(40), an echolucent circular artefact within the sheath separating the sheath from the nerve due to increased subarachnoid fluid.There has been interest in using contrast enhancedultrasound (CEUS) to help identify and recognise the anatomy surrounding the optic nerve, which is a small structure. The incorrect identification of artefacts as part of the sheath by an inexperienced sonographer is a criticism of the technique. A small proof of concept study, using a second generation contrast agent (Sonovue®, Bracco SpA), found …show more content…
• The time spent in active scanning should be minimised.Once the optimum view has been obtained, store the image either as a frame or a video loop and remove the probe from the eye. Measurements can then be performed without unnecessary exposure of the eye to ultrasound energy.
• Use the caliper function on the ultrasound to enable precise measurement. First locate a point 3 mm posterior to the optic disk. At this point place the calipers at 90 degrees to the axis of the optic nerve to measure the diameter of optic nerve and optic nerve sheath
• Take the average of two or three measurements for each side. A 1996 study by Helmke & Hansen suggested the optimal scanning orientation was longitudinal (axial), as this was associated with the least inter-observer variability (37). However, aside from the variability findings, there was no significant difference in measurements between the two orientations. Most patients will be scanned supine, or with a 20° to 30° head up tilt. A Nepalese study, which included 287 patients, examined correlation between ONSD and acute mountain sickness. This study suggested ONSD does not change with patient positioning (34). This was supported by results from
The anatomy of the optic nerve sheath : The intraorbital section of the optic nerve extends from the globe, where it inserts medially, to the optic canal located in the lesser wing of the sphenoid bone. It is encased by a meningeal sheath consisting of dura mater, arachnoid mater and pia mater. Cerebrospinal fluid is contained in the trabeculated subarachnoid space and is continuously and slowly filtered. As a result the optic nerve sheath is in direct communication with the intracranial subarachnoid space. It is this relationship that forms the physiological basis for using the optic nerve sheath as a surrogate for intracranial pressure measurement. The anatomical relationships underpinning the …show more content…
The anterior chamber is anechoic, as generally is the lens, while the iris appears bright and echogenic. The choroid and retina may be seen as a thin grey layer at the posterior aspect of the globe. The optic nerve is the ‘black stripe’ running away from the posterior aspect of the globe and optic disc, and should ideally be positioned in the centre of the ultrasound screen. The nerve sheath, as seen on ultrasound examination, has a high reflectivity compared to the homeogenous appearance of the nerve, and should be relatively easy to distinguish. If the optic nerve sheath is markedly dilated, it may be possible to diagnose this from visual estimation alone. In general, however, the software calipers should be used to ensure accurate measurement and recording. In severely raised intracranial pressure, it may be possible to visualisa a ‘crescent sign’(40), an echolucent circular artefact within the sheath separating the sheath from the nerve due to increased subarachnoid fluid.There has been interest in using contrast enhancedultrasound (CEUS) to help identify and recognise the anatomy surrounding the optic nerve, which is a small structure. The incorrect identification of artefacts as part of the sheath by an inexperienced sonographer is a criticism of the technique. A small proof of concept study, using a second generation contrast agent (Sonovue®, Bracco SpA), found …show more content…
• The time spent in active scanning should be minimised.Once the optimum view has been obtained, store the image either as a frame or a video loop and remove the probe from the eye. Measurements can then be performed without unnecessary exposure of the eye to ultrasound energy.
• Use the caliper function on the ultrasound to enable precise measurement. First locate a point 3 mm posterior to the optic disk. At this point place the calipers at 90 degrees to the axis of the optic nerve to measure the diameter of optic nerve and optic nerve sheath
• Take the average of two or three measurements for each side. A 1996 study by Helmke & Hansen suggested the optimal scanning orientation was longitudinal (axial), as this was associated with the least inter-observer variability (37). However, aside from the variability findings, there was no significant difference in measurements between the two orientations. Most patients will be scanned supine, or with a 20° to 30° head up tilt. A Nepalese study, which included 287 patients, examined correlation between ONSD and acute mountain sickness. This study suggested ONSD does not change with patient positioning (34). This was supported by results from