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…
use of ultrasound to measure ONSD can be readily appreciated on MRI (Figure 1).(3 The optic nerve sheath is bound more loosely to the optic nerve closer to the globe. This loose binding creates a much larger, and potentially more distensible, subarachnoid space in this region, which can appear bulbous on ultrasound (35). While papilloedema may take time to develop, dilation of the optic nerve sheath occurs much earlier and may be a nearinstantaneous manifestation of raised intracranial pressure (36,37).
The history of imaging the optic nerve The first report of ultrasound imaging of the eye was in 1956, but it was early cadaver studies which implicated the optic nerve sheath in the measurement of intracranial pressure. One such study noted the “bulbous portion of the optic nerve was seen to bulge or inflate somewhat as the intracranial pressure was created” with the infusion of crystalloid into the brain (38). The authors also noted this appeared to occur anteriorly, where the nerve sheath was at its thinnest and most expandable. These early studies did not measure the ONSD, relying on imprecise visual clues, and were hampered by the limited ultrasound modes available. These early modes made it difficult to locate a distinct point for measurement at a reproducible distance behind the globe.As ultrasound modalities improved, the focus of most studies was the optimum distance behind the globe at which to best measure ONSD. A 1996 study using modern ultrasonographic techniques showed that ONSD increased by up to 60% at a distance of 3mmbehind the globe in comparison to only 35% at 10 mm (37). This has been confirmed in subsequent studies, indicating that a position 3 mm behind the globe is preferred for measurement (39). Measurements made at this point are more reproducible since ultrasound contrast is greater at this depth with a linear probe. Consistent with this, the optic nerve sheath is at its most distensible anteriorly, where it is potentially most reflective of raised intracranial pressure.
The sonographic appearance of the optic nerve sheath On ultrasound, the globe is visualised as a round, dark, fluid filled structure (see Figure 2).
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…
good correlation between CEUS and MRI. This study suggests, by using nontoxic contrast, exact measurements can be more quickly and easily delineated, which may lessen the effect of operator inexperience (41).
Pros and cons of measurement of ONSD by ultrasound Sonographic ONSD assessment brings some clinical advantages but also some downsides that need to be considered when adopting the technique
Advantages include:
• reproducibility of measurements
• the non-invasive nature of the technique
• ready availability of equipment
• portability of equipment
• rapid performance
• relatively low costs
• avoidance of ionising radiation
• avoidance of patient transport for imaging
Disadvantages include : The primary clinical disadvantage, given the relative novelty of the technique, lies in the ongoing lack of a uniform cut-off value for the diagnosis of raised intracranial pressure . Practical disadvantages are manageable and relate primarily to the need to acquire competence in the scanning technique to optimise accuracy, the potential risk of pressure injury to the globe if technique is poor, and the potential for injury resulting from thermal and non-thermal effects of ultrasound. Ultrasound is generally acknowledged to be a safe technique (42). The largely hypothetical risks of ultrasound centre on the potential biological consequences of interaction between the scanned tissues and the ultrasound wave. These consequences may be thermal or non-thermal, and are measured by the safety indices Thermal Index (TI) and Mechanical Index (MI), which are displayed in real-time on the screen of most modern ultrasound machines. Ultrasound ispresumed to be safe when the values of the TI and MI are less than 1.0 .The TI is the ratio of the power used to the power required to produce a temperature rise of 1°C (43) Ultrasound energy from the probe passes into scanned tissues and is reflected from tissue interfaces; some energy is absorbed and converted to thermal energy, elevating the temperature of local tissues. Scanning time should be minimised to prevent possible thermal injury. It is advised that tissue temperature increase should be kept below 1.5 °C. The MI gives an approximate figure of the risk of the non-thermal effects. These include cavitation, which is the expansion and contraction of tissue gas bubbles during the cycle, and streaming, referring to the movement of complex fluids brought about by the ultrasound energy. The MI gives an approximate figure of the risk of the non-thermal effects. Optic nerve sheath ultrasound should not be used in the presence of evident or suspected rupture of the globe, or when there is significant periorbital injury. The technique is likely to be of limited incremental value in patients with chronically raised intracranial pressure or long-standing papilloedema.
The technique of optic nerve sheath ultrasound Although individual clinicians may vary in certain aspects of their examination technique, there are some general principles which will help optimise ocular ultrasound for assessment of the ONSD:
• Select the high frequency linear array probe on the ultrasound machine as this provides the best compromise between footprint and resolution of superficial structures.
• Apply ultrasound gel liberally to the closed eyelid. If desired, a clear thin dressing (e.g. IV cannula dressing) can be used as a barrier between the closed eyelid and the gel medium although this is not strictly necessary.
• Resting the probe hand on a bony structure such as the forehead or brow ridge stabilises the image and lowers the risk of inadvertent pressure on the globe.
• Place the ultrasound probe lightly over the gel in a transverse orientation initially. There should neither be any direct contact of the probe with the eyelid nor pressure exerted on the globe. The probe marker should be orientated laterally (Figure 3).
• With small, subtle movements scan from side to side (i.e. temporal to nasal), slowly angling the probe superiorly or inferiorly to bring the optic nerve into view. The nerve will appear as a ‘black stripe’ running posteriorly from the rear of the globe. The goal is to centre this on the monitor. If the lens or iris is not seen in your image, the imaging plane is likely off-axis and may result in an underestimation of ONSD.
• Both eyes should be scanned, in case of unilateral papilloedema.
• 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
a healthy adult study by Romagnuolo (44). In that study three investigators measured the ONSD in 10 separate volunteers and concluded the diameter measured by ultrasound does not change significantly with either standard Trendelenburg or reverse Trendelenburg, in comparison with a baseline supine position. The data on the impact of body position on ONSD should not be extrapolated beyond the clinical settings which have been studied, and more work remains to be done.
Differential diagnosis Although uncommon in critical care practice, there are alternative causes for a rise in ONSD and these should be kept in mind when diagnosing raised intracranial pressure ina patient with increased ONSD. The differential diagnosis of increased ONSD includes:
• raised intracranial pressure
• optic neuritis
• arachnoid cyst of the optic nerve
• anterior orbital masses
• cavernous sinus masses
• trauma to the optic nerve
• optic nerve sheath meningiosm
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…
use of ultrasound to measure ONSD can be readily appreciated on MRI (Figure 1).(3 The optic nerve sheath is bound more loosely to the optic nerve closer to the globe. This loose binding creates a much larger, and potentially more distensible, subarachnoid space in this region, which can appear bulbous on ultrasound (35). While papilloedema may take time to develop, dilation of the optic nerve sheath occurs much earlier and may be a nearinstantaneous manifestation of raised intracranial pressure (36,37).
The history of imaging the optic nerve The first report of ultrasound imaging of the eye was in 1956, but it was early cadaver studies which implicated the optic nerve sheath in the measurement of intracranial pressure. One such study noted the “bulbous portion of the optic nerve was seen to bulge or inflate somewhat as the intracranial pressure was created” with the infusion of crystalloid into the brain (38). The authors also noted this appeared to occur anteriorly, where the nerve sheath was at its thinnest and most expandable. These early studies did not measure the ONSD, relying on imprecise visual clues, and were hampered by the limited ultrasound modes available. These early modes made it difficult to locate a distinct point for measurement at a reproducible distance behind the globe.As ultrasound modalities improved, the focus of most studies was the optimum distance behind the globe at which to best measure ONSD. A 1996 study using modern ultrasonographic techniques showed that ONSD increased by up to 60% at a distance of 3mmbehind the globe in comparison to only 35% at 10 mm (37). This has been confirmed in subsequent studies, indicating that a position 3 mm behind the globe is preferred for measurement (39). Measurements made at this point are more reproducible since ultrasound contrast is greater at this depth with a linear probe. Consistent with this, the optic nerve sheath is at its most distensible anteriorly, where it is potentially most reflective of raised intracranial pressure.
The sonographic appearance of the optic nerve sheath On ultrasound, the globe is visualised as a round, dark, fluid filled structure (see Figure 2).
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…
good correlation between CEUS and MRI. This study suggests, by using nontoxic contrast, exact measurements can be more quickly and easily delineated, which may lessen the effect of operator inexperience (41).
Pros and cons of measurement of ONSD by ultrasound Sonographic ONSD assessment brings some clinical advantages but also some downsides that need to be considered when adopting the technique
Advantages include:
• reproducibility of measurements
• the non-invasive nature of the technique
• ready availability of equipment
• portability of equipment
• rapid performance
• relatively low costs
• avoidance of ionising radiation
• avoidance of patient transport for imaging
Disadvantages include : The primary clinical disadvantage, given the relative novelty of the technique, lies in the ongoing lack of a uniform cut-off value for the diagnosis of raised intracranial pressure . Practical disadvantages are manageable and relate primarily to the need to acquire competence in the scanning technique to optimise accuracy, the potential risk of pressure injury to the globe if technique is poor, and the potential for injury resulting from thermal and non-thermal effects of ultrasound. Ultrasound is generally acknowledged to be a safe technique (42). The largely hypothetical risks of ultrasound centre on the potential biological consequences of interaction between the scanned tissues and the ultrasound wave. These consequences may be thermal or non-thermal, and are measured by the safety indices Thermal Index (TI) and Mechanical Index (MI), which are displayed in real-time on the screen of most modern ultrasound machines. Ultrasound ispresumed to be safe when the values of the TI and MI are less than 1.0 .The TI is the ratio of the power used to the power required to produce a temperature rise of 1°C (43) Ultrasound energy from the probe passes into scanned tissues and is reflected from tissue interfaces; some energy is absorbed and converted to thermal energy, elevating the temperature of local tissues. Scanning time should be minimised to prevent possible thermal injury. It is advised that tissue temperature increase should be kept below 1.5 °C. The MI gives an approximate figure of the risk of the non-thermal effects. These include cavitation, which is the expansion and contraction of tissue gas bubbles during the cycle, and streaming, referring to the movement of complex fluids brought about by the ultrasound energy. The MI gives an approximate figure of the risk of the non-thermal effects. Optic nerve sheath ultrasound should not be used in the presence of evident or suspected rupture of the globe, or when there is significant periorbital injury. The technique is likely to be of limited incremental value in patients with chronically raised intracranial pressure or long-standing papilloedema.
The technique of optic nerve sheath ultrasound Although individual clinicians may vary in certain aspects of their examination technique, there are some general principles which will help optimise ocular ultrasound for assessment of the ONSD:
• Select the high frequency linear array probe on the ultrasound machine as this provides the best compromise between footprint and resolution of superficial structures.
• Apply ultrasound gel liberally to the closed eyelid. If desired, a clear thin dressing (e.g. IV cannula dressing) can be used as a barrier between the closed eyelid and the gel medium although this is not strictly necessary.
• Resting the probe hand on a bony structure such as the forehead or brow ridge stabilises the image and lowers the risk of inadvertent pressure on the globe.
• Place the ultrasound probe lightly over the gel in a transverse orientation initially. There should neither be any direct contact of the probe with the eyelid nor pressure exerted on the globe. The probe marker should be orientated laterally (Figure 3).
• With small, subtle movements scan from side to side (i.e. temporal to nasal), slowly angling the probe superiorly or inferiorly to bring the optic nerve into view. The nerve will appear as a ‘black stripe’ running posteriorly from the rear of the globe. The goal is to centre this on the monitor. If the lens or iris is not seen in your image, the imaging plane is likely off-axis and may result in an underestimation of ONSD.
• Both eyes should be scanned, in case of unilateral papilloedema.
• 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
a healthy adult study by Romagnuolo (44). In that study three investigators measured the ONSD in 10 separate volunteers and concluded the diameter measured by ultrasound does not change significantly with either standard Trendelenburg or reverse Trendelenburg, in comparison with a baseline supine position. The data on the impact of body position on ONSD should not be extrapolated beyond the clinical settings which have been studied, and more work remains to be done.
Differential diagnosis Although uncommon in critical care practice, there are alternative causes for a rise in ONSD and these should be kept in mind when diagnosing raised intracranial pressure ina patient with increased ONSD. The differential diagnosis of increased ONSD includes:
• raised intracranial pressure
• optic neuritis
• arachnoid cyst of the optic nerve
• anterior orbital masses
• cavernous sinus masses
• trauma to the optic nerve
• optic nerve sheath meningiosm