Critical Care Horizons 22

Optic Nerve Sheath Ultrasound for the BedsideDiagnosis of Intracranial Hypertension: Pitfallsand PotentialClaire Shevlin1*

1Locum Consultant, Intensive Care Unit, Craigavon Area Hospital, 68 Lurgan Road, Portadown, United Kingdom*Corresponding author’s email: claireshevlin@gmail.com

Date published: 26/06/2015

AbstractRaised intracranial pressure is a complication not just of traumatic brain injury and other acutecerebral insults, but also of a number of general medical conditions. Bedside diagnosis can be difficult;early clinical signs may be misinterpreted, and reliance on cross-sectional imaging studies may furtherdelay diagnosis. Ultrasound is a readily available imaging modality in most critical care areas, andexamination of the optic nerve sheath by bedside ultrasound allows detection of changes in diameterwhich may indicate intracranial hypertension. This paper reviews optic nerve sheath anatomy as abasis for its potential to provide a window on changes within the intracranial cerebrospinal fluid(CSF) space, the technique of sonographic measurement of optic nerve sheath diameter (ONSD),the evidence for correlation with intracranial pressure, and the comparison of ONSD with the ‘goldstandard’ method of intracranial pressure assessment.

Keywords: Intracranial hypertension; Bedside ultrasound; Optic nerve sheath diameter; Ocularultrasound

Competing Interests: The author declares that she has no competing interests

Released under a Creative Commons CC BY-NC-ND 4.0 license except for content otherwise specified

DOI: To be confirmed

Introduction

Bedside ultrasound scanning, particularly echocardiographyand thoracoabdominal sonography, is widely used in theintensive care unit and emergency department to excludelife-threatening pathology. The use of ultrasound of theoptic nerve and its sheath to diagnose time-critical raisedintracranial pressure following traumatic brain injury is lesswidely practiced, but has been described in several smallstudies over the past two decades.

The diameter of the optic nerve sheath has been found tobe a strong predictor of raised intracranial pressure, with ahigh sensitivity and specificity in multiple studies and in asystematic review [1]. Raised intracranial pressure is a commonemergency following brain injury, with prompt diagnosishaving a significant impact on morbidity and mortality [2]

Ultrasound measurement of the optic nerve sheathdiameter (ONSD) allows repeated noninvasive assessments ofintracranial pressure and facilitates evaluation of the responseto treatment. As invasive intracranial pressure monitoringis typically restricted to neurosurgical centres, this mode of

investigation is particularly suited to patients suspected ofraised intracranial pressure prior to transfer for definitivetreatment, as well as patients who continue to be cared forin non-neurosurgical critical care units. In addition to itsdiagnostic goal, there is some, albeit limited, evidence tosuggest that ONSD can also be used for prognostication [3, 4].

The majority of articles on ONSD measurement are inthe setting of raised intracranial pressure secondary totraumatic brain injury. However, a few studies haveused this measurement to diagnose or assess the severityof other pathologies, including meningitis, stroke, hepaticencephalopathy, epilepsy, and acute mountain sickness [5–9].

The potential value of this technique is reflected in thesignificant number of studies performed to date. Unfortunately,most have small patient numbers and hence low power,and could be criticised for potential observer bias. Effortsare ongoing to define the ONSD indicating ‘true’ raisedintracranial pressure, the best sonographic approach tovisualise the optic nerve sheath, the optimum axis to assess theoptic nerve sheath, and the impact of operator experience onmeasurement variability.

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Search strategy

PubMed and Medline databases from 1950 through to April2015 were searched, using the OvidSp interface. Thereference terms used (along with synonyms and closely relatedwords) were optic nerve sheath, optic nerve sheath diameter,optic ultrasound, optic nerve sonography, and optic nerveultrasonography. These terms were searched for separatelyand in combination with the terms intracranial pressure andpapilloedema. Further studies were identified by examiningthe reference lists of included articles. This yielded a total ofover 100 articles. After review, forty-two articles were judgedto be directly relevant to the topic discussed and are referenced.

The anatomy of the optic nerve sheath

The intraorbital section of the optic nerve extends from theglobe, where it inserts medially, to the optic canal located in thelesser wing of the sphenoid bone. It is encased by a meningealsheath consisting of dura mater, arachnoid mater and piamater. Cerebrospinal fluid is contained in the trabeculatedsubarachnoid space and is continuously and slowly filtered.As a result the optic nerve sheath is in direct communicationwith the intracranial subarachnoid space. It is this relationshipthat forms the physiological basis for using the optic nervesheath as a surrogate for intracranial pressure measurement.

The anatomical relationships underpinning the use ofultrasound to measure ONSD can be readily appreciated onMRI (Figure 1).

Figure 1. MRI anatomy of the optic nerve and sheath

Case courtesy of Dr Frank Gaillard, Radiopaedia.org. Fromthe case Optic nerve and chiasm (MRI anatomy)

The optic nerve sheath is bound more loosely to the opticnerve closer to the globe. This loose binding creates a muchlarger, and potentially more distensible, subarachnoid spacein this region, which can appear bulbous on ultrasound [10].While papilloedema may take time to develop, dilation of theoptic nerve sheath occurs much earlier and may be a near-instantaneous manifestation of raised intracranial pressure[11, 12].

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 opticnerve sheath in the measurement of intracranial pressure. Onesuch study noted the “bulbous portion of the optic nervewas seen to bulge or inflate somewhat as the intracranialpressure was created” with the infusion of crystalloid intothe brain [13]. The authors also noted this appeared to occuranteriorly, where the nerve sheath was at its thinnest andmost expandable. These early studies did not measure theONSD, relying on imprecise visual clues, and were hamperedby the limited ultrasound modes available. These early modesmade it difficult to locate a distinct point for measurement at areproducible distance behind the globe.

As ultrasound modalities improved, the focus of moststudies was the optimum distance behind the globe atwhich to best measure ONSD. A 1996 study using modernultrasonographic techniques showed that ONSD increased byup to 60% at a distance of 3 mm behind the globe in comparisonto only 35% at 10 mm [12]. This has been confirmed insubsequent studies, indicating that a position 3 mm behind theglobe is preferred for measurement [14]. Measurements madeat this point are more reproducible since ultrasound contrastis greater at this depth with a linear probe. Consistent withthis, the optic nerve sheath is at its most distensible anteriorly,where it is potentially most reflective of raised intracranialpressure.

The sonographic appearance of the optic nerve sheath

On ultrasound, the globe is visualised as a round, dark, fluidfilled structure (see Figure 2). The anterior chamber is anechoic,as generally is the lens, while the iris appears bright andechogenic. The choroid and retina may be seen as a thin greylayer at the posterior aspect of the globe. The optic nerve isthe ‘black stripe’ running away from the posterior aspect ofthe globe and optic disc, and should ideally be positioned inthe centre of the ultrasound screen. The nerve sheath, as seenon ultrasound examination, has a high reflectivity comparedto the homeogenous appearance of the nerve, and should berelatively easy to distinguish.

If the optic nerve sheath is markedly dilated, it may bepossible to diagnose this from visual estimation alone. Ingeneral, however, the software calipers should be used toensure accurate measurement and recording. In severelyraised intracranial pressure, it may be possible to visualisea ‘crescent sign’[15], an echolucent circular artefact within thesheath separating the sheath from the nerve due to increasedsubarachnoid fluid.

There has been interest in using contrast enhancedultrasound (CEUS) to help identify and recognise the anatomysurrounding the optic nerve, which is a small structure. Theincorrect identification of artefacts as part of the sheath byan inexperienced sonographer is a criticism of the technique.A small proof of concept study, using a second generationcontrast agent (Sonovue®, Bracco SpA), found good correlationbetween CEUS and MRI. This study suggests, by using non-toxic contrast, exact measurements can be more quickly andeasily delineated, which may lessen the effect of operatorinexperience [16].

Pros and cons of measurement of ONSD by ultrasound

Sonographic ONSD assessment brings some clinical advan-tages but also some downsides that need to be consideredwhen adopting the technique.

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Figure 2. Ultrasound anatomy on transverse imaging

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

The primary clinical disadvantage, given the relativenovelty of the technique, lies in the ongoing lack of a uniformcut-off value for the diagnosis of raised intracranial pressure(see below). Practical disadvantages are manageable and relateprimarily to the need to acquire competence in the scanningtechnique to optimise accuracy, the potential risk of pressureinjury to the globe if technique is poor, and the potentialfor injury resulting from thermal and non-thermal effects ofultrasound.

Ultrasound is generally acknowledged to be a safetechnique [17]. The largely hypothetical risks of ultrasoundcentre on the potential biological consequences of interactionbetween the scanned tissues and the ultrasound wave. Theseconsequences may be thermal or non-thermal, and aremeasured by the safety indices Thermal Index (TI) andMechanical Index (MI), which are displayed in real-time on thescreen of most modern ultrasound machines. Ultrasound ispresumed to be safe when the values of the TI and MI are lessthan 1.0 [18].The TI is the ratio of the power used to the powerrequired to produce a temperature rise of 1°C [18]

Ultrasound energy from the probe passes into scannedtissues and is reflected from tissue interfaces; some energyis absorbed and converted to thermal energy, elevatingthe temperature of local tissues. Scanning time should be

minimised to prevent possible thermal injury. It is advisedthat tissue temperature increase should be kept below 1.5 °C.

The MI gives an approximate figure of the risk of thenon-thermal effects. These include cavitation, which is theexpansion and contraction of tissue gas bubbles during thecycle, and streaming, referring to the movement of complexfluids brought about by the ultrasound energy. The MI givesan approximate figure of the risk of the non-thermal effects.

Optic nerve sheath ultrasound should not be used in thepresence of evident or suspected rupture of the globe, or whenthere is significant periorbital injury. The technique is likelyto be of limited incremental value in patients with chronicallyraised intracranial pressure or long-standing papilloedema.

The technique of optic nerve sheath ultrasound

Although individual clinicians may vary in certain aspects oftheir examination technique, there are some general principleswhich will help optimise ocular ultrasound for assessment ofthe ONSD:

• Select the high frequency linear array probe on theultrasound machine as this provides the best compromisebetween footprint and resolution of superficial structures.

• Apply ultrasound gel liberally to the closed eyelid. Ifdesired, a clear thin dressing (e.g. IV cannula dressing)can be used as a barrier between the closed eyelid and thegel medium although this is not strictly necessary.

• Resting the probe hand on a bony structure such as theforehead or brow ridge stabilises the image and lowersthe risk of inadvertent pressure on the globe.

• Place the ultrasound probe lightly over the gel in atransverse orientation initially. There should neitherbe any direct contact of the probe with the eyelid norpressure exerted on the globe. The probe marker shouldbe orientated laterally (Figure 3a).

• With small, subtle movements scan from side to side (i.e.temporal to nasal), slowly angling the probe superiorly orinferiorly to bring the optic nerve into view. The nerve willappear as a ‘black stripe’ running posteriorly from the rearof the globe. The goal is to centre this on the monitor. Ifthe lens or iris is not seen in your image, the imaging planeis likely off-axis and may result in an underestimation ofONSD.

• The globe should also be scanned in the parasagittal plane,with the probe marker superiorly, towards the patient’sforehead (Figure 3b).

• Both eyes should be scanned, in case of unilateralpapilloedema.

• The time spent in active scanning should be minimised.Once the optimum view has been obtained, store theimage either as a frame or a video loop and remove theprobe from the eye. Measurements can then be performedwithout unnecessary exposure of the eye to ultrasoundenergy.

• Use the caliper function on the ultrasound to enableprecise measurement. First locate a point 3 mm posteriorto the optic disk. At this point place the calipers at 90degrees to the axis of the optic nerve to measure thediameter of optic nerve and optic nerve sheath (Figure4).

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(a) Transverse imaging (b) Parasagittal imaging

Figure 3. Ultrasound probe application for ONSD measurement

• Take the average of two or three measurements for eachside.

A 1996 study by Helmke & Hansen suggested theoptimal scanning orientation was longitudinal (axial), asthis was associated with the least inter-observer variability[12]. However, aside from the variability findings, there wasno significant difference in measurements between the twoorientations.

Most patients will be scanned supine, or with a 20° to 30°head up tilt. A Nepalese study, which included 287 patients,examined the correlation between ONSD and acute mountainsickness. This study suggested ONSD does not change withpatient positioning [9]. This was supported by results from ahealthy adult study by Romagnuolo [19]. In that study threeinvestigators measured the ONSD in 10 separate volunteersand concluded the diameter measured by ultrasound does notchange significantly with either standard Trendelenburg orreverse Trendelenburg, in comparison with a baseline supineposition. The data on the impact of body position on ONSDshould not be extrapolated beyond the clinical settings whichhave been studied, and more work remains to be done.

Differential diagnosis

Although uncommon in critical care practice, there arealternative causes for a rise in ONSD and these should bekept in mind when diagnosing raised intracranial pressure ina patient with increased ONSD. The differential diagnosis ofincreased 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 meningioma

Establishing the ONSD cut-off for diagnosis of raisedintracranial pressure

Evidence from multiple studies supports correlation betweenONSD and opening pressure of cerebrospinal fluid orintracranial pressure in both adults and children [2, 14, 20–22]. Table 1 summarises the main clinical studies reporting anassociation between ONSD and intracranial pressure. Somestudies show a high sensitivity and negative predictive valuefor the detection of further increases in intracranial pressure[2, 23]. A more recent study attempted to investigate theimpact of brainstem death on ONSD, as compared to patientsin a state of coma [24]. In this study, ONSD was measuredin 29 brainstem dead patients, 19 comatose patients (11 withindications of raised intracranial pressure), 20 patients withestablished neurological disease and 40 healthy subjects ascontrols. While the comatose and brainstem dead patientsshowed a markedly increased ONSD in comparison to the othertwo groups, there was limited evidence to suggest brainstemdeath could be reliably distinguished from coma on the basisof ONSD.

On review of earlier studies, a tentative association emergedbetween an ONSD of 5.0 mm and the presence of raisedintracranial pressure. In a study of 59 emergency departmentpatients, Tayal [2] found an ONSD of 5.0 mm had a 100%sensitivity to detect patients with raised intracranial pressure.Another emergency department study by Qayyum, found asensitivity and specificity of 100% and 75%, respectively, for acutoff of 5.0 mm, with positive and negative predictive valuesof 94.5 and 100% [23].

However, there remains controversy about the exactdiameter of the optic nerve sheath that best predicts elevatedintracranial pressure. The bulk of studies recruited less than100 patients and indicate a 5 mm cut-off. However, otherstudies have found evidence for different optimal cut-offs.Rajajee and colleagues used 5.2 mm as their upper limit ofnormal measurement [25], Soldatos used 5.7 mm [26], andBäuerle used 5.8 mm [20]. The variation in cut-off acrossstudies is significant, and ranges from 4.8 mm to 6.0 mm[25, 27–29]. This makes it more difficult to establish a ‘set’measurement above which raised intracranial pressure can bediagnosed based on ONSD.

There are multiple explanations for the differences in theupper limit of normal. The studies from 1996-2015 focused on

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Figure 4. Caliper positions for ONSD measurement

heterogeneous patient populations. The majority of studieswere done on ICU and emergency department patients andcompared a control group with a group that might reasonablybe expected to have a high intracranial pressure. Not allstudies attempted to stratify results by gender or ventilatorystatus, both factors which may affect results. A significantproportion were performed in medical inpatients requiring alumbar puncture for neurological disease, or in outpatientsundergoing treatment for intracranial hypertension. While itwas standard in most studies (where stated) to use a 7.5 MHzlinear ultrasound probe, this was not always the case. Lastly,the ‘confirmation’ of raised intracranial pressure was done witha variety of different methods – MRI, CT, invasive intracranialpressure monitors, and CSF opening pressure. This makescomparison between studies difficult.

Of note, ethnic differences may need to be taken intoaccount when measuring ONSD as a surrogate measure of ICP.A study in Chinese patients correlated an elevated openingpressure on lumbar puncture with a significantly lower ONSDthan in Caucasian populations [30].

ONSD measurement has not been widely examined inpaediatric patients and no strong evidence to diagnose raisedintracranial pressure exists. One study, in 64 paediatric patients,reported a very low specificity of ONSD for raised intracranialpressure [31].

Also, a small porcine study noted a lack of correlationbetween ONSD changes and central venous pressure. Itis possible ONSD alone is a poor indicator of change inintracranial pressure when the rise in intracranial pressure

is solely influenced by a rise in central venous pressure [32].

Comparison of ONSD by ultrasound with othermethods of assessing intracranial pressure

Studies of sonographic ONSD have mainly correlated findingswith clinical and radiological signs and symptoms ofelevated intracranial pressure. Intraventricular measurementis the gold standard for measuring intracranial pressure.These measurement devices carry many risks, includinghaemorrhage and infection. These complications partlyaccount for increasing interest in non invasive methodssuch as neuroimaging, transcranial Doppler sonography,ONSD ultrasonography and CT/MRI. In addition, invasivemethods may not be available to individual patients due tocontraindications, such as coagulopathy, thrombocytopaenia,or the lack of local facilities.

The correlation of ONSD with intraventricular pressuremonitoring has not been widely examined, probably because ofthe invasive nature of the latter and the limited pool of patientsavailable. Kimberly correlated ventriculostomy intracranialpressure measurements with ONSD [21]. They found a positivecorrelation with a Spearman rank correlation coefficient ofONSD and ICP of 0.59 (p < 0.0005). Similarly, the area underthe ROC curve for ability of ONSD to distinguish an ICP greaterthan 20 cm H2O was 0.93. Their results indicated a reasonablyhigh specificity of 93% and sensitivity of 88%.

This compares well with the data from clinical andradiological studies. For example, Blaivas correlated ONSD

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Author (ref) Patients (n) Comparison groups (mean/range ONSD) Optimal cut-off (sensitivity/specificity) Population

Hansen and Helmke 1997 [11] 39Control range 2.7 - 4 mm

Patients with raised ICP 3.6 - 6.8 mm5 mm

ICU, children

7.5 MHz probe

Blaivas et al 2003 [33] 35Patients without raised ICP, mean 4.42 mm

Patients with raised ICP, mean 6.27 mm

5.0 mm

Sensitivity 100%, specificity 95%

ED, adults

10 MHz probe

Geeraerts et al 2007 [27] 31Patients without raised ICP, up to 5.1 mm

Patients with raised ICP, up to 6.3 mm

5.7 mm

Sensitivity 100%

ICU, adults

7.5 MHz probe

Tayal et al 2007 [2] 59 Patients with raised ICP, >5mm5 mm

Sensitivity 100%, specificity 63%

ED, adults

7.5 MHz probe

Kimberly et al 2008 [21] 15Patients without raised ICP, mean 4.4 mm

Patients with raised ICP, mean 5.4 mm

5 mm

Sensitivity 88%, specificity 93%

ED/ICU adults

10 MHz probe

Soldatos et al 2008 [26] 76Control without raised ICP, mean 3.6 mm

Patients with raised ICP, mean 6.1 mm

5.7 mm

Sensitivity 74%, specificity 100%ICU, adults

Geeraerts et al 2008 [28] 37 Head injury patients requiring ICP monitor5.9 mm, AUC ROC 0.91

Sensitivity 90%, specificity 84%

ICU, adults

7.5 MHz probe

Goel et al 2008 [34] 100Patients without raised ICP, mean 3.5 mm

Patients with raised ICP, mean 5.8 mm

5 mm

Sensitivity 98.6%, specificity 92.8%

Trauma, adults

7.5 MHz probe

Watanabe 2008 [35] 12Pre-v-post-op chronic subdural collections

Mean 6.1 mm pre-op, 4.8 mm post-opNo cut-off given

Neurosurgery

MRI used

Moretti et al 2009 [36] 535.2 mm

Sensitivity 93%, specificity 74%

ICU, adults

7.5 MHz probe

Moretti et al 2009 [22] 63

Controls, mean 4.9 mm

Patients without raised ICP, mean 5 mm

Patients with raised ICP, mean 6.2 mm

5.2 mm

Sensitivity 94%, specificity 76%

ICU, adults

7.5 MHz probe

Bäuerle et al 2011 [20] 10Controls, mean 5.4 mm

Patients with raised ICP, mean 6.4 mm

5.8 mm

Sensitivity 90%, specificty 84%

Non-ICU neuro

Adults

Rajajee et al 2011 [25] 65Patients without raised ICP, mean 4 mm

Patients with raised ICP 5.3 mm

4.8 mm

Sensitivity 96%, specificity 94%ICU, adults

Strumwasser et al 2011 [29] 10 Trauma patients requiring ICP monitoring6 mm

Sensitivity 26%, specificity 38%ICU, adults

Cammarata et al 2011 [37] 21

Controls, mean 5.51 mm

Patients without raised ICP, mean 5.52 mm

Patients with raised ICP, mean 7 mm

No cut-off stated

r=0.74 for correlation of ONSD with ICPICU, adults

Amini et al 2013 [38] 50Patients without raised ICP, mean 4.6 mm

Patients with raised ICP, mean 6.7 mm

5.5 mm

Sensitivity 100%, specificity 100%Neuro, adults

Qayyum et al 2013 [23] 24Patients with suspected raised ICP

Compared against CT findings

5 mm

Sensitivity 100%, specificity 75%

Positive predictive value 95.4%

ED, adults

Caffery et al 2014 [39] 51ONSD against opening pressure on LP with

20 cmH2O chosen

5 mm pre-selected cut-off

Sensitivity 75%, specificity 44%

AUC on ROC 0.69

ED, adults

Non-trauma

Shirodkar 2014 [6] 101Control, male 4.8 mm, female 4.6 mm

Patient with rasied ICP, mean 5.4 mm

4.6 mm for females:

sensitivity 84.6%, specificity 100%

4.8 mm for males:

sensitivity 75%, specificity 100%

Medical, adults

Mehrpour 2015 [40] 32 Patients with raised ICP, mean 6.24 mm5.7 mm

Sensitivity 100%

Neuro, adults

7.5 MHz probe

Wang et al 2015 [30] 279No high opening pressure 3.55 mm

With high opening pressure 4.58 mm

4.1 mm

Sensitivity 95%, specificity 92%

AUC ROC curve 0.965

Neuro, Chinese adults

9-3 probe

Topcuoglu et al 2015 [24] 29

Controls, mean 4.69 mm

Neurological disease, mean 4.36 mm

Coma raised ICP, mean 5.89 mm

Coma no raised ICP, mean 5.16 mm

Brainstem dead, mean 6.09 mm

No cut-off (attempt to identify ONSD

difference between brainstem dead and

coma with raised ICP

ICU, adults

Table 1. Summary of clinical studies investigating ONSD

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with radiological signs of raised intracranial pressure, asdefined by the presence of mass effect with midline shift greateror equal to 3 mm, a collapsed third ventricle, hydrocephalusor the effacement of sulci with evidence of significant oedema,and found a sensitivity of 100% and specificity of 95% forONSD compared to CT, with positive and negative predictivevalues of 93% and 100% respectively [33]. With regard toneuroimaging, Soldatos found good correlation of ONSD withCT findings in brain-injured adults (r = 0.68, P = 0.002) [26].MRI is often spoken of as a reference test for ONSD, as ithas higher spatial resolution, and the images offer a morerepresentative calculation of the mean diameter, than CT. WhileLagrèze and colleagues contend the accuracy of MRI exceedssonographic methods for determining ONSD [41], Bauerleshowed good scan-rescan reproducibility and good observeragreement in 15 healthy volunteers [20].

Transcranial doppler (TCD) has also been investigated, butsonography windows were often inadequate. While somestudies have generally found it to be a poor predictor ofintracranial pressure, others have found the method basedon two depth TCD more reliable than ONSD measurement[42–44]. This last study enrolled 92 patients and comparedboth non-invasive methods of measurement with CSF pressureobtained by lumbar puncture, using a cut-off for ICP of 14.7mmHg and 5 mm for ONSD. The sensitivity and specificity ofTCD were better than ONSD (sensitivity 37% for ONSD, 68%for TCD, specificity 58.5% for ONSD, 84.3% for TCD). The areaunder the ROC curve was 0.57 for ONSD and 0.87 for TCD.Skill acquisition, acoustic window availability, and ease of usemay favour ultrasound assessment of ONSD by critical careclinicians over TCD, in spite of potential diagnostic advantagesof TCD.

ONSD in practice

From the available evidence, it is reasonable to concludethat optic nerve ultrasound has utility as part of the non-invasive assessment of intracranial pressure, and may offerpotential means of reducing complications associated withintraventricular intracranial pressure monitors by bettertargeting these devices. Naturally, the technique will notbe viable in some patients, such as those with severe oculartrauma, and it must be borne in mind that a number ofaetiologies other than raised intracranial pressure can leadto dilatation of the optic nerve sheath.

Increased ONSD may also be of prognostic significance.Legrand found that ONSD, as measured on brain CT in 77patients, was independently associated with ICU mortality,albeit at greater than 7 mm [4].

It may be argued the main value of this techniquelies in early evaluation, during the initial assessment andresuscitation or transport phases, where cross-sectionalimaging is unavailable. Similarly, the technique may be ofvalue in bedside assessment in the ICU, although studies areneeded to determine if this is a viable strategy to reduce theneed for CT or MRI imaging.

For medical patients at risk of raised intracranial pressure,such as those with liver failure, this straightforward bedsideassessment may be helpful in risk stratification and decision-making around the timing of, and need for, CT imaging. Thisassessment would also be beneficial in these patients prior toprocedures such as central line insertion, as part of the decision-making process around target vein and patient position duringaccess.

Conclusion

The studies to date have generally been small and poorlypowered with the inevitable result that many questions remainunanswered. There is a clear opportunity for the critical carecommunity to collaborate on larger scale studies to evaluatethe potential impact of pathways using ultrasound assessmentof ONSD.

ONSD values greater than 5 mm, and certainly greater than5.8 mm, have been shown to be highly specific and sensitivefor the presence of raised intracranial pressure. Elevated ICPshould be considered in the presence of an ONSD greater than5mm, and if greater than 5.5 mm urgent consideration shouldbe given to medical management pending further diagnosticworkup. In adolescents and non-Caucasian populations, thethresholds are likely to vary and additional data is requiredbefore recommendations can be made.

It is important that physicians using this technique areproficient in clinical ultrasound to minimise errors and adverseeffects due to poor scanning technique. Fortunately ONSDmeasurement by ultrasound is readily taught. For governance,training, and quality monitoring purposes the indication forperformance of ONSD assessment should be recorded alongwith the technique and findings. These should be reviewed inlight of results of additional investigations such as CT or MRI.

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