Optic nerve sonography in the diagnostic evaluation of adult brain injury

Introduction  

The optic nerve sheath diameter (ONSD) may be increased in brain-injured patients, especially children, with intracranial hypertension. We investigated whether measurements of ONSD correlated with simultaneous noninvasive and invasive measurements of the intracranial pressure (ICP) in brain-injured adults.     

超声测量视神经鞘直径评估颅内压增高的临床研究进展

颅内压监测在神经重症的治疗中十分重要,目前主要有无创和有创两种方法,各具优点,有创监测颅内压虽为颅内压监测的金标准,但缺点明显,无法广泛应用。视神经鞘具有特殊的解剖结构,颅内压增高时视神经鞘直径(ONSD)会出现增粗,故超声监测ONSD判断颅内压升高是一种无创、简便、易行且可以床旁重复操作的方法。本文就国内外超声监测ONSD判断颅高压研究进展进行综述。     

Noninvasive surrogates of intracranial pressure: another piece added with magnetic resonance imaging of the cerebrospinal fluid thickness surrounding the optic nerve

Optic nerve sheath diameter enlargement measured by ultrasound is known to be associated with raised intracranial pressure (ICP). Magnetic resonance imaging (MRI) of the cerebrospinal fluid (CSF) surrounding the optic nerve has been found, in an elegant study published in Critical Care, to correlate well with lumbar CSF opening pressure, confirming previously published studies. A simple and rapid T2-weighted fat-suppressed sequence was used to noninvasively measure the CSF width surrounding the optic nerve, allowing one to predict lumbar CSF pressure with relatively good performances, taking into consideration the body mass index and the mean arterial blood pressure. Based on these results and when brain MRI is indicated in situations at risk for increased ICP, the optic CSF thickness measurement could be systematically performed and used as a surrogate marker of raised ICP.Several acute life-threatening conditions such as liver failure, bacterial meningitis, meningoencephalitis, posterior reversible encephalopathy, or post-resuscitation syndrome may be associated with a significant risk of raised intracranial pressure (ICP) [1-5]. Brain magnetic resonance imaging (MRI) is often performed in these ICU patients presenting with coma for diagnosis but also for prognosis.We have known for years that the prognosis in neurocritical patients (traumatic brain injury or stroke) is affected by the incidence of raised ICP, and is particularly affected by the time spent with ICP >25 mmHg [6]. In other medical conditions, however, invasive ICP is rarely measured. Having a noninvasive estimate of ICP is certainly of great interest to detect patients at risk of raised ICP and to initiate specific treatments aiming at decreasing ICP, and/or to decide to monitor ICP invasively.In the absence of obstruction in the cerebrospinal fluid (CSF) circulation, the subarachnoid space surrounding the optic nerve is submitted to the same pressure as the intracranial compartment. Owing to the cul-de-sac anatomy of the optic nerve, CSF can accumulate in the retro-orbital part of the optic nerve, and the sheath can inflate in the presence of raised ICP [7]. Several publications have shown than the optic nerve sheath diameter measured using ultrasound [8-10] or MRI [11,12] correlates well with invasive ICP.In an elegant study, Xie and colleagues have shown that the CSF width surrounding the optic nerve measured using a simple T2-weighted fast-recovery fast-spin echo fat-suppressed sequence correlates well with lumbar CSF pressure in 72 patients with various neurological conditions (CSF hypotension, meningitis, multiple sclerosis, and so forth) [13]. Very similar results showing that the MRI optic nerve sheath, but not the optic nerve itself, correlated well with invasive ICP in comatose traumatic brain injury patients have been published previously in Critical Care[11].Using a stepwise multivariate linear regression analysis, Xie and colleagues have developed a relatively simple formula based on the CSF width surrounding the optic nerve, body mass index, and blood pressure to predict ICP noninvasively [13]. In the ICP range from 3 to 26 mmHg, CSF widths 3 mm, 9 mm or 15 mm behind the retina significantly correlated with the lumbar CSF opening pressure (r² = 0.82 to 0.88) We must, however, note that significant raised ICP (ICP >20 mmHg) was only present in eight of the 72 patients. Moreover, lumbar CSF pressure and MRI were performed with a 24-hour delay. We should also highlight that the relationship between optic nerve sheath enlargement and ICP may not be linear. Hansen and colleagues have clearly shown in an experimental study that a plateau can be reached with a maximum enlargement of the sheath occurring for high ICP values (above 35 to 40 mmHg), and that capability for retraction of the sheath can be altered after exposing the optic nerve trabecula to high pressure and distension [14]. These limitations of the study should be taken into consideration before generalisation of these results in ICU patients with significant raised ICP.A major interest of the Beijing Intracranial and Intraocular Pressure Study Group is that body mass index has been considered in the prediction model [13]. Anatomy can obviously affect the size of the optic nerve and its sheath. However, normative data of the optic nerve complex, considering the age and the height of the subject, are lacking. Determining these data should be probably be an initial step in the process of studying the possible utilization of the optic nerve CSF width to predict raised ICP.The T2-weighted fat-suppressed MRI sequence lasts less than 5 minutes and is considered a standard sequence. Moreover, measuring the CSF width takes less than 1 minute. Considering the very interesting results from the Beijing Intracranial and Intraocular Pressure Study Group and the simplicity of optic nerve width measurement, we could wonder whether this pmeter should be systematically measured when MRI is performed in comatose patients. This measurement could offer the possibility to detect raised ICP using a simple formula taking into consideration the optic nerve width, the body mass index and mean arterial blood pressure. Early detection of raised ICP based on MRI is certainly of great interest to initiate specific treatments and to refer patients to a specialised critical care unit.     

Alterations in optic nerve sheath diameter according to cerebrovascular disease sub-groups

Background

ONSD (optic nerve sheath diameter) is a method used for indirect measurement of the increased intracranial pressure. In previous studies, the relation between the increased intracranial pressure and ONSD was analyzed in the patients suffering from cerebrovascular diseases (CVD). In our study, the patients suffering from ischemic CVD were categorized into 4 subgroups according to Oxfordshire Community Stroke Project classification (OCSP); the relationship between each group and ONSD, and the influence on each eye were analyzed.

Optic nerve sheath diameter measurement: a means of detecting raised ICP in adult traumatic and non-traumatic neurosurgical patients

Introduction

Bedside ultrasound measurement of optic nerve sheath diameter (ONSD) is emerging as a non-invasive technique to evaluate and predict raised intracranial pressure (ICP). It has been shown in previous literature that ONSD measurement has good correlation with surrogate findings of raised ICP such as clinical and radiological findings suggestive of raised ICP.

Using MRI of the optic nerve sheath to detect elevated intracranial pressure

The current gold standard for the diagnosis of elevated intracranial pressure (ICP) remains invasive monitoring. Given that invasive monitoring is not always available or clinically feasible, there is growing interest in non-invasive methods of assessing ICP using diagnostic modalities such as ultrasound or magnetic resonance imaging (MRI). Increased ICP is transmitted through the cerebrospinal fluid surrounding the optic nerve, causing distention of the optic nerve sheath diameter (ONSD). In this issue of Critical Care, Geeraerts and colleagues describe a non-invasive method of diagnosing elevated ICP using MRI to measure the ONSD. They report a positive correlation between measurements of the ONSD on MRI and invasive ICP measurements. If the findings of this study can be replicated in larger populations, this technique may be a useful non-invasive screening test for elevated ICP in select populations.     

超声测量视神经鞘直径预测颅内压增高的围手术期应用进展

视神经鞘(ONS)是颅内硬脑膜的直接延续,内有横梁式的蛛网膜下腔。当患者颅内压(ICP)升高时,脑脊液会经蛛网膜滤出使视神经鞘增宽,因此可以用视神经鞘直径(ONSD)预测ICP增高。目前,围手术期ICP监测手段较少,超声测量ONSD预测ICP的技术因具有无创、床旁、快速等优势在临床上被广泛应用,将该技术应用于围术期患者中可以提供术中ICP监测。本文就ICP监测现状、ONSD与ICP的关系、ONSD预测ICP增高的临界值及ONSD预测围术期ICP增高的应用前景作一综述,以期为围手术期应用超声测量ONSD预测ICP增高提供支持。     

Ultrasonography of optic nerve sheath diameter for detection of raised intracranial pressure: a systematic review and meta-analysis

Purpose

To evaluate the diagnostic accuracy of ultrasonography of optic nerve sheath diameter (ONSD) for assessment of intracranial hypertension.

Methods

Systematic review without language restriction based on electronic databases, with manual review of literature and conference proceedings until July 2010. Studies were eligible if they compared ultrasonography of ONSD with intracranial pressure (ICP) monitoring. Data were extracted independently by three authors. Random-effects meta-analysis and meta-regression were performed.

Results

Six studies including 231 patients were reviewed. No significant heterogeneity was detected for sensitivity, specificity, positive and negative likelihood ratios or diagnostic odds ratio. For detection of raised intracranial pressure, pooled sensitivity was 0.90 [95% confidence interval (CI) 0.80?C0.95; p for heterogeneity, p _het?=?0.09], pooled specificity was 0.85 (95% CI 0.73?C0.93, p _het?=?0.13), and the pooled diagnostic odds ratio was 51 (95% CI 22?C121). The area under the summary receiver-operating characteristic (SROC) curve was 0.94 (95% CI 0.91?C0.96).

Conclusions

Ultrasonography of ONSD shows a good level of diagnostic accuracy for detecting intracranial hypertension. In clinical decision-making, this technique may help physicians decide to transfer patients to specialized centers or to place an invasive device when specific recommendations for this placement do not exist.     

Optic nerve sheath diameter on initial brain CT,raised intracranial pressure and mortality after severe TBI: an interesting link needing confirmation

Optic nerve sheath diameter (ONSD) enlargement on initial computed tomography (CT) scan has been found to be associated with increased mortality after severe traumatic brain injury. This could offer the possibility to detect patients with raised intracranial pressure requiring urgent therapeutic interventions and/or invasive intracranial monitoring to guide the treatment. The method to measure ONSD using CT scan, however, needs further confirmation. Moreover, the link between ONSD enlargement on initial CT scan and raised intracranial pressure also needs to be confirmed by further studies.In a very interesting study performed on 77 severe traumatic brain injury patients, Legrand and colleagues found that the optic nerve sheath diameter (ONSD) measured on the initial brain computed tomography(CT) scan (performed within the first 3 hours of injury) was a very good predictor of ICU mortality [1]. In the multivariate analysis, ONSD >7.3 mm was independently associated with ICU mortality, and performed better than age >32 years, anisocoria at admission, and basal cistern compression on initial CT scan.The optic nerve is surrounded by a dural sheath that can inflate in cases of raised pressure in the cerebrospinal fluid. An enlarged ONSD, measured using ocular sonography, has been found in patients with raised intracranial pressure (ICP) [2]. Even if in Legrand and colleagues'' study the ICP was measured in only 9% of the patients, we can assume that the strong association between ONSD enlargement of initial CT and mortality was related to raised ICP occurring very early after trauma, as suggested by the fact that ONSD enlargement was also associated with other signs of raised ICP in the first CT scan as basal cistern effacement and midline shift. This is probably the major interest of this study: ONSD measurement on initial CT scan could offer the possibility to detect patients with raised ICP needing urgent therapeutic interventions and/or invasive intracranial monitoring to guide the treatment.Our enthusiasm must be tempered, however, as the method to measure ONSD using CT scan needs clarification and confirmation. ONSD has been measured 3 mm behind the globe - where the dural sheath is distensible, as has previously been determined using sonography [2-4] and magnetic resonance imaging [5]. In Legrand and colleagues'' study, ONSD has been measured on a millimetric slice brain CT scan but only in one plane. As suggested by Unsold and colleagues [6], since the optic nerve has a sinuous course in the horizontal and the vertical plane, a section of the nerve in a single plane can conduce one to overestimate ONSD. Actually, the values of ONSD in Legrand and colleagues'' study are larger than values obtained with ultrasound or magnetic resonance imaging or even with CT [7]. Moreover, the precise limits of the sheath and the orbital fat surrounding the sheath can be very difficult to determine. This study probably needs further confirmation of the reliability of the ONSD measurement, after realignment in the optic nerve plane and measurement in several axes.     

床旁超声测量视神经鞘直径对颅脑损伤患者颅内压增高的诊断价值

[目的]探讨利用床旁超声测量视神经鞘直径在颅脑损伤患者中检出颅内压（ICP）增高者的应用价值.[方法]35例颅脑损伤（ICP正常22例,ICP增高13例）患者及10名健康志愿者行床旁超声检查测量视神经鞘直径,并对颅脑损伤患者进行ICP测定.比较三组之间视神经鞘直径,计算视神经鞘直径与ICP的相关性,利用ROC曲线探究预测ICP增高的最佳阈值及准确性.[结果]ICP增高组视神经鞘直径显著高于ICP正常组及对照组（P〈0.05）,ICP正常组视神经鞘直径与对照组差异无统计学意义（P〉0.05）,视神经鞘直径与ICP之间存在正相关关系（r=0.733,P〈0.01）.ROC曲线显示视神经鞘直径≥5.0 mm作为判断标准效果最佳,此时敏感度86.4%（19/22）、特异度84.6%（11/13）、准确度85.7%（30/35）、阳性预测值90.5%（19/21）、阴性预测值78.9%（11/14）.[结论]床旁超声测量视神经鞘直径能够较好预测ICP水平,当视神经鞘直径≥5.0mm提示ICP增高.     

Correlation of optic nerve sheath diameter measurements by computed tomography and magnetic resonance imaging

BackgroundTraditionally, intracranial pressure is measured by direct ventriculostomy, which is invasive. Noninvasive measures such as bedside ultrasound and magnetic resonance imaging have been advocated and utilized recently to assess the intracranial pressure. The role of this study is to determine the degree of agreement between measurements of the optic nerve sheath diameter by computed tomography (CT) and magnetic resonance imaging (MRI).Materials and MethodsRetrospective chart review of 100 consecutive patients who had both MRI and CT scan of the head from January 1, 2011, until March 31, 2013, at our center was performed. A discrepancy of 0.2 mm between the 2 measurements was set as acceptable difference. The measurements of optic nerve sheath diameter (ONSD) were compared for agreement between the 2 modalities using the method by Bland and Altman.ResultsA total of 100 patients with both MRI and CT scan of the head were selected. Of these 100 patients, 24 were male and 76 were female. The average age was 63 years. No ONSD abnormality was detected in any of the patients. The discrepancy in measurements of the ONSD between CT and MRI in transverse plane was less than the predetermined cut-off value of 0.2 mm. Within-subject variance was estimated at 0.0058 for both CT and MRI.ConclusionComparable results without significant discrepancy as predetermined by the study groups were obtained from CT scan. Measurement of ONSD by CT scan can be used to indirectly asses the intracranial pressure in addition to clinical assessment and other signs of increased intracranial pressure on CT scan.     

重症颅脑损伤患者不同颅内压与视神经鞘直径、血清S100β、NSE的相关性

目的探究重症颅脑损伤患者的颅内压与视神经鞘直径、血清中枢神经特异性蛋白(S100β)、神经元特异性烯醇化酶(NSE)的相关性。方法回顾性选取该院2017年10月至2019年12月收治的80例重症颅脑损伤患者,根据腰椎穿刺颅内压测量分为升高组(41例)和对照组(39例)。分别测量两组患者视神经鞘直径、血清S100β、NSE水平。采取Spearman分析进行双变量相关分析。结果升高组右眼直径、左眼直径和两眼平均值均明显高于对照组,差异有统计学意义(P<0.05)。升高组血清S100β、NES水平明显高于对照组,差异有统计学意义(P<0.05)。颅内压的升高与视神经鞘直径、血清S100β和NSE呈正相关(r=0.848、0.865、0.866,P<0.05)。结论当重症颅脑损伤患者出现颅内压升高时,视神经鞘直径、血清S100β、NSE水平也会随之升高,与颅内压的升高呈正相关。     

Ocular sonography in patients with raised intracranial pressure: the papilloedema revisited

Invasive devices are recommended for the early detection of raised intracranial pressure (ICP) after severe traumatic brain injury. Owing to contraindication or local issues, however, invasive ICP monitoring is not always possible. Moreover, a significant proportion of moderate traumatic brain injury patients (managed without invasive ICP) will develop raised ICP. Reliable noninvasive ICP techniques are therefore needed. Soldatos and colleagues report the usefulness of ocular sonography in the diagnosis of raised ICP. Focusing on cerebrospinal fluid accumulation around the retrobulbar optic nerve, they show interesting results for the optic nerve sheath diameter in the diagnosis of raised ICP. If confirmed by further studies, and despite important limitations related to sonography, this technique could serve as a screening test in patients at risk for raised ICP, when invasive monitoring is not possible or is not clearly recommended.     

Optic nerve sheath diameter on computed tomography is correlated with simultaneously measured intracranial pressure in patients with severe traumatic brain injury

Purpose

Assess the relationship between optic nerve sheath diameter (ONSD) measured on bedside portable computed tomography (CT) scans and simultaneously measured intracranial pressure (ICP) in patients with severe traumatic brain injury.

Methods

Retrospective cohort study of 57 patients admitted between 2009 and 2013. Linear and logistic regression were used to model the correlation and discrimination between ONSD and ICP or intracranial hypertension, respectively.

Results

The cohort had a mean age of 40 years (SD 16) and a median admission Glasgow coma score of 7 (IQR 4–10). The between-rater agreement by intraclass coefficient was 0.89 (95 % CI 0.83–0.93, P < 0.001). The mean ONSD was 6.7 mm (SD 0.75) and the mean ICP during CT was 21.3 mmHg (SD 8.4). Using linear regression, there was a strong correlation between ICP and ONSD (r = 0.74, P < 0.001). ONSD had an area under the curve to discriminate elevated ICP (≥20 mmHg vs. <20 mmHg) of 0.83 (95 % CI 0.73–0.94). Using a cutoff of 6.0 mm, ONSD had a sensitivity of 97 %, specificity of 42 %, positive predictive value of 67 %, and a negative predictive value of 92 %. Comparing linear regression models, ONSD was a much stronger predictor of ICP (R ² of 0.56) compared to other CT features (R ² of 0.21).

Conclusions

Simultaneous measurement of ONSD on CT and ICP were strongly correlated and ONSD was discriminative for intracranial hypertension. ONSD was much more predictive of ICP than other CT features. There was excellent agreement between raters in measuring ONSD.     

Sonography of the optic nerve sheath beyond the hyperacute stage of intracerebral hemorrhage

Purpose

To evaluate the feasibility and utility of serial measuring of the optic nerve sheath diameter beyond the hyperacute and acute stage of intracerebral hemorrhage (ICH).

Methods

Four patients with extensive ICH in the left basal ganglia were followed using ultrasound (US) and cerebral CT scans.

Results

Optic nerve sheath diameter values assessed beyond the acute stage of ICH showed a high correlation (ρ = 0.84, p = 0.0022) with midline shift of the third ventricle seen on CT scans.

Conclusions

Optic nerve sonography can be useful to evaluate patients with extensive ICH beyond the acute stage and help monitoring clinical evolution in these patients, when ICP monitoring is not feasible.     

Real time optic nerve sheath diameter measurement during lumbar puncture

Measurement of optic nerve sheath diameter (ONSD) using point of care ultrasound has been used to indirectly assess the intracranial pressure (ICP) particularly in conditions where it is raised. Direct pressure measurements using probes reaching the ventricle system correlated with ONSD using ultrasound. Attempts were made to measure the ONSD pre and post lumbar puncture (LP) after draining cerebrospinal fluid (CSF) as well as post ventricular shunt placement. We report ONSD measurement and demonstrate dynamic changes during LP in a patient with known idiopathic intracranial hypertension (IIH).     

Sonographic evaluation of optic nerve diameter in children with raised intracranial pressure.

OBJECTIVE: The optic nerve is part of the central nervous system, and a rise in intracranial pressure (ICP) can directly affect the perioptic nerve space, leading to an increase in nerve diameter. Transorbital sonography is a safe and easy-to-perform method to measure optic nerve diameter for rapid diagnosis of increased ICP. METHODS: The optic nerve diameter was measured in 3 individual occasions by transorbital sonography in the transverse view, 3 mm posterior to the papilla in both eyes, and the mean was calculated. Two groups were examined: a control group of children with normal ICP and normal ophthalmologic and optic nerve examination results and a case group with increased ICP as determined by an alternative method. RESULTS: The sample consisted of 156 children, of which 78 (50%) were in the case group and 78 (50%) were in the control group. Eighty-four subjects (53.7%) were male, and 72 (46.1%) were female. The mean +/- SD ages were 6.9 +/- 5.6 years in the case group and 6.8 +/- 5.5 years in the control group. The mean optic nerve diameters were 5.6 +/- 0.6 mm (range, 4.55 +/- 0.7 to 7.6 +/- 0.6 mm) in the case group and 3.3 +/- 0.6 mm (range, 2 +/- 0.6 to 4.35 +/- 0.6 mm) in the control group. The difference in the means was significant at P < .001. CONCLUSIONS: Optic nerve diameter, measured by transorbital sonography, was significantly greater in pediatric patients with increased ICP compared with a control group. Transorbital sonography can be used to identify pediatric patients with raised ICP.     

Comparison of accuracy of optic nerve ultrasound for the detection of intracranial hypertension in the setting of acutely fluctuating vs stable intracranial pressure: post-hoc analysis of data from a prospective, blinded single center study

ABSTRACT: INTRODUCTION: Optic nerve sheath diameter (ONSD) measurement with bedside ultrasound has been shown in many studies to accurately detect high intracranial pressure (ICP). The accuracy of point-in-time ONSD measurement in the presence of ongoing fluctuation of ICP between high and normal is not known. Recent laboratory investigation suggests that reversal of optic nerve sheath distension may be impaired following bouts of intracranial hypertension. Our objective was to compare the accuracy of ONSD measurement in the setting of fluctuating versus stable ICP. METHODS: This was a retrospective analysis of data from prospective study comparing ONSD to invasive ICP. Patients with invasive ICP monitors in the ICU underwent ONSD measurement with simultaneous blinded recording of ICP from the invasive monitor. Three measurements were made in each eye. Significant acute ICP fluctuation (SAIF) was defined in two different ways; as the presence of ICP both above and below 20 mmHg within a cluster of six measurements (Definition 1) and as a magnitude of fluctuation >10 mmHg within the cluster (Definition 2). The accuracy of point-in-time ONSD measurements for the detection of concurrent ICP >20 mmHg within clusters fulfilling a specific definition of SAIF was compared to the accuracy of ONSD measurements within clusters not fulfilling the particular definition by comparison of independent receiver operating characteristic (ROC) curves. RESULTS: A total of 613 concurrent ONSD-ICP measurements in 109 clusters were made in 73 patients. Twenty-three (21%) clusters fulfilled SAIF Definition 1 and 17 (16%) SAIF Definition 2. For Definition 1, the difference in the area under the curve (AUC) of ROC curves for groups with and without fluctuation was 0.10 (P = 0.0001). There was a fall in the specificity from 98% (95% CI 96 to 99%) to 74% (63 to 83%) and in the positive predictive value from 89% (80 to 95%) to 76% (66 to 84%) with fluctuation. For Definition 2, also, there was a significant difference between the AUC of ROC curves of groups with fluctuation-magnitude >10 mmHg and those with fluctuation-magnitude 5 to 10 mmHg (0.06, P = 0.04) as well as <5 mmHg (0.07, P = 0.01). CONCLUSIONS: Specificity and PPV of ONSD for ICP >20 mmHg are substantially decreased in patients demonstrating acute fluctuation of ICP between high and normal. This may be because of delayed reversal of nerve sheath distension.     

Non-invasive assessment of intracranial pressure using ocular sonography in neurocritical care patients

Objective  To assess the relationship between optic nerve sheath diameter (ONSD) and intracranial pressure (ICP) in neurocritical care patients. Design  Prospective, observational study. Setting  Surgical critical care unit, level 1 trauma center. Patients  A total number of 37 adult patients requiring sedation and ICP monitoring after severe traumatic brain injury, subarachnoid hemorrhage, intracranial hematoma, or stroke. Measurements and main results  Optic nerve sheath diameter was measured with a 7.5 MHz linear ultrasound probe. ICP was measured invasively via a parenchymal device. Simultaneous measurements were performed atleast once a day during the first 2 days after ICP insertion and in cases of acute changes. There was a significant relationship between ONSD and ICP (78 simultaneous measures, r = 0.71, P < 0.0001). Changes in ICP were strongly correlated with changes in ONSD (39 measures, r = 0.73, P < 0.0001). Enlarged ONSD was a suitable predictor of elevated ICP (>20 mmHg) (area under ROC curve = 0.91). When ONSD was less than 5.86 mm, the negative likehood ratio for raised ICP was 0.06. Conclusion  In sedated neurocritical care patients, non-invasive sonographic measurements of ONSD are correlated with invasive ICP, and the probability to have raised ICP if ONSD is less than 5.86 mm is very low. This method could be used as a screening test when raised ICP is suspected. The authors received no financial support for this work.