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|Year : 2019 | Volume
| Issue : 3 | Page : 772-776
Ultrasound-measured optic nerve sheath diameter correlates well with cerebrospinal fluid pressure
Salil Gupta1, Aditya Pachisia2
1 Department of Neurology, Army Hospital (Research and Referral), New Delhi, India
2 Department of Medicine, Military Hospital, Palampur, Himachal Pradesh, India
|Date of Web Publication||23-Jul-2019|
Dr. Salil Gupta
Department of Neurology, Army Hospital (Research and Referral), Delhi Cantt, New Delhi - 110 010
Source of Support: None, Conflict of Interest: None
Background: Intracranial pressure (ICP) can be raised in many neurological conditions and must be treated early to prevent poor clinical outcomes. To detect the rising ICP, it is important to measure it repeatedly using a tool that is noninvasive, safe, accurate, and portable with minimal inter- and intraobserver variability. The aim of our study was to correlate cerebrospinal fluid (CSF) pressure with ultrasound (US)-measured optic nerve sheath diameter (ONSD) and find out a measurement which correlates best with CSF pressure of >20 cm of water.
Materials and Methods: All patients in whom lumbar puncture (LP) was indicated and CSF pressure could be measured using manometer were included in the study. Ocular US was used to measure the ONSD.
Results: A hundred patients were included in the study out of which 81% were males. The mean age was 60.5 years (±15.6) with a range of 26–90 years. Significant positive correlation was found between the ONSD and CSF pressure. Correlation coefficient (r) was 0.715 (P < 0.001). Receiver's operating characteristic curve was used to find out the ONSD value correlating with a CSF pressure of >20 cm of water. An ONSD >0.63 cm had a sensitivity of 77.3% [95% confidence interval (CI) 54.6–92.1] and specificity of 92.3% (95% CI 84.0–97.1) in predicting a CSF pressure of >20 cm of water (likelihood ratio [LR] + 10.05, LR − 0.25).
Conclusion: There is a positive correlation of US-measured ONSD and LP-measured CSF pressure. ONSD of >0.63 cm suggests a CSF pressure of >20 cm of water.
Keywords: CSF pressure, lumbar puncture, optic nerve sheath diameter, ultrasound
Key Message: Progressively rising intracranial pressure requires a repeated assessment using a safe, accurate and noninvasive tool with minimal inter- and intraobserver variability. Ultrasound measured optic nerve sheath diameter correlates well with lumbar puncture measured cerebrospinal fluid pressure. An optic nerve sheath diameter of >0.63 cm suggests a cerebrospinal fluid pressure of greater than 20 cm of water.
|How to cite this article:|
Gupta S, Pachisia A. Ultrasound-measured optic nerve sheath diameter correlates well with cerebrospinal fluid pressure. Neurol India 2019;67:772-6
Intracranial pressure (ICP) can be raised in a host of neurological conditions, and if it is not detected early, it can lead to poor clinical outcomes and mortality. Mostly physicians use clinical surrogates such as headache, vomiting, a low Glasgow Coma Scale, or papilledema to conclude that raised ICP is present. However, this approach is prone to errors and a vital therapeutic window may be missed if raised ICP is not detected on time. To detect the rising ICP, it is important to measure it repeatedly. In view of this, a noninvasive, safe, accurate, and portable tool with minimal inter- and intraobserver variability would be highly desirable in helping initiate timely ICP-lowering strategies. Currently, ICP can be measured accurately by invasive measures such as intraventricular and intraparenchymal catheters or a manometer connected to a lumbar puncture (LP) needle placed in the subarachnoid space. However, these are invasive methods and are limited by the lack of availability of technical expertise universally; they are expensive and require highly specialized equipment; and, they carry the risk of infection and hemorrhage. Repeated measurements may not be possible especially if a manometer is used for recording. Ultrasound (US) has become widely available as a safe, point-of-care tool capable of picking up real-time changes. It is available in most emergency departments and intensive care units. Several studies in the last decade have documented the association of raised ICP as suggested by computed tomography (CT) or magnetic resonance imaging (MRI) brain, with the optic nerve sheath diameter (ONSD) measured by US.,, The US-guided measurement of ONSD is simple, easy to perform, reproducible, and with low intra- and interobserver variability., However, there are only a few studies which have documented the relationship using actual measurement of ICP and correlated it with ONSD at the same time. The aim of our study was to correlate cerebrospinal fluid (CSF) pressure with the US-measured ONSD and find out an ONSD measurement which correlates best with a CSF pressure of >20 cm of water.
| » Materials and Methods|| |
This prospective observational study was conducted at a tertiary care teaching hospital over 1 year. All patients in whom lumbar puncture (LP) was indicated and CSF pressure could be measured were included in the study. Patients were excluded if they were hemodynamically unstable, had undergone previous shunt surgery or any other neurosurgical procedure, had suppurative conditions or bony deformity over the lumbar spine, meningocele, bleeding diathesis, or any other condition where LP could not be done or the pressure could not be measured due to poor flow into the manometer. Patients were also excluded if they had inflammatory eye swelling or external ophthalmoparesis due to any condition which might interfere in the measurement of ONSD.
To calculate the sample size, we assumed that the sensitivity of CSF pressure measurement using a manometer was nearly 100%. Assuming the desired sensitivity of 90% in the new test and keeping type I (α) error at 0.05 and power of study at 0.8, we arrived at a sample size of 100. All selected patients first underwent US measurement of ONSD. The sonographic procedure was carried out in all patients by the second author. Ocular US was performed with a 7.5-MHz linear probe of Mindray DP-8500 in B mode. A 21A linear probe was used to obtain axial cross-sectional images of the optic nerve, and ONSD was measured at a depth of 3 mm from the distal-most point of the optic nerve in the globe. For each subject, the sonographer performed three measurements on each eye. The resulting six measurements were then averaged to yield a mean ONSD, to minimize intraoperator variability. [Figure 1] shows the image of ONSD measured with US in one of our patients. LP was done in the left lateral position with hips and knee joints flexed within a few minutes of measuring ONSD. The puncture site between L3 and L4 was cleansed with spirit and iodine and infiltrated with 2% lignocaine. The LP needle was inserted in the space. The legs were then extended. A central venous pressure (CVP) manometer with calibrations in centimetres was fixed to a stand in such a way that the zero mark of the manometer was at the level of the spine. Once the needle was in the subarachnoid space, the stylet was withdrawn and the tubing of CVP manometer was attached to the needle. CSF was allowed to flow into the manometer till it stabilized (with minimum movement with respiration) at a particular level. The reading in the manometer was recorded as CSF pressure in centimetres of water.
The study had the approval of the institute's ethics committee. Informed written consent was taken from all patients or their next of kin. Data analysis was performed using SPSS (Statistical Package for the Social Sciences) version 20. Qualitative data were expressed in frequency and percentage (%). Quantitative data were expressed using mean, standard deviation, or median. Pearson's correlation coefficient was used to find out the relationship between ONSD and CSF pressure. A CSF pressure of >20 cm of water was considered as raised ICP. Receiver's operating characteristic (ROC) curve was used to find the cutoff value of mean ONSD with CSF pressure of >20 cm of water. A P value of <0.05 was taken as significant.
| » Results|| |
A hundred patients were included in the study, out of which 81% were males. The mean age was 60.5 years (±15.6) with a range of 26–90 years. [Table 1] shows the likely etiology for which the LP with CSF analysis was done. Scatter plot was used to depict the correlation between CSF pressure and average ONSD [Figure 2]. Significant positive correlation was found between the average ONSD and CSF pressure. Correlation coefficient (r) was 0.715 (P < 0.001). ROC curve was used to find out an ONSD value correlating with a CSF pressure of >20 cm of water [Figure 3]. An ONSD >0.63 cm had a sensitivity of 77.3% [95% confidence interval (CI) 54.6–92.1] and specificity of 92.3% (95% CI 84.0–97.1) in predicting a CSF pressure of >20 cm of water (LR + 10.05, LR − 0.25).
|Table 1: Likely etiological profile of the patients included in th e study|
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|Figure 2: Scatter plot showing the correlation between ONSD and CSF pressure|
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|Figure 3: ROC curve for ONSD value correlating with a CSF pressure >20 cm of water. Area under the ROC curve = 0.930; Standard error = 0.039; 95% Confidence interval = 0.861 to 0.971|
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| » Discussion|| |
We have demonstrated a significant positive correlation between ONSD and CSF pressure. Our study has shown that when ONSD is >0.63 cm, it has 77.3% sensitivity and 92.3% specificity for a CSF pressure of >20 cm of water. US was used to measure ONSD, whereas LP was used to document CSF pressure. Lenfeldt et al., have shown earlier that CSF pressure, as assessed by LP, is an accurate technique to determine ICP in patients with communicating CSF systems.
ONSD measurement has been used increasingly in the past decade as a surrogate marker of ICP. MRI, CT, and US have been used to take measurements. However, there has been no consensus regarding the normal value of ONSD and what measurement is representative of raised ICP. Published meta-analyses have suggested that cutoff values from 4.8 to 5.9 mm indicate increased ICP., An Indian study has demonstrated that the average ONSD in their control group age 18–40 years was 4.6 and 4.8 mm in female and male subjects, respectively. It has been demonstrated by Dubourg et al., in their systematic review and meta-analysis that the normal values of ONSD, as measured by US in an adult, a child aged 1–15 years, and an infant are <5, <4.5, and <4 mm, respectively. They also concluded that patients with ONSD > 5.00–5.70 mm had ICP > 20 mm with the diagnostic odds ratio of 51, sensitivity of 90% (95% CI 80%–95%), and specificity 85% (95% CI 73%–93%).
There are very few studies which have correlated ONSD with the actual measurement of CSF pressure, as shown in [Table 2]. Moretti et al., showed that the optimal ONSD cut-off point to define ICP (>20 mmHg) was 5.2 mm with a 93.1% sensitivity and 73.9% specificity in 63 patients with spontaneous intracerebral hemorrhage. Geeraerts et al., reported that adults with a high ICP (defined as ICP >20 mmHg for more than 30 min in the first 48 h after trauma), had a higher ONSD value (6.3 ± 0.6 mm) than those with normal ICP (5.1 ± 0.7 mm) or the control group (4.9 ± 0.3mm). Irazuzta et al., recorded ONSD and CSF pressure by performing LP in all their 13 patients between 12 and 18 years of age with suspected idiopathic intracranial hypertension. Ten of their patients had raised ICP, defined as CSF pressure of >20 cm of water, and all these patients had an ONSD ≥4.5 mm. Maissan et al., researched whether ONSD and ICP simultaneously changed during tracheal manipulation in patients with traumatic brain injury (TBI). In all their 18 patients, ICP increased above 20 mmHg during manipulation of the trachea, and this increase was directly associated with a dilation of ONSD of >5.0 mm. After tracheal manipulation stopped, ICP and ONSD decreased immediately to baseline levels. The correlation between ICP and ONSD was high (r = 0.80); at a cutoff of ≥5.0 mm ONSD, a sensitivity of 94%, a specificity of 98%, and an area under the curve of 0.99 (95% CI 0.97–1.00) for detecting elevated ICP were determined. Caffery et al., compared the relationship of ONSD with ICP measured by opening pressure manometry on LP. Their cohort consisted of 51 adult patients presenting to the emergency department and requiring LP. The sensitivity of ONSD greater than or equal to 5 mm for identifying elevated ICP defined as >20 cm of water was 75% (95% CI 53%–90%) with a specificity of 44% (25%–65%). The area under the ROC curve was 0.69 (0.54–0.84), suggesting a relationship between ONSD and ICP. Frumin et al., found out that ONSD ≥5.2 mm was a good predictor of ICP (>20 mmHg) with a sensitivity of 83.3% (95% CI 35.9%–99.6%) and specificity of 100% (95%CI 84.6%–100%). They measured ICP through an external ventricular device. Kimberly et al., found a significant correlation of ONSD, as measured by US, with ICP, as measured by ventriculostomy in their 15 patients. Using ROC curve analysis, they found that an ONSD >5 mm correlated with ICP >20 cm H2O with a sensitivity of 88% (95% CI 47%–99%) and specificity of 93% (95% CI 78%–99%). More recently, Wang et al., used a mathematical modeling method to demonstrate a correlation between ONSD and CSF pressure.
|Table 2: Studies correlating ONSD with actual measurement of CSF pressure|
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Although, all the studies quoted above have compared the ONSD as measured by US with the ICP, they are different in many ways including the number of patients selected, their age group, the etiology of the underlying condition, the technique of measuring ONSD, and the method of recording ICP. Our study contributes to the growing body of evidence suggesting a good positive correlation of ONSD as measured by US with LP-assessed CSF pressure. We suggest an average ONSD of 0.63 cm as a marker of raised ICP, defined as a CSF pressure of >20 cm of water.
The strength of our study, when compared to the previously published studies, lies in the larger number of study subjects recruited, the direct measurement of CSF pressure performed, and a single sonographer recording all the ONSD measurement. In addition, our patients had varied underlying etiology with the ICP ranging from normal to raised. The weakness of our study was that we did not include any condition causing a mass effect, such as traumatic brain injury (TBI), malignant infarcts, intracerebral hemorrhage, or obstructive hydrocephalus.
In conclusion, our study has shown a good positive correlation of US-measured ONSD with ICP, as measured by CSF manometry. It has also shown that an ONSD of >0.63 cm suggests a CSF pressure of > 20 cm of water with 77.3% sensitivity and 92.3% specificity. Our findings can be helpful to the practicing clinician in diagnosing raised ICP using a non-invasive tool at bedside. They may also help in following up a patient by serial recordings to assess the effect of therapeutic measures on raised ICP. The researcher may find the results helpful in planning studies where serial measurements of ICP are needed.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2]