Monitoring Changes in Noninvasive Intracranial Pressure Markers Using Ocular Ultrasonography in Adults Undergoing Ventriculoperitoneal Shunt Surgery: A Prospective Observational Study

Abstract

Background

Optic nerve sheath diameter (ONSD) reflects the change in cerebrospinal fluid pressure. ONSD/eyeball transverse diameter (ETD) is a new ultrasonic indicator, devoid of influences of age, gender, and ethnicity. We studied the changes in the ocular sonographic parameters: ONSD, ETD, and ONSD/ETD ratio in patients of hydrocephalus before and after ventriculoperitoneal (VP) shunt placement and the factors associated with their changes.

Methods

The parameters of ONSD, ETD, and ONSD/ETD were measured in 40 patients by a single observer with Sonosite machine at three time points: before induction of anesthesia, immediately after extubation, and at 24 hours of extubation to detect changes. We examined the association between changes in clinical characters before and after shunt insertion with the ocular sonographic parameters.

Results

ONSD at 24 hours after extubation, that is, 5.29 ± 0.26 mm, was substantially lower compared to the baseline value of 5.40 ± 0.31 mm (p < 0.01). Mean ONSD/ETD ratio at 24 hours after extubation, that is, 0.228 ± 0.01, was significantly reduced compared to the baseline value of 0.233 ± 0.01 (p < 0.01). No differences in the parameters were observed between the baseline and immediately after extubation. We did not observe any substantial changes in ETD across the three time points. The reduction in ONSD and ONSD/ETD parameters from baseline to 24 hours was significantly associated with headache (p = 0.00) and nausea and vomiting (p = 0.002).

Conclusion

ONSD and ONSD/ETD ratio, both useful bedside parameters, reflect intracranial pressure (ICP) changes in patients undergoing VP shunt surgery. A decrease in the postoperative value of these parameters compared to preoperative values was consistent with relief of clinical symptoms, reflecting a reduction in ICP from successful surgery.

Keywords

noninvasive intracranial pressure monitoring - ocular sonography - ventriculo-peritoneal shunt surgery - hydrocephalus

Background

Ocular ultrasonography (USG) has emerged as a reliable bedside tool to diagnose raised intracranial pressure (ICP). Optic nerve sheath diameter (ONSD) measured by ocular USG, 3 mm behind the eyeball with a linear probe, is a promising surrogate marker for raised ICP. Optic nerve sheath (ONS) is a direct continuation of the dura matter, with subarachnoid space containing cerebrospinal fluid (CSF), which is in direct continuity with intracranial CSF compartment. This communication between perioptic nerve CSF and cerebral CSF allows noninvasive assessment of ICP. When ICP increases, the pressure in the ONS increases linearly, which distends the ONS within seconds of ICP elevation and can be measured by USG. ONSD measurement is a point-of-care tool, easy-to-use, reliable, sensitive, with minimal or no complication and is a potential preferable alternative to other noninvasive modalities like computed tomography (CT)/magnetic resonance imaging for raised ICP detection and monitoring.[1] [2] Nonetheless, factors affecting ONSD are still not elucidated and the threshold value to diagnose raised ICP is unclear. The cutoff values for ONSD to predict raised ICP is debatable. Literature suggests that ONSD cutoff of 4.8 mm has the highest accuracy to detect raised ICP in traumatic brain injury.[3]

The newly studied sonographic ratio of ONSD and eye transverse diameter (ETD), that is, ONSD/ETD, is a novel marker to diagnose raised ICP and seems to have a greater reliability than ONSD or ETD alone.[4] This ratio is reported to be independent of factors such as gender, ethnicity, age, and body mass index in recently conducted studies.[5] [6] Normal values of USG-derived ONSD/ETD is reported to range from 0.10 to 0.23 mm in healthy adults. A cutoff value more than 0.22 mm is reported to have a high sensitivity and specificity to detect raised ICP.[7]

Ventriculoperitoneal (VP) shunt is inserted for CSF diversion to relieve raised ICP in patients of hydrocephalus.[8] Improvement in the clinical sign of raised ICP, reflects in the ocular sonography measurements (ONSD, ETD, ONSD/ETD) as well. Nonetheless, the temporal relationship between ICP and ocular sonographic indicator variabilities in adult patients undergoing VP shunt surgeries is not studied extensively and there is a dearth of literature on the same. Some previous studies have evaluated the association of ONSD/ETD ratio with invasive ICP monitoring and investigated associations between ONSD, ETD, and ONSD/ETD ratios.[8] [9] [10] [11] Few others evaluated the utility of ONSD/ETD ratio as an indicator to predict prognosis of patients.[12] However, there are limited studies that have compared multiple ocular parameters simultaneously, that is, ONSD, ETD, and ONSD/ETD ratio as indicators to monitor changes before and after VP shunt surgery.

We designed this study in patients of hydrocephalus undergoing VP shunt placement to monitor the changes in ocular sonographic indicators before and after shunt placement. Our primary objective was to evaluate the changes in noninvasive optical parameters of ICP including ONSD, ETD, and ONSD/ETD ratio before and after VP shunt placement. As secondary objectives, we have studied the clinical factors, associated with change in ONSD, ETD, and ONSD/ETD ratio. Correlation between change in ONSD with change in ETD and ONSD/ETD ratio before and after VP shunt surgery was also evaluated.

Methodology

We conducted a prospective observational study in the department of neuroanesthesia at a tertiary care hospital in Delhi. Forty consecutive patients aged 18 to 75 years undergoing VP shunt surgery with features of raised ICP were enrolled for the study after informed consent. Exclusion criteria were age less than 18 years or more than 75 years, American Society of Anesthesiologists (ASA) IV and above, patients with intraocular mass, optic neuritis, ocular trauma, and high myopia, and patient's refusal to participate in the study. Patient's demographic data, Glasgow Coma Scale (GCS), and symptoms of raised ICP (headache, nausea-vomiting) were recorded preoperatively. In the operating room, before anesthesia induction, standard ASA monitors were attached and baseline hemodynamic parameter and oxygen saturation were noted.

Ultrasound was performed by the author (S.B.), who was trained in ultrasound to measure the optical parameters of ONSD and ETD, using the M Turbo ultrasound imaging system (Sonosite). The first ultrasound measurement of ONSD (N1) and ETD (E1) was performed at baseline (before anesthesia induction). Patients were placed in a neutral supine position on the operating table. Ultrasound gel was applied over the closed upper eyelid, taped shut with the help of Tegaderm with no pressure over the eyelids. A linear array of 6 to 13 MHz ultrasound probe was placed on the lateral area of the eyelid, avoiding excessive pressure on the eye. Axial images of each eye were taken in two-dimensional mode, adjusting to display the entry of the optic nerve into the globe. The simultaneous appearance of lens and optic nerve confirmed best plane of the ultrasound probe. The best image was frozen and the transverse diameter of the ONS 3 mm behind the globe was measured using an electronic caliper. We measured ONSD on two sides and took average ONSD for analysis, because increase in ICP may affect ONSD bilaterally.[13] Similarly, the maximum transverse diameter of the eye parallel to the lens (retina to retina) was measured on this plane using electronic calipers. An average of three measurements was taken for each eye at each time point. Subsequently, patients were anesthetized for the surgery as per our institutional protocol. At the end of surgery, sonographic measurement of ONSD (N2) and ETD (E2) were obtained just after extubation in the operating room and at 24 hours after extubation (N3) (E3) in the intensive care unit (ICU). At each time point, we took three measurements for each optical parameter (ONSD and ETD) and reported the average. Every patient was followed up to 24 hours after shunt surgery. Every fifth patient was additionally reviewed by an expert second observer (N.G.) in the operation theater and ICU (before and after shunt surgery) and the sonographic readings were compared with the primary observer as a quality measure. In cases of disagreements, defined by a difference of more than 10% in the values, a reevaluation was conducted. In the postoperative period, the patients were monitored for hemodynamic parameters, GCS, and clinical symptoms (headache, nausea-vomiting) at 24 hours ([Fig. 1]).

Fig. 1 Flow diagram.

Statistical Analysis

Sample size was estimated using the formula for paired test to compare differences between two means. Assuming the standard deviation (SD) of mean differences between presurgery and postsurgery as 0.5[14] [15] and at least a 0.3 SD reduction in postsurgery mean of the sonographic markers (e.g., ONSD), 90% power, and 5% alpha error, a sample size of 33 patients was deemed necessary. We planned to enroll 40 adult patients in the study to account for any potential exclusion or loss to follow-up.

Statistical analysis was done using the STATA software. For continuous variables, mean ± SD or median (interquartile range) were presented depending on symmetric or asymmetric distribution of the data, respectively. For binary or qualitative variables, number and proportions were reported. We reported the mean ± SD of the primary outcomes, that is, ONSD, ETD, and ONSD/ETD ratio measured at baseline, immediately after extubation and at 24-hour after extubation, given their symmetric distribution. To evaluate any differences between baseline values (considered as reference) with the values at immediate post-extubation or 24-hour post-extubation for the outcomes of ONSD, ETD, and ONSD/ETD ratio, we used paired t-test. We also reported the mean difference and its 95% confidence intervals (CIs) between the values of ONSD, ETD, and ONSD/ETD ratio at baseline compared to 24-hour post-extubation. To assess correlation between the ONSD, ETD, and ONSD/ETD ratio, we estimated Pearson's correlation coefficient. To evaluate association between the change in ONSD and ONSD/ETD ratio from preoperative to 24 hours post-extubation with demographic and clinical parameters, we estimated the beta coefficient with its 95% CIs using generalized linear models with identity link. A p-value of < 0.05 was considered to be statistically significant.

Results

We screened a total of 60 patients and enrolled 40 consecutive participants in the study who met the study criteria and consented for participation. All enrolled participants were included in the analyses ([Fig. 1]). The mean age of the participants was 43 years; the general characteristics of the study population are shown in [Table1]. In around half of the participants (48%) the indication for VP shunt placement was traumatic brain injury ([Table 2]).

Table 1 Demographic profile and associated comorbidities (N = 40)

Age (y)	42.8 ± 16.4
Female	22 (55.0)
Height (cm)	165.1 ± 6.1
Body weight (kg)	64.8 ± 9.2
Body mass index (kg/m2)	23.8 ± 3.0
Hypertension	4 (10.0)
Diabetes type 2	6 (15.0)
Hypothyroidism	4 (10.0)
Chronic airway obstructive disease	1 (2.5)

Note: Data is presented as mean ± standard deviation (SD) or n (%).

Table 2 Cerebral pathology among the study population (N = 40)

Traumatic brain injury	19 (47.5)
Subarachnoid hemorrhage	11 (27.5)
Cerebrovascular accident	5 (12.5)
Meningoencephalitis with hydrocephalus	2 (5.0)
Tumor extension into ventricles with hydrocephalus	3 (7.5)

Note: Data is presented as n (%).

The ONSD value at 24 hours after extubation, that is, 5.29 ± 0.26 mm, was substantially lower compared to the baseline value of 5.40 ± 0.31 mm (p < 0.01). The mean ONSD/ETD ratio at 24 hours after extubation, that is, 0.228 ± 0.01, was significantly reduced compared to the baseline value of 0.233 ± 0.01 (p < 0.01). No differences in ONSD or ONSD/ETD ratio were observed between the baseline value and immediately after extubation. We did not observe any significant changes in ETD across the three time points (E1, E2, E3) of measurements ([Table 3]). The difference in means of ONSD between baseline and 24-hour after extubation was 0.11 mm (95% CI 0.08–0.13, p = 0.001, [Table 4]).

Table 3 Values of ONSD, ETD, and ONSD/ETD ratio at different time points presented as mean ± SD

	Baseline	Immediate after extubation	p-Value[a]	24-hour post-extubation	p-Value[b]
ONSD
ONSD average (mm)	5.40 ± 0.31	5.39 ± 0.30	0.11	5.29 ± 0.26	0.00
ETD
ETD average (mm)	23.11 ± 0.29	23.14 ± 0.34	0.02	23.10 ± 0.33	0.67
ONSD/ETD
ONSD/ETD average	0.233 ± 0.01	0.232 ± 0.01	0.06	0.228 ± 0.01	< 0.001

Abbreviations: ETD, eyeball transverse diameter; ONSD, optic nerve sheath diameter; SD, standard deviation.

^a Comparison between baseline mean (reference) and after extubation mean.

^b Comparison between baseline mean (reference) and 24 hours after extubation mean.

Table 4 Difference in mean ± SD of sonographic parameters at different time points

	Baseline	24-hours after extubation	Difference in means (95% CI)	p-Value
ONSD	5.40 ± 0.31	5.29 ± 0.26	0.11 (0.08, 0.13)	0.001
ETD	23.11 ± 0.29	23.10 ± 0.33	0.009 (–0.03, 0.05)	0.67
ONSD/ETD	0.233 ± 0.01	0.228 ± 0.01	0.004 (0.003, 0.006)	0.001

Abbreviations: CI, confidence interval; ETD, eyeball transverse diameter; ONSD, optic nerve sheath diameter; SD, standard deviation.

Of the total study participants, 35/40 (87.5%) experienced headache before surgery, while at 24 hours after surgery 24/40 (60.0%) of the patients had headache. Our analysis showed that the change in ONSD and ONSD/ETD ratio from baseline to 24 hours post-extubation is significantly associated with headache (p = 0.00) and nausea or vomiting (p = 0.002) ([Table 5]). No other demographic or clinical variables had any significant association with change in ONSD and ONSD/ETD ratio ([Table 5]).

Table 5 Association between change in ONSD and ONSD/ETD ratio from baseline to 24 hours post-extubation with demographic and clinical parameters

Parameters	ONSD		ONSD/ETD
	β coefficient (95% CI)	p	β coefficient (95% CI)	p
Age	–0.001 (–0.002 to 0.001)	0.19	–0.001 (–0.001 to 0.001)	0.46
Sex	–0.006 (–0.056 to 0.043)	0.79	0.001 (–0.002 to 0.002)	0.96
ASA	–0.003 (–0.012 to 0.005)	0.42	–0.001 (–0.004 to 0.003)	0.61
BMI	–0.026 (–0.065 to 0.013)	0.19	–0.001 (–0.003 to 0.008)	0.32
Comorbidity	–0.035 (–0.086 to 0.015)	0.17	–0.001 (–0.003 to 0.001)	0.39
GCS 24 h postop	0.002 (–0.009 to 0.014)	0.71	0.001 (–0.004 to 0.006)	0.71
Mean BP	0.002 (–0.002 to 0.007)	0.32	0.002 (–0.003 to 0.004)	0.10
Saturation (O2)	–0.018 (–0.041 to 0.005)	0.13	–0.001 (–0.002 to 0.005)	0.35
Surgical time	0.001 (–0.003 to 0.005)	0.67	–0.001 (–0.002 to 0.002)	0.96
Headache	0.085 (0.041 to 0.128)	0.00	0.004 (0.001 to 0.006)	0.001
Nausea/Vomiting	0.085 (0.032 to 0.139)	0.002	0.004 (0.002 to 0.007)	0.00

Abbreviations: ASA, American Society of Anesthesiologists; BMI, body mass index; BP, blood pressure; CI, confidence interval; ETD, eyeball transverse diameter; GCS, Glasgow Coma Scale; ONSD, optic nerve sheath diameter.

The Pearson's correlation coefficient was estimated for the average of each ocular sonographic parameters measured across the three time points. The Pearson's correlation coefficient between average of ONSD (N1 + N2 + N3/3) and ONSD/ETD ratio (N1/E1 + N2/E2 + N3/E3/3) was 0.96 indicating strong correlation, that between ONSD and ETD was 0.57, suggesting relatively poor correlation. The same between average ETD (E1 + E2 + E3/3) and ONSD/ETD ratio was 0.32, suggesting poor correlation.

Discussion

In this study, we aimed to evaluate the changes in optical parameters of ONSD, ETD, and ONSD/ETD ratio before and after VP shunt placement. Our study findings showed that ONSD and ONSD/ETD at 24 hours after extubation were significantly reduced compared with before surgery. There were no substantial changes in ETD across any time points. The proportion of participants reporting headache and nausea/vomiting was lower after shunt surgery compared with baseline, which corroborated with the reductions in the ONSD and ONSD/ETD ratio values from baseline to 24 hours after extubation.

Traditionally, CT imaging has been the cornerstone of surgical decision-making and follow-up. However, ocular sonography has demonstrated potential for detecting early changes of elevated ICP—often before they are visible on CT scans, which carry the disadvantages of radiation exposure, risk of transportation, higher cost, and delayed imaging. Despite these advantages, the widespread clinical adoption of ocular sonography faces challenges, including the requirement for technical expertise to obtain high-quality images and notable interoperator variabilities.[16] [17]

Another limitation is the inherent two-dimensional reconstruction of the eyeball in bedside sonography, which can lead to measurement inaccuracies. Furthermore, involuntary eye movements, common in patients with trauma, stroke, or gaze palsy, may further compromise the reliability of sonographic assessments.

Salih et al conducted a study among 51 patients aged 16 to 60 years with obstructive hydrocephalus. They obtained serial ONSD readings at baseline, immediately after surgery, and at 6, 12, and 24 hours. They have reported significant reduction in ONSD values (5.710 ± 0.95 vs. 4.76 ± 0.75 mm) post-24 hours of successful VP shunt insertion.[18] Our study results are corroborating with their findings and additionally we have measured ETD and ONSD/ETD at each time point and we observed significant reduction in ONSD/ETD values at 24 hours.

Subramanian et al studied a population aged 10 years and above with hydrocephalus where they observed serial reduction in USG-derived ONSD values after CSF diversion procedure, which was significantly reduced on postop day 7.[10] Our study results are in agreement with theirs as we found significantly reduced values of ONSD and ONSD/ETD at 24 hours on serial measurements.

Chopra et al reported serial reduction in USG-derived ONSD values among 40 patients posted for CSF diversion surgeries in a prospective observational study.[19] The values at baseline, after intubation, post-extubation, and 2 hours thereafter till the next 12 hours demonstrated substantial consistent reduction, findings similar to our observation.

In a recently conducted study by Zhao et al in pediatric population with viral meningitis, they noted ONSD/ETD ratio had significant decline on the third day (p < 0.01) after treatment in the group of patients with raised ICP (> 200 mm of H₂O). Their findings suggested ONSD/ETD can be a reliable indicator to detect ICP changes following therapeutic measures.[20] Our study findings are in alignment.

In a separate study, improvement in neurological symptoms of raised ICP like headache, visual obscuration, nausea, and vomiting after VP shunt surgery were associated with statistically significant reductions in ONSD values from baseline.[15] Their findings are in concordance with our observations of reduced proportion of patients (87.5% vs. 60%) reporting headache after VP shunt surgery at 24 hours.

Breedt et al reported findings which were in contrast to ours. ONSD/ETD, ONSD, and ETD measured in young neurotrauma patients had no correlation with raised ICP. The study was designed as a 3-year retrospective review of a prospectively maintained data base in 74 severe head injury patients. CT Marshall scoring had significant correlation with ONSD and ONSD/ETD.[21]

Our study findings suggest the changes in the observed ocular sonographic parameters (ONSD, ONSD/ETD) are reflection of the reduced ICP following successful VP shunt placement which is further evident by improvement in clinical symptoms.

Considering the observational nature of the study, the findings can potentially be subject to selection and measurement bias. However, our study methods were designed carefully to minimize possible biases. The study participants were enrolled consecutively if they met the inclusion criteria toward minimizing selection bias. As the sonographic measurements were done by a single observer there is possibility of some systematic measurement errors, so an average of three readings were taken and around 20% of the observations were verified by a second observer for quality control to minimize possible biases. We conducted double data entry to minimize any data entry error. Given the low risk of bias, we consider our study findings internally valid.

The study had some limitations. The study population was restricted to the age group of 18 to 75 years and therefore it is not generalizable to other population categories. We did not use the gold-standard intraventricular manometry to measure ICP. Optic nerve or ocular pathology was ruled out based on history and symptomatology, a fundoscopic examination inclusion would have been a better technique. Another drawback is the limited time points of measurement in the postoperative period. A longer follow-up could have provided more useful information about patient outcomes at discharge and association of outcome with our study indicators.

We conclude that both the optical indicators (ONSD and ONSD/ETD) reflect changes in CSF pressures after VP shunt surgery at 24 hours and may be useful for noninvasive ICP measurement and follow-up. However, ETD alone does not seem to be a good indicator for monitoring changes in CSF pressure. The decrease in clinical reports of headache and nausea/vomiting following shunt surgery aligned with the observed reductions in ONSD and ONSD/ETD ratio values from baseline to 24 hours post-extubation, supporting the reliability of the optical sonographic parameters. The present study framework restricts us to comment upon which sonographic parameter fares better. Further prospective studies comparing with gold standard intraventricular ICP may be helpful to determine which parameter is a better indicator of raised ICP and has better temporal relationship with its resolution after surgery.

Conflict of Interest

None declared.

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Publication History

Article published online:
12 December 2025