Intracranial pressure monitoring remains one of the fundamental challenges in neurocritical care, where delayed recognition of elevated ICP can result in catastrophic neurological outcomes. Traditional invasive ICP monitoring techniques, while considered the gold standard, carry inherent risks including infection, hemorrhage and technical complications, making them unsuitable for many clinical scenarios.[1] The urgent need for reliable, non-invasive alternatives has intensified as emergency departments and intensive care units increasingly encounter patients with suspected raised ICP who require immediate assessment but may not meet criteria for invasive monitoring.[2]

The anatomical relationship between the optic nerve and cerebrospinal fluid provides a unique opportunity for non-invasive ICP assessment. The optic nerve sheath, as an extension of meningeal coverings of the brain, responds dynamically to changes in cerebrospinal fluid pressure, with the subarachnoid space surrounding the nerve expanding when ICP increases.[3] This physiological principle forms the foundation for ultrasonographic measurement of optic nerve sheath diameter as a surrogate marker for intracranial pressure changes. Point-of-care ultrasound of ONSD has demonstrated the ability to reflect immediate changes in ICP, with studies showing significant correlations between ONSD measurements and direct ICP monitoring (correlation coefficient of 0.7) [4,5].

Current literature demonstrates considerable promise for ONSD ultrasound across various clinical populations, though most evidence derives from traumatic brain injury cohorts. Recent meta-analyses have established robust diagnostic accuracy parameters, with area-under-the-curve values consistently exceeding 0.92 for detecting elevated ICP.[4,6] However, significant heterogeneity exists in reported cutoff values, ranging from 4.5 mm to 6.3 mm across different studies and populations.[3,7] This variability reflects methodological differences in measurement techniques, patient populations and reference standards used for comparison.

The clinical utility of ONSD measurement extends beyond its diagnostic accuracy to encompass practical advantages that make it particularly suitable for emergency and critical care settings. The technique requires minimal training, demonstrates high inter-observer reliability and can be performed rapidly at the bedside without patient transport or radiation exposure.[1,2]   Studies have confirmed that bedside ONSD measurement can be performed reliably by emergency physicians and intensivists, with measurements taken approximately 5 minutes before and after therapeutic interventions showing immediate responsiveness to ICP changes.[5]

Despite growing evidence supporting ONSD ultrasound in traumatic brain injury populations, there remains a significant knowledge gap regarding its performance in non-traumatic neurological emergencies. Non-traumatic causes of raised ICP, including central nervous system infections, intracranial hemorrhage, acute ischemic stroke and idiopathic intracranial hypertension, present distinct pathophysiological characteristics that may influence the relationship between ONSD and ICP.[6] Recent studies specifically focusing on non-traumatic emergency department populations have reported varying diagnostic accuracy parameters, with some achieving 100% sensitivity and 89.2% specificity using a 6.3 mm cutoff value.[8]

The heterogeneity in reported cutoff values represents a critical limitation for clinical implementation of ONSD ultrasound. While some studies suggest optimal thresholds around 5.0 mm with 74.14% sensitivity and 49.22% specificity, others report superior performance with higher cutoffs approaching 5.7-6.3mm.[2,3,7] These discrepancies likely reflect differences in study populations, measurement methodologies and reference standards, highlighting the need for population-specific validation studies to establish optimal diagnostic thresholds.

Furthermore, the distinction between acute and chronic elevations in ICP may influence ONSD measurements and their interpretation. The presence of sonographic signs such as the crescent sign, which represents the ultrasound correlate of papilledema, may indicate chronicity of pressure elevation and influence diagnostic accuracy.[9] Understanding these nuances becomes particularly important in non-traumatic populations where the temporal evolution of ICP elevation may differ significantly from acute traumatic scenarios.

Given the critical importance of rapid ICP assessment in non-traumatic neurological emergencies and the current gaps in population-specific evidence, there is an urgent need to validate ONSD ultrasound specifically in non-traumatic brain injury patients. This study addresses this knowledge gap by systematically evaluating the diagnostic performance of ultrasound-measured ONSD in detecting radiologically confirmed raised ICP in patients presenting with non-traumatic neurological emergencies, with the goal of establishing evidence-based cutoff values and diagnostic accuracy parameters for this specific population

This was a cross-sectional study carried out in the Department of Radiodiagnosis at a tertiary care centre. The main objective was to evaluate the diagnostic role of ultrasound-based optic nerve sheath diameter (ONSD) measurements in detecting raised intracranial pressure (ICP) in patients with non-traumatic neurological conditions. Patients who presented with clinical suspicion of raised ICP such as headache, vomiting, altered sensorium or signs like papilledema were considered for inclusion. Cases with a history of trauma or ocular pathology affecting the optic nerve anatomy were excluded from the study.

ONSD was measured at the bedside using a Samsung HS-80 ultrasound machine equipped with a high-frequency linear probe. The patient was kept in supine position with eyes closed and a generous amount of gel was applied over the eyelids to avoid applying undue pressure. The transducer was gently placed over the closed upper eyelid and measurements were taken in the axial plane, precisely 3 mm behind the globe. Both eyes were assessed and three readings per eye were recorded, from which the mean value was used for final analysis.

In all patients, non-contrast CT (NCCT) brain was also done using a 128-slice multidetector CT scanner (6th generation, Syngo.via software). Standard radiation safety protocols were followed. Metallic accessories were removed prior to scanning and the head was positioned to reduce radiation exposure to the eyes. Scanning was done from the vertex to the C2 vertebrae using parameters of 120 kV and 250 mAs, with slice thickness ranging between 0.625 mm to 1 mm. The images were reconstructed using the brain algorithm and coronal and sagittal reformats were obtained as needed.

Additionally a MRI brain was performed in select cases where further evaluation was warranted. A 3T GE Discovery MR750w MRI scanner was used and patients were screened carefully for contraindications such as pacemakers or metallic implants. The scan was acquired with a dedicated 32-channel head coil and various standard sequences were included namely T1, T2, FLAIR, DWI, SWI, 3D MPRAGE and T2 CISS. Images were taken from the vertex down to the level of C2. Slice thickness varied from 1 to 5 mm depending on the sequence and field of view ranged from 220 to 240 mm. Gadobutrol (0.1 mmol/kg) was administered when contrast imaging was indicated. Multiplanar reconstructions were done using isotropic data from 3D sequences. The average MRI scan time was around 20–30 minutes, with additional time for contrast sequences if required.

All clinical and radiological data were recorded in Microsoft Excel. The patients’ identities were kept confidential throughout the process. The ultrasound-based ONSD values were then correlated with MRI findings to assess the accuracy of this bedside technique. Data analysis was carried out using SPSS software. Quantitative variables such as ONSD were expressed as mean ± standard deviation. Statistical significance was determined using the independent t-test for comparing ONSD in patients with and without radiological signs of raised ICP. ROC analysis was also done to identify optimal cutoff values, along with calculating sensitivity, specificity, positive and negative predictive values and overall diagnostic accuracy. A p-value less than 0.05 was taken as statistically significant.

Table 1: Baseline Demographic and Clinical Profile of Patients (n = 100)

A total of 100 patients were enrolled and their demographic and clinical characteristics are in Table 1. The mean age of the study population was 48.6 ± 12.3 years with ages ranging from 18 to 76 years. There was a male predominance with 61% males and 39% females. Clinically the most frequently reported symptom was headache, observed in 76% of patients, followed by vomiting (52%) and altered sensorium (43%). Seizures were noted in 21% of cases, while 17% had papilledema on fundoscopy. A Glasgow Coma Scale (GCS) score of less than 8 was seen in 12% of the cohort. The mean systolic and diastolic blood pressures were 132.7 ± 14.6 mmHg and 82.3 ± 9.5 mmHg, respectively. MRI findings confirmed raised ICP in 64% of patients whereas 36% had a normal intracranial pressure.

Table 2: Comparison of Optic Nerve Sheath Diameter (ONSD) Between Patients with MRI findings of Raised ICP

In the present study the optic nerve sheath diameter (ONSD) was compared between patients who had radiologically evident raised intracranial pressure (ICP) and those who did not, based on MRI findings. As shown in Table 1 there was a clear and statistically significant difference in ONSD values between these two groups. Among patients with raised ICP, the mean ONSD on the right side was 5.75 ± 0.37 mm, while the left eye measured 5.80 ± 0.36 mm. In contrast, those without radiological signs of raised ICP had mean ONSD values of 5.36 ± 0.45 mm (right) and 5.48 ± 0.42 mm (left). Difference among the groups was found to be statistically significant with p-values of 0.01 (right eye) and 0.03 (left eye) demonstrating the association between increased ONSD and elevated ICP.

Table 3: ROC Analysis of ONSD for Predicting Raised ICP

Diagnostic evaluation was further carried using Receiver Operating Characteristic (ROC) curve analysis the results of which are presented in Table 3. The Area Under the Curve (AUC) was calculated to be 0.825 for the right eye and 0.884 for the left eye both with a highly significant p-value < 0.0001. These AUC values reflect a strong diagnostic performance of ONSD as a marker for raised ICP. Based on this analysis the optimal cutoff values were determined to be 5.71 mm for the right eye and 5.83 mm for the left eye. At these thresholds both the sides showed a high sensitivity of 90.7% also the specificity was 68% for the right and 76% for the left, suggesting slightly superior performance on the left side.

Table 4: Distribution of Patients Based on ONSD Cutoff Values

As per the distribution pattern presented in Table 4, a total of 76% of patients had right eye ONSD values greater than 5.71 mm, while 64% had left eye ONSD values exceeding 5.83 mm.

Table 5: Diagnostic Performance of ONSD in Predicting Raised ICP

Lastly the detailed diagnostic performance parameters are summarized in Table 5. For the right eye, the sensitivity and specificity at the 5.71 mm cutoff were 90.7% and 68%, respectively, with a positive predictive value (PPV) of 71.43% and a negative predictive value (NPV) of 89.76%. The overall accuracy came out to be 80.01%. The left eye ONSD at the 5.83 mm cutoff showed sensitivity of 90.7%, specificity of 76%, PPV of 75.8% and NPV of 81.9%, with a slightly better overall diagnostic accuracy of 81%. These findings reaffirm that ONSD, especially when assessed with ultrasound, is a reliable, sensitive and non-invasive screening tool for detecting raised ICP in patients with non-traumatic neurological conditions.

Figure 1: A case of 28 Yr female came with complaints of Fever and vomiting

Figure A: B mode ultrasound image of Right Eye ONSD-6.0mm

Figure B: B mode ultrasound image of Left Eye ONSD- 5.8mm

Figure C and D: Axial MRI Brain (T2 and Post contrast) images showing multiple ring-enhancing lesions with surrounding vasogenic edema compressing the 4th ventricle suggesting raised ICP.

Figure 2: 45 yr Female patient came with complain of Loss of consciousness, vomiting

Figure A: B mode ultrasound image of Right Eye ONSD-5.9mm

Figure B: B mode ultrasound image of Left Eye-6.3mm

Figure C& D: Axial NCCT brain shows a large hematoma with surrounding vasogenic edema and sulcal effacement, midline shift and subfalcine herniation showing Raised ICP.

The findings of this study significantly advance our understanding of optic nerve sheath diameter (ONSD) ultrasonography as a diagnostic tool for non-traumatic raised intracranial pressure (ICP), demonstrating robust sensitivity (90.7% bilateral) and moderate specificity (68–76%) at cutoff values of 5.71 mm (right eye) and 5.83 mm (left eye). These results align with recent meta-analyses reporting pooled sensitivity of 0.92 and specificity of 0.90 for ONSD in detecting elevated ICP,[4,6] while providing critical population-specific validation for non-traumatic etiologies. The observed asymmetry in diagnostic performance between eyes higher specificity for the left eye (76% vs. 68%) despite identical sensitivity may reflect anatomical variations in sheath compliance or technical factors such as probe positioning, though further investigation is warranted [10,11].

Diagnostic Thresholds and Pathophysiological Considerations

The optimal ONSD cutoffs identified here (5.71–5.83 mm) occupy a middle ground between previously reported values ranging from 4.8 mm in resource-limited settings to 6.3 mm in emergency department cohorts [6,12]. This variability likely stems from differences in measurement protocols, reference standards and patient demographics. For instance, studies using invasive ICP monitoring typically report lower thresholds (e.g., 5.0–5.2 mm),[4,13] whereas those correlating with CT/MRI findings, as in this study, often yield higher values due to the indirect nature of radiological ICP assessment.[8,14] Notably, the 5.83 mm left-eye cutoff demonstrated diagnostic accuracy comparable to invasive methods (AUC 0.884), supporting its utility in settings where gold-standard monitoring is unavailable.

The strong correlation between ONSD and radiological signs of ICP elevation (p = 0.01–0.03) underscores the shared pathophysiology between optic nerve sheath distension and global intracranial hypertension. As a meningeal extension, the sheath’s subarachnoid space transmits pressure changes rapidly, with animal models showing ONSD adjustments within minutes of ICP fluctuations. [4,12] However, the moderate specificity observed here (68–76%) suggests confounding factors in non-traumatic populations, such as chronic hypertension-induced sheath fibrosis or localized optic nerve pathologies, which may limit sheath expansion despite elevated ICP.[9,15]

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Clinical Implications and Protocol Standardization

The high negative predictive value (81.9–89.8%) positions ONSD ultrasonography as an exceptional rule-out test for intracranial hypertension, particularly valuable in emergency triage. In scenarios where immediate CT/MRI is logistically challenging such as hemodynamically unstable patients or resource-limited settings a normal ONSD measurement could safely defer advanced imaging.[8,12] Conversely the 71.4–75.8% positive predictive value necessitates confirmatory testing, aligning with guidelines recommending ONSD as an adjunct rather than standalone diagnostic.[4,13]

Interobserver variability, a historical limitation of ONSD ultrasonography, appears mitigated by standardized protocols. The 3 mm retrobulbar measurement site and triplicate averaging used here mirror techniques validated in reliability studies showing intraclass correlation coefficients >0.88.[10,11]  Emerging protocols like CLOSED (Color Doppler–Low power–Optic disk clarity–Safety–Elevated frequency–Dual measurements) further enhance reproducibility, achieving <0.3 mm interoperator variability in recent trials.[10,16] Such standardization is critical for clinical adoption, particularly given the steep learning curve studies suggest 17–20 supervised scans suffice for novice proficiency.[11,17]

While this study strengthens the evidence base for ONSD in non-traumatic ICP assessment, several limitations merit consideration. The reliance on CT/MRI as reference standards introduces potential misclassification bias, as radiographic signs (e.g., effaced cisterns, midline shift) correlate imperfectly with direct ICP measurements.[13,14]

Additionally, the single-center design and exclusion of pediatric populations limit generalizability. Future multicenter studies employing invasive ICP monitoring across diverse etiologies (e.g., hepatic encephalopathy, autoimmune CNS disorders) could refine cutoff values and validate disease-specific thresholds.

Technical innovations may further enhance ONSD’s diagnostic precision. Ratios adjusting for ocular anatomy (e.g., ONSD/eyeball transverse diameter) show promise in reducing interindividual variability, with preliminary data suggesting improved specificity in comatose patients.[2,9] Similarly, dynamic assessments such as postural changes or Valsalva maneuvers could help differentiate fixed sheath dilation from pressure-reactive changes.[9,18]

This study conclusively demonstrates that ultrasonographic ONSD measurement achieves high diagnostic accuracy for intracranial hypertension in non-traumatic brain injury, with performance metrics supporting its integration into routine neurocritical care. When combined with clinical assessment, ONSD ultrasonography enables rapid risk stratification, guiding timely interventions to mitigate secondary brain injury. Standardization of measurement protocols and ongoing technological refinements will solidify its role as an indispensable tool in the neuromonitoring arsenal.

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