The optic nerve is surrounded by subarachnoid space and any change in the intracranial pressure distends the perioptic subarachnoid space, causing distension of the intraorbital part of the optic nerve sheath. Hence, measurement of optic nerve sheath diameter (ONSD) is gaining prominence as a non-invasive tool for detecting raised intracranial pressure (ICP). The ONSD can be measured using transorbital ultrasonography (USG), Computerized Tomographic Scan (CT Scan), or Magnetic Resonance imaging (MRI). Before using the ONSD routinely for detecting raised ICP, it is necessary to establish baseline values in normal populations. While several studies exist for foreign populations, Indian studies remain sparse, mostly utilizing USG. This study presents CT scan measurements of ONSD in normal Indian adults.

MATERI ALS AND METHODS

This prospective observational study included 1 00 adult patients undergoing CT scans as part of routine investigation for non-neurological conditions of the head and neck, with no symptoms or signs of intracranial lesions, following institutional ethics approval. Brain CT scans were performed using a Philips Ingenuity 64-slice scanner following standard protocols. Measurement of ONSD for both eyes was recorded at 3 mm and 8 mm behind the globe on the Medsynapse Picture Archiving and Communication System (PACS) using electronic calipers.

STATISTICAL ANALYSIS

Continuous variables were summarized as mean ± SD with range. Paired t-tests compared right vs. left ONSD and bilateral 3 mm vs. 8 mm means. Welch's independent-samples t-test compared male and female participants. Pearson correlation examined relationships between measurements. Two-sided p < 0.05 was considered statistically significant.

RESULTS

The cohort comprised 55 males and 45 females. ONSD values at 3 mm behind the globe were 4.42 ± 0.83 mm in the right eye and 4.84 ± 0.88 mm in the left eye. At 8 mm behind the globe, values were 4.04 ± 0.81 mm in the right eye and 4.46 ± 0.96 mm in the left eye.

The mean bilateral ONSD was 4.63 ± 0.70 mm at 3 mm behind the globe and 4.25 ± 0.69 mm at 8 mm behind the globe. The measurement at 8 mm was significantly smaller than at 3 mm (p = 0.0001 5). ONSD in the left eye was significantly higher than in the right eye (p = 0.001 0). No significant differences were observed across sexes (3 mm: male 4.60 mm vs. female 4.66 mm, p = 0.672; 8 mm: male 4.1 9 mm vs. female 4.32 mm, p = 0.351 ) or age groups.

TABLE 1 : MEAN ONSD VALUES IN DIFFERENT AGE GROUPS

DISCUSSION

ONSD measurement is established as a effective surrogate for indirect ICP evaluation. CT scans offer high availability and precise computer-assisted measurement using electronic calipers without operator-dependent variance seen in USG. Most authors agree that 3 mm behind the globe is the optimal measurement point due to maximum subarachnoid space distensibility.

In foreign CT studies, Vaiman et al. (201 5) reported ONSD ranges of 4.91 ± 1 .51 mm (right) and 5.1 7 ± 1 .34 mm (left) at 3 mm. Itanyi et al. (2021 ) observed 4.4 ± 0.5 mm at 3 mm in Nigerians. In Indian USG studies, Shirodkar et al. (201 4) reported mean ONSD of 4.6 mm in females and 4.8 mm in males, while Kalim et al. (2022) reported 4.28 ± 0.28 mm. Our CT values (4.63 mm at 3 mm) align well with these estimates.

Limitations: Small sample size; restricted to adult subjects; eyeball transverse diameter (ETD) ratio was not evaluated.

CONCLUSIONS

In normal Indian adults, mean ONSD measured on CT scan is 4.63 mm at 3 mm behind the globe and 4.25 mm at 8 mm behind the globe. Left eye measurements are slightly larger. Since CT scan is routinely performed in emergency neurosurgical admissions, standard ONSD measurement via PACS software provides a rapid, non-invasive benchmark for identifying elevated ICP (>5 mm threshold).

Financial Support and Sponsorship: Nil

Conflict of Interest: None declared

Ethical Approval: Institutional Review Board at Chettinad Academy of Research & CARE (IHEC-I/01 4/04/2026, dated 1 5-04-2026).

AI Technology Declaration: No Artificial Intelligence (AI)-Assisted Technology was used in manuscript preparation.

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