Normative Values of Evans’ Index and Cranial Dimensions on Brain CT Scans: Age- and Sex-Related Variations in a Southeastern Nigerian Hospital Population

Background: Evans’ Index (EI) is a widely used linear radiologic measure of ventricular size on computed tomography (CT) and magnetic resonance imaging. Despite the increasing use of volumetric neuroimaging techniques, EI remains a practical and commonly applied marker of ventricular enlargement in routine clinical practice. However, age- and sex-specific normative reference values are limited in sub-Saharan Africa, where population-specific data are essential for accurate interpretation. This study aimed to establish age- and sex-specific normative values for Evans’ Index and cranial dimensions in a southeastern Nigerian population using CT scans reported as normal despite clinical indications. Methods: This retrospective cross-sectional study analyzed 676 normal non-contrast cranial CT scans. Maximum anterior horn diameter (MAHD) and maximum internal cranial diameter (MICD) were measured at the level of the frontal horns, and EI was calculated as MAHD/MICD. The results from the non-normally distributed data were presented as median, interquartile range (IQR), and percentile-based normative ranges (2.5th–97.5th percentiles). Associations with age and sex were assessed using non-parametric tests. Results: Of the 676 participants, 268 (39.6%) were male and 408 (60.4%) were female. The overall median Evans’ Index (EI) was 0.28 (IQR 0.05), with a 95% normative range of 0.22–0.32. Females had significantly higher median EI (0.29 vs 0.26) and larger MAHD compared with males, whereas males had significantly larger MICD (all p < 0.001, Mann–Whitney U test). EI demonstrated a strong positive correlation with age (Spearman’s ρ = 0.746; p < 0.001). Significant differences in EI, MAHD, and MICD were observed across age groups (Kruskal–Wallis p < 0.001 for all). Median EI increased progressively across age categories (0.24 in 1–18 years, 0.26 in 19–50 years, 0.29 in 51–80 years, and 0.30 in ≥81 years), with corresponding increases in MAHD and a modest decline in MICD in older age groups. Conclusions: Evans’ Index and cranial dimensions in southeastern Nigeria vary significantly by age and sex. These population-specific normative values are consistent with commonly used EI thresholds (≥0.30 for ventriculomegaly) while emphasizing the need for age- and sex-stratified interpretation in clinical practice.