Age- and sex-specific reference values of Evans’ Index and cranial dimensions on brain CT in a south-eastern Nigerian hospital population

Abstract

Background

Evans’ Index (EI) is a widely used linear radiological 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 hospital-based 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 hospital-based reference values for Evans’ Index and cranial dimensions in a south-eastern Nigerian population using CT scans reported as normal despite clinical indications.

Methods

This retrospective cross-sectional study analysed 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. Results from the non-normally distributed data were presented as median with quartile ranges (Q1–Q3) and percentile-based reference ranges (2.5th–97.5th percentiles). Associations with age and sex were assessed using non-parametric tests, and measurement reproducibility was evaluated using intraclass correlation coefficient analysis.

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 (Q1–Q3: 0.25–0.30), with a 95% reference 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), and multivariable regression analysis confirmed age as an independent predictor of EI. 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 south-eastern Nigeria vary significantly by age and sex. These hospital-based reference values are consistent with commonly used EI thresholds (≥ 0.30 for ventriculomegaly) while emphasising the need for age- and sex-stratified interpretation in clinical practice. These hospital-based reference values are derived from radiologically normal CT scans obtained during clinically indicated imaging and may not fully represent community-based healthy populations.

Background

Hydrocephalus is characterized by pathological enlargement of the cerebral ventricles resulting from an imbalance between cerebrospinal fluid production and absorption [1, 2]. Ventricular dilatation may also occur in non-obstructive conditions such as cerebral atrophy and normal ageing [3, 4]. Accurate assessment of ventricular size is therefore fundamental to neuroimaging-based diagnosis and clinical decision-making.

Computed tomography (CT) and magnetic resonance imaging (MRI) provide reliable modalities for evaluating ventricular size [5, 6]. Among several linear indices, Evans’ Index (EI), first described by Evans in 1942, is defined as the ratio of the maximum width of the frontal horns of the lateral ventricles to the maximum internal cranial diameter at the same axial level [2].

Despite advances in volumetric MRI techniques, EI remains widely used in routine clinical practice because of its simplicity, reproducibility, and applicability, particularly in resource-limited settings where advanced volumetric analysis may not be readily available. Furthermore, contemporary neuroimaging guidelines for idiopathic normal pressure hydrocephalus (NPH) continue to incorporate EI as an important imaging marker when interpreted alongside clinical and other radiological findings [7, 8].

Traditionally, an EI value ≥ 0.30 has been considered indicative of ventricular enlargement and has been incorporated into diagnostic criteria for hydrocephalus, including normal pressure hydrocephalus (NPH) [2, 7]. However, emerging evidence suggests that a single universal cut-off may not adequately account for physiological ageing and inter-individual variability [9, 10].

Recent studies increasingly advocate contextual and age-adjusted interpretation of EI, particularly in elderly populations and in the assessment of idiopathic normal pressure hydrocephalus [8–11]. Age-related ventricular enlargement has been consistently demonstrated in morphometric and volumetric studies [12–14]. Currà et al. reported progressive ventricular expansion after the seventh decade, with earlier changes observed in men [13], while Ambarki et al. demonstrated correlations between linear indices and volumetric ventricular enlargement in healthy elderly individuals [14].

Hospital-based CT morphometric studies from Europe, Asia, and Africa have further demonstrated regional and ethnic variability in EI and related cranial morphometric parameters [15–20]. In Africa, CT-based reference value studies have been conducted in Nigeria [18], Ghana [19], and Ethiopia [20], while studies from India and Kashmir have reported comparable data in Asian populations [16, 17].

More recent African neuroimaging studies have continued to demonstrate demographic variability in ventricular morphometry across populations, further emphasizing the need for locally derived reference values. In addition, studies comparing Evans’ Index with other ventricular indices, such as the bicaudate index, have highlighted the continuing relevance of simple linear measurements in routine neuroimaging practice, particularly in low-resource settings where volumetric techniques may not be routinely available [16].

However, many of these studies lacked percentile-based reference ranges, detailed age stratification, or sex-specific reference intervals. Although Nigerian studies from northern populations [18] and more recently from Delta State in southern Nigeria [21] have reported reference Evans’ Index values in adults, comparable age- and sex-specific reference values incorporating both paediatric and elderly populations remain limited for south-eastern Nigeria.

Given recognized anthropometric and cranial morphometric variations across ethnic and geographic groups, reliance on non-local reference values may lead to misclassification of ventricular size. In addition, the continued reliance on CT imaging in low-resource settings underscores the need for robust, population-specific reference data derived from readily applicable linear indices.

Establishing demographic-specific Evans’ Index reference values contributes to precision neuroimaging by improving the contextual interpretation of ventricular enlargement across diverse populations. Such demographic-specific reference values align with emerging precision medicine approaches that emphasize individualized interpretation of diagnostic imaging findings.

By providing age- and sex-specific reference values from an underrepresented African population, this study contributes to the growing body of evidence supporting demographic calibration of global neuroimaging reference standards and promotes more context-sensitive interpretation of ventricular morphometry across diverse populations. In low-resource settings where advanced volumetric MRI techniques and dedicated image-processing software are often unavailable or are not routinely available, simple linear indices such as Evans’ Index continue to play an important role in routine neuro-diagnostic decision-making, including the evaluation of hydrocephalus and normal pressure hydrocephalus.

Furthermore, population-specific reference values may improve differentiation between physiological ageing and pathological ventriculomegaly, thereby reducing the risk of diagnostic misclassification and supporting more accurate clinical assessment.

This study therefore aimed to establish age- and sex-specific percentile-based reference values for Evans’ Index (EI), maximum anterior horn diameter (MAHD), and maximum internal cranial diameter (MICD) in individuals with neurologically indicated but radiologically normal brain CT scans in south-eastern Nigeria, and to evaluate the influence of age and sex on these parameters.

Methods and materials

Study design and setting

This retrospective cross-sectional study was conducted at a private diagnostic imaging centre in Owerri, Imo State, south-eastern Nigeria. Ethical approval for this study was obtained from the Imo State University Teaching Hospital Research Ethics Committee (Approval No: IMSUTH/REC/2025/074; Approved on: 15 April 2025).

Study population

Cranial CT scans performed for various clinical indications between January and December 2021 were reviewed. A total of 676 individuals whose CT scans were independently reported as normal by at least two consultant radiologists were included in the analysis. No formal a priori power calculation was performed because this was a retrospective study; instead, all eligible CT scans that met the inclusion criteria during the study period were included to maximize statistical precision.

Inclusion criteria

• Individuals of any age or sex.

• Brain CT scans reported as normal despite neurological symptoms or head injury.

Exclusion criteria

• Radiological evidence of hydrocephalus.

• Space-occupying lesions, intracranial haemorrhage, or cerebral infarction.

• Congenital cranial abnormalities.

• CT scans with severe motion artefacts, metallic artefacts, patient rotation, prior cranial surgery, or poor image quality were excluded.

Imaging technique and measurements

All scans were acquired using a Siemens Somatom Emotion 16-slice CT scanner (Siemens Healthineers, Erlangen, Germany) with 5 mm axial slices obtained from the skull base to the vertex. Measurements were performed on axial images at the level of the frontal horns using KPACS radiology software in accordance with previously described Evans’ Index morphometric techniques [2, 14, 18].

The primary measurements were performed by a consultant radiologist with 10 years of experience in diagnostic radiology, while inter-observer reproducibility assessments were conducted independently by a second consultant radiologist with 9 years of experience. Digital electronic callipers embedded within the software were used, and all measurements were recorded in millimetres (mm). MAHD was measured as the maximum transverse distance between the outermost margins of the frontal horns of the lateral ventricles, while MICD was measured as the maximum internal diameter between the inner tables of the skull at the same axial level. Evans’ Index was calculated as the ratio of MAHD to MICD.

Figure 1 illustrates the measurement technique used in this study.

Fig. 1.

Axial CT image demonstrating the measurement of Evans’ Index at the level of the frontal horns of the lateral ventricles

To assess measurement reproducibility, a random subset of 50 CT scans was independently re-measured by two observers. Intra-observer reliability was assessed by repeat measurement after one week by the same observer, blinded to the initial measurements. Intraclass correlation coefficients were calculated for MAHD, MICD, and Evans’ Index.

Statistical analysis

Data were analysed using IBM SPSS version 20 (IBM Corp., Armonk, NY, USA). Normality was assessed using the Shapiro–Wilk test. As the data were non-normally distributed, continuous variables are presented as median with quartile ranges (Q1–Q3), where the interquartile range represents the difference between the 75th percentile (Q3) and the 25th percentile (Q1). Reference ranges were defined using the 2.5th and 97.5th percentiles.

Age groups were selected to reflect broad physiological, neuroanatomical, developmental, and ageing stages across the lifespan while ensuring adequate subgroup sample sizes for robust non-parametric analyses. The age thresholds were chosen to distinguish paediatric/adolescent individuals (1–18 years), younger and middle-aged adults (19–50 years), older adults in whom age-related ventricular changes become increasingly evident (51–80 years), and the very elderly population (≥ 81 years), in whom ventricular enlargement associated with advanced ageing has been more frequently reported.

Group comparisons by sex were performed using the Mann–Whitney U test, whereas differences across the four age groups were assessed using the Kruskal–Wallis test with Bonferroni-adjusted post hoc analyses. As the data were non-normally distributed, Kruskal–Wallis analysis was performed using mean ranks rather than arithmetic means. Associations between age and the study variables were evaluated using Spearman’s rank correlation coefficient, with corresponding 95% confidence intervals estimated using Fisher’s z-transformation.

Multivariable linear regression analysis was performed to evaluate the independent effects of age and sex on Evans’ Index. Regression coefficients (β), 95% confidence intervals, and p-values were reported. Linear regression assumptions were assessed by inspection of standardized residual plots, histograms, and normal probability plots. No substantial deviations from linearity, normality, or homoscedasticity were observed, supporting the suitability of the linear regression model. Statistical significance was set at p < 0.05.

Results

Table 1 summarizes the demographic characteristics of the study population. A total of 676 CT scans met the inclusion criteria with a male-to-female ratio of 1:1.5. Participants ranged from 1 year to over 80 years of age, with the largest proportion in the 19–50-year age group (45.1%).

Table 1.

Demographic characteristics of the study population (N = 676)

Variable	Category	Frequency (n)	Percentage (%)
Sex	Male	268	39.6
	Female	408	60.4
Age group (years)	1–18	59	8.7
	19–50	305	45.1
	51–80	242	35.8
	≥ 81	70	10.4

Table 2 presents the inter- and intra-observer reliability analyses for Evans’ Index and cranial measurements. Excellent reproducibility was observed for all variables, with inter-observer ICC values ranging from 0.90 to 0.93 and intra-observer ICC values ranging from 0.95 to 0.97. These findings indicate a high degree of measurement consistency and support the reliability of the morphometric assessments used in this study.

Table 2.

Inter- and intra-observer reliability of Evans’ Index and cranial measurements

Variable	Inter-Observer ICC (95% CI)	Intra-Observer ICC (95% CI)
Evans’ Index	0.90 (0.83–0.94)	0.95 (0.90–0.97)
MAHD	0.91 (0.85–0.95)	0.95 (0.91–0.97)
MICD	0.93 (0.88–0.96)	0.97 (0.94–0.98)

ICC: Intraclass Correlation Coefficient; CI: Confidence Interval; MAHD: Maximum Anterior Horn Diameter; MICD: Maximum Internal Cranial Diameter

In Fig. 2, the Evans’ Index distribution in the study population shows a slight negative skew (skewness = − 0.27; mean = 0.27, median = 0.28, SD = 0.03). The overlaid normal curve demonstrates mild deviation from normality, which was confirmed by the Shapiro–Wilk test (p < 0.001).

Fig. 2.

Histogram of Evans’ Index on axial CT scan

In Table 3; Age-specific analysis demonstrated a progressive increase in Evans’ Index across the lifespan, with median values rising from 0.24 (Q1–Q3: 0.22–0.26) in individuals aged 1–18 years to 0.30 (Q1–Q3: 0.30–0.32) among those aged ≥ 81 years. A similar age-related increase was observed for MAHD, whereas MICD increased from childhood to adulthood and subsequently declined in the oldest age group.

Table 3.

Age-specific median values and percentile-based reference ranges of Evans’ Index and cranial dimensions

Variable	Age Group (years)	Median (Q1–Q3)	Reference Range (2.5th–97.5th percentile)
Evans’ Index	1–18	0.24 (0.22–0.26)	0.21–0.28
	19–50	0.26 (0.24–0.28)	0.22–0.30
	51–80	0.29 (0.28–0.31)	0.23–0.33
	≥ 81	0.30 (0.30–0.32)	0.29–0.32
MAHD (mm)	1–18	30.17 (27.00–31.38)	26.60–33.27
	19–50	33.34 (31.13–36.21)	27.60–38.34
	51–80	37.26 (36.14–38.29)	30.43–39.60
	≥ 81	35.80 (34.80–39.85)	34.80–40.67
MICD (mm)	1–18	120.52 (119.80–122.60)	114.49–136.75
	19–50	129.21 (124.68–132.41)	120.90–136.13
	51–80	129.36 (121.34–131.65)	117.70–134.31
	≥ 81	120.72 (117.60–125.04)	117.60–127.07

Figure 3 presents a boxplot showing the distribution of Evans’ Index values across age groups (1–18, 19–50, 51–80, and ≥ 81 years), demonstrating a progressive increase in ventricular size with advancing age.

Fig. 3.

Distribution of Evans’ Index across age groups

In Table 4, the sex-specific analysis showed that females had higher median EI and MAHD values (p < 0.001), while males had a significantly larger MICD value (p < 0.001).

Table 4.

Sex-Specific Median values and percentile-based reference ranges of Evans’ Index and cranial dimensions

Variable	Sex	Median (Q1–Q3)	Reference range (2.5th–97.5th percentile)
Evans’ Index	Male	0.26 (0.23–0.28)	0.21–0.31
	Female	0.29 (0.27–0.30)	0.22–0.32
	Overall	0.28 (0.25–0.30)	0.22–0.32
MAHD (mm)	Male	32.61 (30.26–36.23)	26.60–38.98
	Female	36.28 (34.80–37.78)	29.16–40.31
	Overall	35.48 (31.84–37.33)	27.13–39.95
MICD (mm)	Male	129.31 (121.33–133.48)	117.32–136.57
	Female	126.54 (121.65–130.86)	117.60–133.26
	Overall	127.44 (121.44–131.40)	117.60–135.89

Figure 4 presents a boxplot showing the distribution of Evans’ Index values among males and females, with females demonstrating higher median Evans’ Index values than males.

Fig. 4.

Distribution of Evans’ Index according to sex

Table 5 presents the mean ranks from the Kruskal–Wallis test across the four age groups. Mean ranks for Evans’ Index and MAHD rise progressively with advancing age, reflecting the strong age-related trend, whereas MICD mean ranks peak in the 19–50 years’ group and decline in older age groups. Post hoc pairwise comparisons demonstrated statistically significant differences between most age groups after Bonferroni correction. (adjusted α = 0.008).

Table 5.

Kruskal–Wallis mean ranks of Evans’ Index, MAHD, and MICD across age groups

Age group (years)	N	EI Mean Rank	MAHD Mean Rank	MICD Mean Rank
1–18	59	144.90	93.37	191.46
19–50	305	240.98	258.93	402.11
51–80	242	443.41	466.85	348.45
≥ 81	70	563.91	448.11	150.88
χ² (df = 3)		300.104	270.191	131.057
p-value		< 0.001	< 0.001	< 0.001

Table 6 presents the Spearman rank correlations between age and the study variables. Age showed a strong positive association with Evans’ Index (ρ = 0.746, 95% CI: 0.710–0.777; p < 0.001) and maximum anterior horn diameter (MAHD; ρ = 0.654, 95% CI: 0.609–0.695; p < 0.001), and a weak negative association with maximum internal cranial diameter (MICD; ρ = −0.240, 95% CI: −0.309 to − 0.167; p < 0.001).

Table 6.

Correlation between age and Evans’ Index and cranial dimensions

Variable	Spearman’s ρ	95% CI	p-value
Evans’ Index	0.746	0.710–0.777	< 0.001
MAHD	0.654	0.609–0.695	< 0.001
MICD	−0.240	−0.309 to − 0.167	< 0.001

Table 7 presents a multivariable linear regression analysis to evaluate the independent effects of age and sex on Evans’ Index. Increasing age was independently associated with higher Evans’ Index values (β = 0.00092, 95% CI: 0.00083–0.00100; p < 0.001). Although females had higher median Evans’ Index values in the univariate analysis, sex was not an independent predictor after adjustment for age (β = 0.00372, 95% CI: −0.00024–0.00768; p = 0.066). The overall model explained 51.8% of the variation in Evans’ Index (R² = 0.518).

Table 7.

Multivariable linear regression analysis predicting Evans’ Index

Predictor	β Coefficient	95% CI	p-value
Age (years)	0.00092	0.00083–0.00100	< 0.001
Female sex	0.00372	−0.00024–0.00768	0.066
Constant	0.226	0.222–0.230	< 0.001

Model statistics: R² = 0.518; Adjusted R² = 0.516; F = 361.2; p < 0.001

Discussion

This study establishes age- and sex-specific percentile-based reference values for Evans’ Index (EI) and cranial dimensions in a south-eastern Nigerian hospital population with radiologically normal CT scans. The overall median Evans’ Index (EI) of 0.28 (95% reference range: 0.22–0.32) was slightly higher than values reported in northern Nigeria [18], Ghana [19], Ethiopia [20], and Delta State in southern Nigeria [21], although the upper limit remained close to the widely accepted threshold of 0.30 for ventriculomegaly [2, 9]. The slightly higher median EI observed in the present study may reflect regional differences in cranial morphology, population-specific demographic characteristics, and the inclusion of both paediatric and very elderly participants.

The recent Delta State study further highlights regional variation in Evans’ Index measurements within Nigeria and reinforces the need for locally derived reference values across diverse geographic and ethnic populations. Similar age-related increases in Evans’ Index have been reported in studies from Ghana and Ethiopia, while investigations from India have demonstrated the continued utility of Evans’ Index as a practical ventricular morphometric measure in routine neuroimaging. Studies involving European elderly populations have likewise highlighted the limitations of applying a single universal Evans’ Index threshold across all age groups and have advocated age-adjusted interpretation, particularly in older adults [9, 11]. Collectively, these findings support the growing recognition that ventricular morphometry varies across populations and that locally derived reference values may improve the accuracy of neuroimaging interpretation. Variations in reported Evans’ Index values and reference ranges are likely attributable to differences in ethnic cranial morphometry, demographic structure, age composition, imaging protocols, and study design. Consequently, the use of population-specific reference values may reduce diagnostic misclassification and improve the accuracy of clinical decision-making when evaluating ventricular enlargement across diverse populations.

The histogram (Fig. 2), age-specific reference values, and mean-rank comparisons (Table 5) demonstrated a progressive increase in Evans’ Index and maximum anterior horn diameter (MAHD) with advancing age. Median Evans’ Index increased from 0.24 in individuals aged 1–18 years to 0.30 among those aged ≥ 81 years, while MAHD increased from 30.17 mm in the youngest age group to peak values in older adults. In contrast, maximum internal cranial diameter (MICD) increased from childhood to adulthood but declined substantially in the oldest age group. These findings suggest that the age-related increase in Evans’ Index is driven not only by ventricular enlargement but also by a relative reduction in internal cranial dimensions in advanced age. This pattern is consistent with age-related cerebral atrophy, progressive loss of brain parenchymal volume, and ventricular expansion that accompany normal ageing and neurodegenerative processes [11–14].

The strong positive correlation between age and Evans’ Index (ρ = 0.746, 95% CI: 0.710–0.777) indicates a substantial age-related effect, suggesting clinically meaningful ventricular enlargement with advancing age. Similar age-related increases in ventricular dimensions have been reported in studies from Ghana, Ethiopia, and European elderly populations [11, 13, 19, 20]. Currà et al. [13] demonstrated progressive ventricular enlargement after the seventh decade of life, while Ambarki et al. [14] reported significant associations between Evans’ Index and ventricular volume measurements in healthy elderly individuals. The close agreement between the present findings and those reported in previous morphometric and volumetric studies supports the validity of the observed age-related trends. The age-related changes identified in this study are therefore likely to reflect physiological brain ageing rather than pathological ventriculomegaly alone. Clinically, these findings reinforce the importance of interpreting Evans’ Index within the context of patient age, particularly among older adults in whom values approaching 0.30 may reflect age-related structural changes rather than overt hydrocephalus.

Multivariable linear regression analysis demonstrated that age remained an independent predictor of Evans’ Index after adjustment for sex, whereas sex was not independently associated with Evans’ Index. This finding suggests that age-related structural brain changes are the primary drivers of ventricular enlargement in this population and may partly account for the sex differences observed in the univariate analyses. The results further support the importance of age-adjusted interpretation of Evans’ Index in clinical practice and reinforce concerns regarding the use of a single universal threshold across diverse demographic groups.

Statistically significant sex differences were observed, with females exhibiting higher median Evans’ Index (EI) and maximum anterior horn diameter (MAHD) values despite having smaller maximum internal cranial diameters (MICD) (all p < 0.001). This finding contrasts with that of Hamidu et al. [18], who reported no significant sex differences in EI in a northern Nigerian cohort. The discrepancy may reflect differences in population characteristics, age distribution, sample composition, or regional cranial morphometric variation between the two studies. These findings underscore the importance of sex-specific interpretation of ventricular indices, particularly when applying fixed diagnostic thresholds across diverse populations. However, while the observed differences were notable, the absolute differences in median EI values were modest and should therefore be interpreted within the appropriate clinical context.

The observed sex differences may reflect underlying biological variations in cranial morphology, ventricular anatomy, hormonal influences, and patterns of cerebral ageing. Males generally have larger cranial dimensions, whereas differences in ventricular size and age-related structural brain changes may influence Evans’ Index measurements. Variability in hormonal exposure, particularly the effects of sex hormones on brain development and ageing, may also contribute to sex-specific differences in ventricular morphometry. These mechanisms may partly explain the higher Evans’ Index values observed among females in the present study despite their smaller internal cranial diameters. From a clinical perspective, these findings suggest that sex-specific reference values may improve the interpretation of ventricular enlargement and reduce the risk of diagnostic misclassification in routine neuroimaging practice.

Although Evans’ Index remains one of the most widely used linear measures of ventricular size, it is not without limitations. Other linear indices, such as the bicaudate index, have been proposed to improve the assessment of ventricular enlargement, while MRI-based volumetric techniques provide more comprehensive evaluation of ventricular anatomy and cerebrospinal fluid spaces. Nevertheless, volumetric methods require advanced imaging protocols, specialized software, and greater technical expertise, which may limit their routine use in many clinical settings. In contrast, Evans’ Index remains simple, rapid, reproducible, and readily applicable on both CT and MRI. Its continued incorporation into diagnostic criteria for hydrocephalus and normal pressure hydrocephalus underscores its ongoing clinical relevance.

Consequently, although modern volumetric approaches offer greater anatomical precision, Evans’ Index continues to serve as a practical and valuable tool for the assessment of ventricular enlargement, particularly in resource-limited settings where advanced neuroimaging resources may not be routinely available.

The results further highlight the limitations of applying a single universal EI cut-off across all populations. Although the traditional threshold of ≥ 0.30 remains a useful clinical reference point, age- and sex-stratified reference values, particularly percentile-based thresholds, may enhance diagnostic precision in the assessment of ventriculomegaly, hydrocephalus, and cerebral atrophy. However, EI values approaching or exceeding 0.30 in older adults should be interpreted cautiously within the broader clinical and radiological context rather than being considered independently diagnostic of pathology. The age-adjusted cut-offs proposed by Brix et al. [9], together with the observations by Missori et al. [10] that pathological EI values are rarely reached in normal ageing, support a more nuanced and context-sensitive interpretation.

From a clinical perspective, the findings of this study suggest that Evans’ Index should be interpreted within the context of patient age, sex, clinical presentation, and complementary neuroimaging findings rather than relying solely on a universal threshold. The age-specific reference values generated in this study demonstrate that Evans’ Index values increase progressively with advancing age and may approach 0.30 in otherwise radiologically normal elderly individuals. Consequently, EI values near or slightly above this threshold in older adults should be interpreted cautiously and correlated with clinical and radiological findings before a diagnosis of ventriculomegaly or hydrocephalus is considered. Conversely, similar values in younger individuals may warrant further evaluation. These findings may assist clinicians in distinguishing physiological ageing from pathological ventricular enlargement, particularly in resource-limited settings where advanced volumetric imaging techniques are not routinely available.

Collectively, these findings reinforce the importance of integrating demographic characteristics, clinical presentation, and complementary imaging findings when assessing ventricular enlargement in both routine practice and research settings.

Limitations

This study has several limitations. It was retrospective and hospital-based, including individuals referred for clinically indicated CT imaging rather than a community-derived healthy population. Consequently, the findings may not fully represent the general healthy population. Although scans were independently reported as radiologically normal, subtle subclinical neurodegenerative, traumatic, or age-related changes may have remained undetected. Because CT scans were obtained for clinical indications, the possibility of undetected subclinical pathology cannot be completely excluded despite radiological normality.

In addition, the unequal age and sex distribution of participants may have influenced subgroup-specific estimates. The distribution of participants across age groups was also uneven, with smaller numbers in the paediatric/adolescent and very elderly categories compared with the adult age groups. Although non-parametric statistical methods were used and significant age-related trends were consistently observed, the findings for the smallest age groups should be interpreted with appropriate caution. Although both sex groups were adequately represented, the higher proportion of female participants may have influenced sex-specific estimates and may limit the generalizability of the findings to populations with more balanced sex distributions.

The use of 5 mm axial slices may have introduced partial volume averaging effects and minor measurement imprecision compared with thinner-slice multi-detector CT protocols (1–2 mm). Detailed acquisition parameters such as tube current (mAs), field of view, and reconstruction algorithms were not available from archived imaging records and could not be retrospectively retrieved.

However, 5 mm protocols remain widely used in routine clinical practice in many low-resource settings and are consistent with methodologies employed in several comparable African CT morphometric studies. Measurements were obtained using standardized digital calliper techniques, and excellent inter- and intra-observer reliability was demonstrated. Nevertheless, minor variability inherent to linear morphometric measurements cannot be completely excluded.

Strengths

Despite these limitations, the study has notable strengths. These include a relatively large sample size, strict confirmation of radiological normality by consultant radiologists, comprehensive stratification by age and sex, incorporation of reproducibility testing, and both visual and statistical demonstration of distribution patterns and age-related trends. The study also provides clinically relevant hospital-based reference values for Evans’ Index and cranial dimensions in a previously underrepresented south-eastern Nigerian population, thereby contributing to more context-sensitive neuroimaging interpretation in resource-limited settings.

Conclusion

Hospital-based reference values of Evans’ Index and cranial dimensions in south-eastern Nigeria vary significantly according to age and sex. The percentile-based reference ranges generated in this study provide locally relevant benchmarks that may improve the interpretation of ventricular size in clinical practice. These findings support more context-sensitive neuroimaging assessment in the evaluation of ventriculomegaly, hydrocephalus, and cerebral atrophy, particularly in resource-limited settings. Age-stratified reference values are recommended, as Evans’ Index values approaching or exceeding 0.30 in older adults may reflect physiological ageing and should be interpreted within the broader clinical and radiological context.

Abbreviations

CT

Computed tomography

EI

Evans’ Index

Q1–Q3

First and third quartiles

MICD

Maximum internal cranial diameter

MRI

Magnetic resonance imaging

NPH

Normal pressure hydrocephalus

ρ

Spearman’s rank correlation coefficient

Author contributions

E.I. contributed to the study design, literature review, data acquisition and write up of first and final draft. C.A.I. contributed to the study design, literature review, analysis, and write up of first and final draft. M.A. contributed to result interpretation and final draft. All authors read and approved the final article submitted.

Funding

Provided and supported by Imo State University Teaching Hospital, Orlu. The hospital had no role in study design, data collection, analysis, interpretation or manuscript write-up.

Data availability

The datasets generated and/or analyzed during the current study are not publicly available due to ethical restrictions on sharing anonymized clinical imaging data. However, they will be available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Ethical approval for this study was obtained from the Imo State University Teaching Hospital Research Ethics Committee (Approval No: IMSUTH/REC/2025/074; Approved on: 15 April 2025). As this was a retrospective review of anonymized imaging data, the requirement for informed consent was waived by the Ethics Committee. All procedures performed in this study were conducted in accordance with the ethical standards of the institutional research committee and with the 1964 Helsinki Declaration and its later amendments.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.