What are the normal ventricular index measurements (e.g., Evans index, frontal horn index, lateral ventricle width, third‑ventricle width, and bicaudate ratio)?

Normal Ventricular Index Measurements

The Evans index should be ≤0.3 in adults under 70 years of age, with values >0.3 considered pathological and indicative of ventriculomegaly. 1, 2

Evans Index (Frontal Horn Ratio)

Definition and Measurement:

• The Evans index is calculated as the maximal width of the frontal horns of the lateral ventricles divided by the maximal internal diameter of the skull at the same level 1
• This measurement is obtained on axial CT or MRI images 1

Normal Values by Age:

Adults

• Mean value: 0.27–0.28 (SD 0.04) in population-based studies 3, 2, 4
• Upper limit of normal: 0.3 for individuals <70 years 1, 3, 4
• Age ≥70 years: Mean increases to 0.30 (SD 0.03), with 20.6% of healthy elderly having values >0.3 2
• Recommended cutoff for elderly (≥70 years): 0.34 may be more appropriate than 0.3 3
• Range in healthy adults: 0.11 to 0.46 2

Pediatric Population

• Children: 0.218 to 0.312 (10th to 90th percentile) 5
• Age <3 years: Higher values than older children 6
• Age ≥3 years: Lower values, stabilizing through adolescence 6

Gender and Age Considerations:

• Males have slightly higher values than females, though the difference is not always statistically significant 3, 4
• Values increase progressively with age in adults 3, 2, 4
• Among men aged ≥80 years, mean Evans index reaches 0.3 2

Frontal Horn Index (Bifrontal Index)

Normal Values:

• Pediatric: 0.265 to 0.380 (10th to 90th percentile) 5
• This index decreases with age in children 5

Lateral Ventricle Width Measurements

Anterior Horn Width:

• Normal: <3 mm 7, 8
• 95th percentile at term: ≤3 mm 8
• Abnormal: >6 mm 7, 8

Levene Ventricular Index (Neonates/Infants):

• Measured horizontally from midline falx to lateral wall of anterior horn at the level of foramen of Monro 7
• Normal values (97th percentile):
  • 27 weeks gestation: ~14 mm 7
  • 31 weeks gestation: ~15 mm 7
  • 33 weeks gestation: ~16 mm 7
• Intervention threshold: 97th percentile + 4 mm (approximately 18–20 mm) 7, 8

Septum-Caudate Distance:

• Left side typically greater than right 6
• Does not vary significantly between age groups in children 6

Cella Media (Body of Lateral Ventricle):

• Width does not differ significantly between younger (<3 years) and older (≥3 years) children 6

Third Ventricle Width

Normal Values:

• Adults: Not precisely defined in guidelines, but moderate enlargement noted in idiopathic normal pressure hydrocephalus 1
• Neonates/Infants: >3 mm is abnormal 7
• Pediatric: Larger in children <3 years compared to older children 6

Bicaudate Index (Bicaudate Ratio)

Definition:

• Minimum intercaudate distance divided by brain width at the same level 3

Normal Values:

Adults

• Mean: 0.11 (range varies by study) 3
• Values increase with age 3
• Males and females show statistically significant differences 3

Pediatric

• Normal range: 0.059 to 0.152 (10th to 90th percentile) 5
• Correlation with age: -0.26 (decreases with age) 5

Additional Ventricular Indices

Huckman Number (Pediatric):

• Normal: 3.3 to 5.0 cm (10th to 90th percentile) 5
• Positive correlation with age (r=0.34) 5

Ventricular/Biparietal Ratio (V/BP Ratio):

• Used in posthemorrhagic hydrocephalus assessment 9
• Mild hydrocephalus: 0.26–0.40 9
• Moderate hydrocephalus: 0.40–0.60 9
• Severe hydrocephalus: 0.60–0.90 9
• Extreme hydrocephalus: 0.91–1.0 9

Occipital Horn Measurement:

• Normal: 11.6 mm ± 2 mm 7
• Abnormal: >16 mm 7

Critical Clinical Caveats

Interpretation Pitfalls:

• Ventricular size alone does not predict neurodevelopmental outcomes; associated brain pathology (periventricular leukomalacia, intraventricular hemorrhage) is the primary determinant of prognosis 10
• Stable mild-to-moderate ventriculomegaly in asymptomatic infants does not mandate intervention 11, 10
• Athletic training can increase left ventricular end-diastolic dimension to >55 mm in 45% of male athletes, which must be distinguished from pathological enlargement 12
• Ventricular asymmetry can be a normal anatomical variant; both ventricles should be measured 8

Measurement Reliability:

• All indices are operator-dependent and require standardized technique 8
• Linear measurements may underestimate ventricular volume in asymmetrically shaped ventricles 13
• MRI provides superior assessment compared to ultrasound for prognostication in neonates 10