Evans Index
Ratio of frontal horn width to maximal skull diameter, screening for ventriculomegaly and hydrocephalus.
The Evans index is the ratio of the maximal width of the frontal horns of the lateral ventricles to the maximal internal diameter of the skull on the same axial CT or MRI slice. It is the most widely used linear index of ventricular size.
📐 Formula
Evans index = frontal horn width / internal skull diameter
🎯 When to Use
- Screening for ventriculomegaly on axial CT or MRI.
- Supporting a diagnosis of normal pressure hydrocephalus (NPH) alongside clinical and imaging features.
- Following ventricular size over time.
⚙️ How It Works
- Measure the maximal distance between the tips of the frontal horns.
- Measure the maximal internal diameter of the skull on the same slice.
- Divide the first by the second to obtain a dimensionless ratio.
📊 Interpreting the Result
- ≤ 0.30 — within normal limits.
- > 0.30 — ventriculomegaly; supports hydrocephalus / NPH when correlated with clinical features and the callosal angle.
- Both measurements should be taken on the same slice for validity.
⚠️ Limitations & Pitfalls
- A single linear ratio; does not distinguish hydrocephalus from ex-vacuo dilatation due to atrophy.
- Increases with age; interpret cautiously in the elderly.
- Should be combined with the callosal angle and DESH sign when assessing NPH.
📚 References
- Evans WA. An encephalographic ratio for estimating ventricular enlargement and cerebral atrophy. Arch Neurol Psychiatry. 1942;47(6):931-937.
- Relkin N, Marmarou A, Klinge P, et al. Diagnosing idiopathic normal-pressure hydrocephalus. Neurosurgery. 2005;57(3 Suppl):S4-S16.
⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.