Diagnosis & Imaging
Evans index
The Evans index is a measurement on brain imaging used to assess whether the ventricles are enlarged.
In short: Evans index
- It is a ratio comparing ventricle width to skull width.
- A value above 0.3 generally indicates ventricular enlargement.
- It helps quantify ventriculomegaly.
- It is used in evaluating hydrocephalus, including NPH.
What it is
The Evans index is a measurement made on brain imaging (CT or MRI) that compares the width of the frontal horns of the lateral ventricles to the widest inner width of the skull. It is used to assess whether the ventricles are enlarged.
Understanding Evans index
The index is calculated as a ratio: the maximum width of the frontal horns divided by the maximum inner width of the skull at the same level. A value greater than 0.3 is generally taken to indicate ventricular enlargement (ventriculomegaly). The Evans index provides a simple, standardized way to quantify ventricle size, useful in diagnosing and discussing hydrocephalus, including normal pressure hydrocephalus (NPH), where ventricular enlargement is a key feature. Like any single measurement, it is interpreted alongside the clinical picture and other findings, since enlarged ventricles can have causes other than active hydrocephalus.
Why it matters
Understanding the Evans index helps families interpret imaging reports that mention it, and see how it provides a standardized measure of whether the ventricles are enlarged — a key part of evaluating hydrocephalus.
Common questions
What is the Evans index?
A ratio of ventricle width to skull width on imaging.
What value indicates enlargement?
Generally above 0.3.
What is it used for?
Quantifying ventricular enlargement in hydrocephalus, including NPH.
Bottom line
The Evans index is an imaging ratio, with values above 0.3 generally indicating enlarged ventricles, used in evaluating hydrocephalus.
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