Evans index
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The Evans index can be used as a marker of ventriculomegaly.
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Usage
It is useful as a marker of ventricular volume and thus has been proposed as a helpful biomarker in the diagnosis of normal pressure hydrocephalus (NPH) 1. Unfortunately, it is a very rough marker of ventriculomegaly and varies greatly depending on the location and angle of the slice 2. It is also not specific for normal pressure hydrocephalus, and has been reported in other conditions such as progressive supranuclear palsy or dementia with Lewy bodies 6. As such, the Evans index has little role to play in day-to-day reporting 4.
Measurement
Evans index is the ratio of the maximum width of the frontal horns of the lateral ventricles and the maximal internal diameter of the skull at the same level employed in axial CT and MRI images.
Interpretation
Interpretation of the Evans index is as follows 5:
0.20-0.25: normal
0.25-0.30: possible or early ventriculomegaly
>0.30: ventriculomegaly
The ratio can be higher in a normal healthy elderly individual, and varies with age and sex 3.
History and etymology
The ratio was first proposed by William A Evans Jr, in 1942 5.
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