Evans' Index

Evans' Index (Evans, 1942) is the standard linear surrogate for lateral ventricular size on CT or MRI. On the axial slice showing the widest extent of the frontal horns, the maximum frontal horn width (outer edge to outer edge, or the maximal distance between the lateral margins of the frontal horns) is divided by the maximum internal diameter of the skull at the same level (inner table to inner table, typically at the level of the anterior falx). A ratio of 0.3 or greater indicates ventriculomegaly and is one of the core radiological criteria used to support a diagnosis of idiopathic normal pressure hydrocephalus (iNPH) — alongside disproportionately enlarged subarachnoid space (DESH) and callosal angle — as well as a general screen for hydrocephalus of any cause. The index does not by itself distinguish enlargement from hydrocephalus versus enlargement from atrophy (ex vacuo dilatation); callosal angle, DESH, and clinical context (gait disturbance, urinary incontinence, cognitive decline) help make that distinction.