The Evans' index is the ratio of 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. This ratio varies with age and sex.
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. As such Evans' index has little role to play in day-to-day reporting 4.
A normal Evans' index is less than 0.3, but it can be higher in a normal healthy elderly individual 3.
- 1. Ng SE, Low AM, Tang KK et-al. Value of quantitative MRI biomarkers (Evans' index, aqueductal flow rate, and apparent diffusion coefficient) in idiopathic normal pressure hydrocephalus. J Magn Reson Imaging. 2009;30 (4): 708-15. doi:10.1002/jmri.21865 - Pubmed citation
- 2. Toma AK, Holl E, Kitchen ND et-al. Evans' index revisited: the need for an alternative in normal pressure hydrocephalus. Neurosurgery. 2011;68 (4): 939-44. doi:10.1227/NEU.0b013e318208f5e0 - Pubmed citation
- 3. Brix M, Brix WE, Brix SA et-al. The Evans' Index revisited: New cut-off levels for use in radiological assessment of ventricular enlargement in the elderly. (2017) European journal of radiology. doi:10.1016/j.ejrad.2017.07.013 - Pubmed
- 4. Neikter J, Neikter AS, Neikter HmP, Neikter TM, Neikter SG, Neikter ZD, Neikter FD, Neikter. Ventricular Volume Is More Strongly Associated with Clinical Improvement Than the Evans Index after Shunting in Idiopathic Normal Pressure Hydrocephalus. (2020) AJNR. American journal of neuroradiology. doi:10.3174/ajnr.A6620 - Pubmed