title: Evans' index revisited: the need for an alternative in normal pressure hydrocephalus.
abstract: BACKGROUND
  The international guidelines for the diagnosis of normal pressure hydrocephalus (NPH) define ventricular enlargement as Evans' index greater than 0.3.
  
  
  OBJECTIVE
  To establish whether there is a correlation between Evans' index and ventricular volume (VV) in NPH and whether choosing different planes for the measurements could produce significantly different results.
  
  
  METHODS
  Pre-shunt insertion, thin-section CT scans of the brains of 10 patients with shunt-responsive NPH were reviewed retrospectively, measuring Evans' index, frontal horn index, VV, and total intracranial volume (ICV). The ventricular/intracranial volume index (VV/ICV) was calculated. Correlation between each of the linear indices and VV and VV/ICV was done.
  
  
  RESULTS
  Significant differences were found in the index values calculated at different planes. The frontal horn index at a plane 16 mm parallel to the anterior commissure-posterior commissure (AC-PC) plane showed best correlation with VV and VV/ICV (r: 0.658 and 0.587, respectively). Evans' index, also obtained at a plane 16 mm parallel to the AC-PC plane, showed best correlation with VV and VV/ICV (r: 0.619 and 0.498, respectively).
  
  
  CONCLUSIONS
  Evans' index value can vary significantly in a patient with NPH, depending on the level of the brain CT scan image at which the frontal horns and maximal inner skull diameters are measured. Evans' index is not an ideal method for estimating the VV in NPH patients. Volumetric measurements represent the logical accurate estimate of true ventricular size as well as the size of the other intracranial compartments.
authors.affiliation: Victor Horsley Department of Neurosurgery, National Hospital for Neurology and Neurosurgery, Queen Square, London, United Kingdom. Ahmedktoma@yahoo.com
authors.forename: Ahmed K
authors.initials: AK
authors.lastname: Toma
authors.forename: Etienne
authors.initials: E
authors.lastname: Holl
authors.forename: Neil D
authors.initials: ND
authors.lastname: Kitchen
authors.forename: Laurence D
authors.initials: LD
authors.lastname: Watkins
mesh_terms: D001921:Brain / Q000000981:diagnostic imaging*; D006801:Humans; D006850:Hydrocephalus, Normal Pressure / Q000000981:diagnostic imaging*; D009929:Organ Size; D012189:Retrospective Studies; D012720:Severity of Illness Index; D014057:Tomography, X-Ray Computed / Q000379:methods / Q000592:standards