title: Evidence that congenital hydrocephalus is a precursor to idiopathic normal pressure hydrocephalus in only a subset of patients.
abstract: OBJECTIVE
  To identify the proportion of patients in a large idiopathic normal pressure hydrocephalus (INPH) cohort with large head circumference (HC) who presumably have congenital hydrocephalus that has not become clinically apparent until late in life.
  
  
  METHODS
  HC was measured in 158 patients diagnosed with communicating INPH and assigned to HC centile range adjusted for height and sex. The proportion of patients with INPH and HC above the 97th, 90th or 50th centiles was compared with the proportion expected in a normal population.
  
  
  RESULTS
  The proportion of patients with HC >90th centile was significantly larger than would be expected in a normal distribution (19.6% vs 10%, p = 0.0001), as was the proportion of patients with HC >97th centile (8.9% vs 3%, p = 0.0001). The relative association between INPH and HC >97th centile was nearly tripled (relative association 2.95; CI 1.36 to 6.41), but the relative association between INPH and HC >50th centile was not significantly higher than predicted (relative association 1.13; CI 0.95 to 1.34).
  
  
  CONCLUSIONS
  A significantly larger proportion of elderly adults with INPH have a HC greater than the 90th or 97th centile than predicted by population norms, supporting the concept that compensated congenital hydrocephalus that does not become symptomatic until late in life is one aetiology of INPH, but is not responsible for all INPH. The mechanism that leads to the development of INPH in most patients remains elusive.
authors.affiliation: Adult Hydrocephalus Program, Department of Neurology, Johns Hopkins Hospital, Baltimore, Maryland 21287, USA. Rwilso21@jhmi.edu
authors.forename: Robin K
authors.initials: RK
authors.lastname: Wilson
authors.forename: Michael A
authors.initials: MA
authors.lastname: Williams
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D002508:Cephalometry; D015331:Cohort Studies; D005260:Female; D006801:Humans; D006849:Hydrocephalus / Q000150:complications*; D006850:Hydrocephalus, Normal Pressure / Q000209:etiology*; D008297:Male; D011237:Predictive Value of Tests; D012189:Retrospective Studies; D012886:Skull / Q000033:anatomy & histology