title: Osmolality of Cerebrospinal Fluid from Patients with Idiopathic Intracranial Hypertension (IIH).
abstract: BACKGROUND
  Idiopathic intracranial hypertension (IIH) is a disorder of increased intracranial fluid pressure (ICP) of unknown etiology. This study aims to investigate osmolality of cerebrospinal fluid (CSF) from patients with IIH.
  
  
  METHODS
  We prospectively collected CSF from individuals referred on suspicion of IIH from 2011-2013. Subjects included as patients fulfilled Friedman and Jacobson's diagnostic criteria for IIH. Individuals in whom intracranial hypertension was refuted were included as controls. Lumbar puncture with ICP measurement was performed at inclusion and repeated for patients after three months of treatment. Osmolality was measured with a Vapor Pressure Osmometer.
  
  
  RESULTS
  We collected 90 CSF samples from 38 newly diagnosed patients and 28 controls. At baseline 27 IIH-samples and at 3 months follow-up 35 IIH-samples were collected from patients. We found no significant differences in osmolality between 1) patients at baseline and controls (p = 0. 86), 2) patients at baseline and after 3 months treatment (p = 0.97), and 3) patients with normalized pressure after 3 months and their baseline values (p = 0.79). Osmolality in individuals with normal ICP from 6-25 cmH2O (n = 41) did not differ significantly from patients with moderately elevated ICP from 26-45 cmH2O (n = 21) (p = 0.86) and patients with high ICP from 46-70 cmH2O (n = 4) (p = 0.32), respectively. There was no correlation between osmolality and ICP, BMI, age and body height, respectively. Mean CSF osmolality was 270 mmol/kg (± 1 SE, 95% confidence interval 267-272) for both patients and controls.
  
  
  CONCLUSIONS
  CSF osmolality was normal in patients with IIH, and there was no relation to treatment, ICP, BMI, age and body height. Mean CSF osmolality was 270 mmol/kg and constitutes a reference for future studies. Changes in CSF osmolality are not responsible for development of IIH. Other underlying pathophysiological mechanisms must be searched.
authors.affiliation: Department of Ophthalmology, Rigshospitalet, University of Copenhagen, Glostrup, Denmark.
authors.forename: Elisabeth A
authors.initials: EA
authors.lastname: Wibroe
authors.affiliation: Danish Headache Center, Department of Neurology, Rigshospitalet, University of Copenhagen, Glostrup, Denmark.
authors.forename: Hanne M
authors.initials: HM
authors.lastname: Yri
authors.affiliation: Danish Headache Center, Department of Neurology, Rigshospitalet, University of Copenhagen, Glostrup, Denmark.
authors.forename: Rigmor H
authors.initials: RH
authors.lastname: Jensen
authors.affiliation: Department of Pediatrics and Adolescent Medicine and department of Neurosurgery, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Morten A
authors.initials: MA
authors.lastname: Wibroe
authors.affiliation: Department of Ophthalmology, Rigshospitalet, University of Copenhagen, Glostrup, Denmark.
authors.forename: Steffen
authors.initials: S
authors.lastname: Hamann
mesh_terms: D000328:Adult; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D007427:Intracranial Pressure / Q000502:physiology; D008297:Male; D009994:Osmolar Concentration; D011446:Prospective Studies; D011559:Pseudotumor Cerebri / Q000134:cerebrospinal fluid* / Q000503:physiopathology; D013129:Spinal Puncture