title: The pressure difference between eye and brain changes with posture.
abstract: The discovery of a posture-dependent effect on the difference between intraocular pressure (IOP) and intracranial pressure (ICP) at the level of lamina cribrosa could have important implications for understanding glaucoma and idiopathic intracranial hypertension and could help explain visual impairments in astronauts exposed to microgravity. The aim of this study was to determine the postural influence on the difference between simultaneously measured ICP and IOP.
  
  
  
  Eleven healthy adult volunteers (age = 46 ± 10 years) were investigated with simultaneous ICP, assessed through lumbar puncture, and IOP measurements when supine, sitting, and in 9° head-down tilt (HDT). The trans-lamina cribrosa pressure difference (TLCPD) was calculated as the difference between the IOP and ICP. To estimate the pressures at the lamina cribrosa, geometrical distances were estimated from magnetic resonance imaging and used to adjust for hydrostatic effects.
  
  
  
  The TLCPD (in millimeters of mercury) between IOP and ICP was 12.3 ± 2.2 for supine, 19.8 ± 4.6 for sitting, and 6.6 ± 2.5 for HDT. The expected 24-hour average TLCPD on earth-assuming 8 hours supine and 16 hours upright-was estimated to be 17.3mmHg. By removing the hydrostatic effects on pressure, a corresponding 24-hour average TLCPD in microgravity environment was simulated to be 6.7mmHg.
  
  
  
  We provide a possible physiological explanation for how microgravity can cause symptoms similar to those seen in patients with elevated ICP. The observed posture dependency of TLCPD also implies that assessment of the difference between IOP and ICP in upright position may offer new understanding of the pathophysiology of idiopathic intracranial hypertension and glaucoma. Ann Neurol 2016;80:269-276.
authors.affiliation: Department of Radiation Sciences, Umeå University, Umeå, Sweden.|Center for Biomedical Engineering and Physics, Umeå University, Umeå, Sweden.
authors.forename: Anders
authors.initials: A
authors.lastname: Eklund
authors.affiliation: Department of Clinical Science, Ophthalmology, Umeå University, Umeå, Sweden.
authors.forename: Gauti
authors.initials: G
authors.lastname: Jóhannesson
authors.affiliation: Department of Pharmacology and Clinical Neuroscience, Umeå University, Umeå, Sweden.
authors.forename: Elias
authors.initials: E
authors.lastname: Johansson
authors.affiliation: Department of Radiation Sciences, Umeå University, Umeå, Sweden.
authors.forename: Petter
authors.initials: P
authors.lastname: Holmlund
authors.affiliation: Department of Radiation Sciences, Umeå University, Umeå, Sweden.
authors.forename: Sara
authors.initials: S
authors.lastname: Qvarlander
authors.affiliation: Department of Radiation Sciences, Umeå University, Umeå, Sweden.
authors.forename: Khalid
authors.initials: K
authors.lastname: Ambarki
authors.affiliation: Department of Radiation Sciences, Umeå University, Umeå, Sweden.|Umeå Center for Functional Brain Imaging, Umeå University, Umeå, Sweden.
authors.forename: Anders
authors.initials: A
authors.lastname: Wåhlin
authors.affiliation: Department of Pharmacology and Clinical Neuroscience, Umeå University, Umeå, Sweden.
authors.forename: Lars-Owe D
authors.initials: LO
authors.lastname: Koskinen
authors.affiliation: Department of Pharmacology and Clinical Neuroscience, Umeå University, Umeå, Sweden.
authors.forename: Jan
authors.initials: J
authors.lastname: Malm
mesh_terms: D005260:Female; D006801:Humans; D007427:Intracranial Pressure / Q000502:physiology*; D007429:Intraocular Pressure / Q000502:physiology*; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D059906:Neuroimaging; D011187:Posture / Q000502:physiology*