title: Optic nerve sheath diameter, intracranial pressure and acute mountain sickness on Mount Everest: a longitudinal cohort study.
abstract: OBJECTIVE
  To investigate the association of optic nerve sheath diameter (ONSD), as a correlate of intracranial pressure (ICP), with acute mountain sickness (AMS).
  
  
  METHODS
  Longitudinal cohort study of mountaineers from sea level to 6400 m.
  
  
  METHODS
  Mount Everest (North side).
  
  
  METHODS
  13 mountaineers (10 men, 3 women; aged 23-52 years) on a British expedition to climb Mount Everest.
  
  
  METHODS
  ONSD was measured ultrasonically, 3 mm behind the globe using B scans recorded with an OTI-Scan 3D scanner (Ophthalmic Technologies, Canada). Serial binocular scans were recorded at sea level, and 2000, 3700, 5200 and 6400 m. All ONSDs were measured by a blinded observer.
  
  
  METHODS
  ONSD, AMS score (using the Lake Louise scoring system), heart rate, and oxygen saturation levels.
  
  
  RESULTS
  All results were analysed by regression analysis with adjustment. ONSD was positively associated with increasing altitude above sea level (0.10 mm increase in ONSD per 1000 m, 95% CI 0.05 to 0.14 mm) and AMS score (0.12 mm per score, 95% CI 0.06 to 0.18 mm); further associations were found with resting heart rate (0.29 mm per 20 beats/min, 95% CI 0.17 to 0.41 mm) and oxygen saturations (0.20 mm per 10% decrease, 95% CI 0.11 to 0.29 mm).
  
  
  CONCLUSIONS
  ONSD increases at high altitude, and this increase is associated with more severe symptoms of AMS. Given the linkage between ONSD and ICP, these results strongly suggest that intracranial pressure plays an important role in the pathophysiology of AMS.
authors.affiliation: Wellcome Research Training Fellow, Nuffield Department of Surgery, Level 6, John Radcliffe Hospital, Oxford OX3 9DU, UK. aisutherland@mac.com
authors.forename: A I
authors.initials: AI
authors.lastname: Sutherland
authors.forename: D S
authors.initials: DS
authors.lastname: Morris
authors.forename: C G
authors.initials: CG
authors.lastname: Owen
authors.forename: A J
authors.initials: AJ
authors.lastname: Bron
authors.forename: R C
authors.initials: RC
authors.lastname: Roach
mesh_terms: D000208:Acute Disease; D000328:Adult; D000531:Altitude; D000532:Altitude Sickness / Q000209:etiology* / Q000503:physiopathology; D004812:Epidemiologic Methods; D005260:Female; D006801:Humans; D007427:Intracranial Pressure / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D009051:Mountaineering / Q000502:physiology*; D009900:Optic Nerve / Q000000981:diagnostic imaging*; D014463:Ultrasonography