Optic nerve sheath diameter, intracranial pressure and acute mountain sickness on Mount Everest: A longitudinal cohort study

Andrew I Sutherland ^1*, Daniel S Morris ², Christopher G Owen ³, Anthony J Bron ⁴ and Robert C Roach ⁵

¹ Nuffield Department of Surgery, John Radcliffe Hospital, University of Oxford, OX3 9DU, United Kingdom
² Royal Victoria Infirmary, Queen Victoria Rd, Newcastle upon Tyne, NE1 4LP, United Kingdom
³ Division of Community Health Sciences, St George's, University of London, SW17 0RE, United Kingdom
⁴ Clinical Research Unit, Nuffield Laboratory of Ophthalmology, University of Oxford, OX2 6AW, United Kingdom
⁵ Research Division, Altitude Research Center, University of Colorado Denver Health Sciences Center,, United States

^* To whom correspondence should be addressed. E-mail: aisutherland{at}mac.com.

Accepted 5 December 2007

Abstract

Objective: To investigate the association of optic nerve sheath diameter (ONSD), as a correlate of intracranial pressure (ICP), to acute mountain sickness (AMS).

Design: Longitudinal cohort study of mountaineers from sea level to 6400m

Setting: Mount Everest (North side)

Participants: 13 mountaineers (10 male, aged 23-52 years) on a British expedition to climb Mount Everest

Interventions: ONSD was measured ultrasonically, 3mm behind the globe using B-scans recorded with an OTI-Scan 3D scanner (Ophthalmic Technologies, Canada). Serial binocular scans were recorded at sea level, 2000m, 3700m, 5200m and 6400m. All ONSDs were measured by a masked observer.

Main Outcome Measures: ONSD, AMS score (using the Lake Louise scoring system), heart rate, and oxygen saturations.

Results: All results were analysed by regression analysis with adjustment. ONSD was positively associated with increasing altitude above sea level (0.1mm increase in ONSD per 1000m, P <0.001) and AMS score (0.12mm per score, 95% CI 0.06-0.18 mm). Further associations were found between resting heart rate (0.29mm per 20 bpm, 95% CI 0.17 to 0.41 mm) and oxygen saturations (0.20mm 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.

Key Words: Optic nerve sheath diameter, acute mountain sickness, altitude, intracranial haemodynamics, intracranial pressure

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