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.
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