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

OBJECTIVE: To investigate the association of optic nerve sheath diameter (ONSD), as a correlate of intracranial pressure (ICP), with acute mountain sickness (AMS).
DESIGN: Longitudinal cohort study of mountaineers from sea level to 6400 m.
SETTING: Mount Everest (North side). PARTICIPANTS: 13 mountaineers (10 men, 3 women; aged 23-52 years) on a British expedition to climb Mount Everest.
INTERVENTIONS: 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. MAIN OUTCOME MEASURES: 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.