Abstract
Optic Nerve Sheath Diameter Correlates With The Presence and Severity of Acute Mountain Sickness: Evidence For Increased Intracranial Pressure
(J Appl Physiol. 2009;106:1207–1211) PJ Fagenholz, JA Gutman, AF Murray, et al
Prepared by Anil Menon, MD, Stanford Wilderness Medicine Fellow, Stanford, CA, USA
It has been postulated that elevated intracranial pressure (ICP) causes acute mountain sickness (AMS). Elevated ICP might result from hypoxia induced cerebral vasodilation or edema. When attempting to correlate AMS with ICP, previous studies have not been able to adequately measure ICP or have been limited by sample size. This cross-sectional study sampled a larger population of climbers while using ultrasonagraphy to measure ICP.
Study participants were solicited from climbers at Pheriche, Nepal (4240 m) and 287 were enrolled. The participants did not climb higher than Pheriche for the 2 weeks preceding participation. One physician examined the patients and assessed AMS with the Lake Louise Score (LLS). A headache and LLS greater then 3 indicated AMS, and occurred in 69 patients (24%).
A second, blinded physician performed the ultrasonagraphy. This physician used a high frequency probe (7-10 MHz) to save 3 cross-sectional images of the optic nerve sheath from each eye in a supine patient whose eyes were closed. Sheath diameter was not calculated in real-time but images were saved and measured by a third observer at a later time. All 6 images were averaged at a point 3 millimeters posterior to the globe. In an effort to reduce interobserver differences, an additional blinded observer measured 40 randomly selected patients.
Optic nerve sheath diameter (ONSD) was correlated with the occurrence and severity of AMS (OR 6.32 P < 0.001) as assessed by the LLS. The authors suggest that ONSD might serve as a clinical and research tool for assessing AMS. A major drawback to this study was the lack of baseline low altitude ONSD measurements in the subjects.