Optic Nerve Sheath Diameter Changes at Altitude

AbstractPurpose: This study aimed to assess variations in Optic Nerve Sheath Diameter (ONSD) as a correlate of Intracranial Pressure (ICP) during ascent to high‐altitude and to explore the relationship between ONSD and Acute Mountain Sickness (AMS). AMS is a high‐altitude illness characterized by headaches, lethargy, nausea and vomiting. At present there is no consensus on the pathophysiology of AMS. There are similarities in symptomology between AMS and raised ICP. A greater understanding of ICP at altitude and its role in AMS may yield new strategies for management of the illness.Methods: We conducted a longitudinal cohort study involving 10 mountaineers climbing Mt Toubkal (4167 m). Participants underwent daily binocular ultrasound B‐scans at interval altitudes during the ascent. Altitude, AMS diagnosis and severity (using Lake Louise Score [LLS]) were recorded concurrently with ultrasound examinations. Ultrasound scans were reviewed by a blinded observer who measured ONSD and assessed image quality (1 = poor, 2 = moderate, 3 = good).Results: During the ascent to 4167 m and return to low‐land, there was no significant variation in left (p = 0.97), right (p = 0.68) or mean (p = 0.86) ONSD. The ratio of ONSD to baseline was significantly (p = 0.038) greater in participants with AMS on Day 5; this was not consistent across all time points. No significant correlation between the ratio of ONSD to baseline and LLS was observed at any time point. Mean image quality was 2.12 (SD ± 0.65); images largely scored in the ‘Moderate’ – ‘Good’ range.Conclusions: This study is reflective of changes in ONSD during slow ascent profiles commonly used in mountaineering. This suggests that ONSD and therefore ICP does not significantly vary during this style of ascent to 4167 m. We did not identify any convincing relationship between AMS and ONSD, adding to previous research suggesting the same. Considering the limitations of this study, it cannot stand alone to disapprove a causal relationship between ONSD and AMS.