Optic nerve sheath diameter at high altitude: standardized measures in healthy volunteers.

Background: Increases in the diameter of the optic nerve sheath (ONSD) on ultrasound are associated with high intracranial pressure (hICP). The normal value varies with altitude and the population studied. The objective of this study is to describe the normal values of the ONSD in a healthy adult population of the city of Bogotá, Colombia, at 2640 meters above sea level (masl).

Patients and methods: A prospective observational study was conducted on a total of 247 healthy individuals recruited from May 2021 to May 2022 who were subjected to the color, low power, optic disk, safety, elevated frequency, dual (CLOSED) protocol for measuring the bilateral ONSD adjusted to the eyeball transverse diameter (ETD).

Results: A total of 230 individuals were analyzed; the average ONSD of the right eye (RE) was 0.449 cm (range 0.288-0.7) and that of the left eye (LE) was 0.454 cm (range 0.285-0.698); the correlation between RE and LE was 0.93 (p < 0.005), and the correlation of the ONSD/ETD ratios for the RE and LE was lower (r² = 0.79, p < 0.005). A total of 10.8% of the studied population had values greater than 0.55 cm.

Conclusions: The median ONSD and ONSD/ETD ratio in the city of Bogotá are similar to those described in other populations; however, approximately 10.8% of the healthy population may present higher values, which would limit the use of ONSD on its own for clinical decision-making, only repeated measurements with significant changes in the ONSD and ONSD/ETD or asymmetries between the measurements of both eyes linked to clinical findings would allow the diagnosis of hICP.