Background/aimsTo assess changes in optic nerve sheath diameter (ONSD) at high altitude and in acute mountain sickness (AMS).MethodsCochrane Library, EMBASE, Google Scholar and PubMed were searched for articles published from their inception to 31st of July 2020. Outcome measures were mean changes of ONSD at high altitude and difference in ONSD change between subjects with and without AMS. Meta-regressions were conducted to investigate the relation of ONSD change to altitude and time spent at that altitude.ResultsEight studies with 248 participants comparing ONSD from sea level to high altitude, and five studies with 454 participants comparing subjects with or without AMS, were included. ONSD increased by 0.14 mm per 1000 m after adjustment for time (95% CI: 0.10 to 0.18; p<0.01). Restricted cubic spline regression revealed an almost linear relation between ONSD change and time within 2 days. ONSD was greater in subjects with AMS (mean difference=0.47; 95% CI: 0.14 to 0.80; p=0.01; I2=89.4%).ConclusionOur analysis shows that ONSD changes correlate with altitude and tend to increase in subjects with AMS. Small study number and high heterogeneity are the limitations of our study. Further large prospective studies are required to verify our findings.

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A 36-year-old woman at 37 weeks' gestation presented to the emergency department with sudden onset of massive vaginal bleeding and abdominal pain just before the visit. She had a history of oligohydramnios in antepartum examination and did not have coagulopathy or history of placental abruption. On physical examination, she was conscious and afebrile, with a pulse rate of 76 beats/min and blood pressure of 87/52 mm Hg. Point-of-care ultrasonography of the abdomen revealed fetal distress, with a fetal heartbeat of 197 beats/min, and others were performed (Figure and Video E1, available online at http://www.annemergmed.com).For the diagnosis and teaching points, see page 137. To view the entire collection of Images in Emergency Medicine, visit www.annemergmed.com Figure. Point-of-care ultrasonography of the abdomen, demonstrating one large hyperechoic mass (asterisk) between the placenta (short arrow) and the uterus (arrowhead), which presented as a typical abruption. The fetus is marked with the long arrow.