A total of 109 subjects (all male; mean age: 19.6 years, SD: 1.7 years) lived at 50 m and traveled to Tibet by plane were recruited to our observational cohort study. To participate in the study, subjects had to be healthy Han Chinese men between the age of 18 and 35 years, and no high-altitude exposure in recent 2 years. All of the subjects completed a self-reported questionnaire (structured case report forms, CRFs) to report their disease status and medical history. We did not include participants who had suffered from cardio-cerebrovascular, respiratory, ophthalmic, or migraine diseases, or had had a headache or cold, or were taking any medications during ascent to high altitude. To motivate and recruit subjects, the purpose of the study was explained in detail to all of the subjects who volunteered for participation, and all of the participants signed an informed consent before the examinations. The Human Ethics Committee of Fudan University approved the protocol.

All of the participants underwent baseline examination 1 week before departure and within 24 h after their arrival at 3,700 m. Demographic (age, body mass index (BMI), smoking and drinking history), physiological [heart rate at rest (HR, beats/min), oxygen saturation (SpO₂, %)], and hematological data [hemoglobin level (Hb, g/L) and red blood cell count (RBC, *10⁹)] were obtained. To measure the severity of HAH and AMS, the Lake Louise Scoring (LLS) system for acclimatization grading was used. Using the self-reported questionnaire, Lake Louise points were assigned on a 0 to 3 scale for headache, gastrointestinal symptoms, fatigue, and dizziness. All subjects with a headache and LLS ≥3 were considered to have AMS (Roach et al., 2018).

All optical coherence tomography (OCT) scans were acquired by the same operator using the Cirrus OCT device (Cirrus 5,000, Carl Zeiss Meditec Inc. Dublin, California, United States; software version 6.5.0772). The Cirrus HD-OCT 5,000 has an A-scan velocity of 27,000 scans/second with a 5 μm axial resolution and a scanning depth of 2 mm. The instrument uses light of 840 nm wavelength, and images of the optic disc and macula. The optic disc scan was centered on the optic disc (optic disc tube 200 × 200 protocol), while macular scan was centered on the fovea (macular cube 512 × 128 protocol). To eliminate binocular confounding factors, we only included the left eye. Only high-quality images (signal strength ≥6) of the left eye in each participant were taken. The scans were used to measure RNFL thickness and macular thickness. RNFL thickness was automatically calculated by the fast RNFL procedure. The software allows the mapping of thickness data on a quadrant-by-quadrant basis. We considered the average values of three different measurements per quadrant (superior, inferior, nasal, and temporal), and the overall data obtained in all of the quadrants were used to determine the average RNFL thickness (Figure 1). OCT software calculates macular retinal thickness as the distance between the first signal from the inner limiting membrane and the signal from the anterior boundary of the retinal pigment epithelium. The map of the early treatment diabetic retinopathy study (ETDRS) grid is composed of nine sectorial thickness measurements in three concentric circles with diameters of 1 mm, 3 mm, and 6 mm. The area bounded by the outer (6 mm) and middle (3 mm) circles forms the outer ring, and the area bounded by the middle (3 mm) and inner circles (1 mm) forms the inner ring. The central 1-mm circular region represents the foveal area (Figure 2).

Continuous data were presented as the means ± standard deviations (SDs), and their normality was assessed by the Shapiro–Wilk test. Categorical variables were presented as percentages. Differences between baseline and high altitude were analyzed using a paired-sample Student’s t test for Gaussian distribution. When Gaussian distribution was not satisfied for continuous variables, the nonparametric Mann–Whitney U test was used to compare the baseline and high-altitude measurements. To evaluate possible correlations between changes at ONH, HAH, or AMS parameters, Spearman correlations were used. The significance level (two-tailed) was set at 0.05.

All of the 109 participants completed the self-reported questionnaire, and systemic and hematological measurements at sea level and at high altitude were obtained. The incidence rates of AMS and HAH following acute exposure to high altitude were 44.0 and 67.0%, respectively. AMS scores within 24 h after arriving at high altitude ranged from Lake Louise scores of 0–10, with an average of 2.4. As for the physiological measurements, SpO₂ decreased from 98.3 to 83.0% (p < 0.001), while HR significantly increased from sea level to high altitude. In terms of the hematological measurements, Hb and RBC showed a significant increase (p < 0.001; Table 1).

In this study, we investigated the retina of the subjects before and after ascent to high altitude. As shown in Table 2, there were evident changes in the thickness of the RNFL and macular thickness in the participants after their ascent to high altitude. In the optic disc, there was also a significant increase in the thickness of the RNFL in the superior and inferior quadrants and mean RNFL (p < 0.05), with an insignificant increase in RNFL thickness in the nasal and temporal quadrants (p > 0.05). There was a significant increase in macular thickness in the outer superior and outer nasal quadrants (p < 0.05). However, the central subfield thickness exhibited no significant decrease (p < 0.05). Changes in thickness of the RNFL in the peripapillary sectors were evident following ascent to high altitude. No significant differences were observed between HAH and non-HAH subjects (Figure 3).

Spearman correlation analyses were used to explore the relationships between the optic disc measurements and HAH severity. The correlation analysis revealed a significant correlation between the optic disc measurements and HAH severity. HAH severity significantly correlated with the ratio of mean thickness (r = 0.246, p = 0.01 in Figure 4A), inferior thickness (r = 0.216, p = 0.02 in Figure 4C), and nasal thickness (r = 0.193, p = 0.04 in Figure 4D). The ratios of superior and temporal RNFL thickness did not display a significant correlation with HAH severity (p > 0.05 in Figures 4B,E). However, the ratios of mean RNFL thickness did not significantly correlate with AMS (Figure 4F), LLS scores (Figure 4G), gastrointestinal symptoms, fatigue, dizziness, hemoglobin, or RBC (Supplementary Figure S1).

This study was undertaken to objectively quantify structural changes in the optic disc and macula during acute, high-altitude exposure to low atmospheric oxygen, low humidity, and strong ultraviolet radiation. In this study, OCT was used to determine alterations in the retinal morphology, including macular thickness and the RNFL. Evaluating changes in the optic disc that are highly related to the structure of the brain can provide a better understanding of the pathophysiology of HAH.

According to the findings of the RNFL parameters of the optic disc during exposure to high altitude, young Chinese males enrolled in this study had a thicker RNFL in the superior and inferior areas and a thinner RNFL in the nasal and temporal areas, which is consistent with histological changes of the RNFL reported in previous studies (Hsu and Tsai, 2008; Ascaso et al., 2012). A week before the subjects entered Tibet, the RNFL thickness in each quadrant of the optic disc exhibited different degrees of thickening after rapid ascent to high altitude, especially in the superior and inferior quadrants, which is in agreement with Tian et al. (2018). However, we did not assess the effects of long-term exposure to high altitude and subsequent return to low altitude using OCT. In the studies by Tian (Tian et al., 2018) and Willmann (Willmann et al., 2011), the optic nerve did not show permanent damage after short-term exposure, which may indicate that HAR is a benign high-altitude illness.

Our results indicated that the thickness of the macula notably increased, and the outer superior and outer nasal zones were much thicker than at baseline. A study on macular thickness conducted by Fischer et al. quantified macular structure in 14 healthy subjects before and after ascent to high altitude; the authors found a minor increase in total retinal thickness (Fischer et al., 2012). Tian et al. adopted OCT to scan the retinal structure of 91 healthy subjects after 1-month exposure to high altitude (4,600 m above the sea level), and their results indicated a significant increase in RNFL thickness in the superior and inferior zones (Tian et al., 2018). However, previous OCT studies on retinal changes associated with high-altitude exposure and AMS showed no significant alteration in any of the ETDRS subfields (Ascaso et al., 2012; Fischer et al., 2012). Our present findings indicated that acute exposure to high altitude did not result in macular edema, but it did increase the perimacular thickness of ETDRS. This indicates that the macular region has a superior self-regulation potential to meet the demands of oxygenation upon acute exposure to high altitude in healthy subjects.

Consistent with the results of other studies (Bian et al., 2013; Shin, 2014), the incidence of HAH, a core symptom of AMS, was 67.0% in our participants. To objectively and quantitatively analyze correlation between parameters of RNFL at ONH and HAH, AMS, or LLS, we monitored changes in the optic disc that were acquired within 24 h of high-altitude exposure, as LLS scores attain their peak values at the same time. According to the proposed model (Figure 5), exposure to high altitude can significantly reduce blood oxygen saturation, thereby inducing hypoxemia (Abdolrahimzadeh et al., 2017). By further stimulating hypoxia through the activation of the corresponding signaling pathway, hypoxemia can result in a significant increase in cerebral blood flow and blood volume, thereby inducing brain swelling and increased ICP (Hackett and Roach, 2001; Rosenblum, 2007; Schoonman et al., 2008; Bailey et al., 2009). The increase in ICP can lead to HAH and elevate the trans-lamina pressure difference (Jonas et al., 2013; Hou et al., 2016). This would directly obstruct axoplasmic flow at ONH (Tso and Hayreh, 1977), and an obstruction to axoplasmic transport could increase RNFL and cause papilledema.

In a large sample, we confirmed that the ratio in the mean nerve fiber layer thickness did not significantly correlate with AMS or LLS scores, which is consistent with the results of previous studies (Willmann et al., 2011; Fischer et al., 2012). Some other studies showed significant positive correlations between AMS and optic disc swelling (Bosch et al., 2008), optic nerve sheath diameter (Bosch et al., 2010), corneal thickness (Bosch et al., 2009), and retinal capillary blood flow (Sutherland et al., 2008). Our study was the first to explore the relationship between changes in RNFL thickness of the peripapillary sectors and HAH; specifically, it illustrated a consistent and robust association between them.

Several limitations to our study should be mentioned. First, the association between alterations of RNFL and HAH was described, but no causal relationship could be determined because of the observational nature of the study. Therefore, longitudinal studies should be conducted to validate our findings in the future. Second, the participants in our study were all young male individuals, which decreases the generalizability of the results. More subjects distributed in different age groups will be recruited in a future study. Third, a longitudinal follow-up study to observe the dynamic changes of the retina at multiple time points and altitudes should be conducted in the future. More parameters of the eyes should be considered, including intraocular pressure (IOP), optic nerve sheath diameter (ONSD), optical coherence tomography angiography (OCTA), and fundus images. Finally, the onset of headache in participants was subjective and not precise. Studies on biomarkers with high sensitivity and specificity for HAH may be desirable in the future investigations.