Abstract
Background:
Third-generation Tibetan descendants in the eastern Himalayas are presumed to possess genetic adaptations that confer resistance to high-altitude hypoxia. However, the impact of rapid ascent on this population particularly among native highlanders, remains poorly documented. Anecdotal evidence and observational data indicate that highlanders ascending rapidly (e.g., >500 m/day) may experience lower incidence and milder forms of these conditions than lowlanders.

Aim:
This study aimed to evaluate physiological effects and incidence of high-altitude illness (HAI) in eastern Himalayan highlanders during rapid ascent to altitudes above 14,300 feet.

Methods:
A cross-sectional observational study was conducted in May 2020 at a government hospital in the Eastern Himalayas. Thirty-three male combatants of eastern Himalayan origin, who had resided at lower altitudes for over two years, were assessed using the LLSQ (Lake Louis Scoring Questionnaire) following acute high-altitude exposure.

Results:
The prevalence of AMS (acute mountain sickness) was 27.3% (n=9), with equal distribution across mild, moderate, and severe categories (n=3 each). High-altitude pulmonary edema (HAPE) and cerebral edema (HACE) each occurred in 9.1% (n=3) and 3 % (n=1) of patients respectively. Headache was the most frequent symptom (36.4%), whereas ataxia was the least (3.0%). Lower oxygen saturation (SpO₂ ≤85%) was significantly associated with AMS (χ² = 15.91, p < 0.001). No fatalities were reported.

Conclusion:
Third-generation Tibetan highlanders remain vulnerable to AMS, HAPE, and HACE during rapid ascent despite presumed genetic tolerance. Pharmacological prophylaxis and pulse oximetry monitoring are recommended in operational settings to mitigate risk and maintain functional capacity. This study represents the first systematic assessment of HAI incidence in Eastern Himalayan highlanders and highlights the need for high-altitude, tailored preventive strategies in primary healthcare.

Significance:
This study holds multidimensional clinical, operational, and scientific importance, particularly in the context of high-altitude medicine and indigenous populations. It debunks the myth of invulnerability in Tibetan highlanders, validates pulse oximetry as a frontline tool and drives evidence-based policy for the safety of thousands of native highlanders in military, tourism, and labour sectors at extreme altitudes.

Key words: High altitude, LLSQ, Tibetans, Highlanders Altitude Sickness, Acute Mountain Sickness, High-Altitude Pulmonary edema, High-Altitude Cerebral edema, Hypoxia, Acclimatization.