Blood clotting can occur from an increase in hemoglobin, which has caused blood to thicken.
This means that an increase in blood pressure can cause fluid to leak from the blood vessels and into the brain, causing it to swell. Common symptoms include vomiting, headaches, continual exhaustion and a shortness of breath. AMS can begin around 8,000 feet. This has occurred as low as 10,000 ft., well below basecamp. HACE is the body's response to a decrease in oxygen levels at high altitudes.