The phrase “reverse altitude sickness” describes feeling unwell after returning to low elevations from a high-altitude environment. While not an official medical diagnosis, the symptoms are recognized by health professionals as High-Altitude De-Acclimatization Syndrome (HADAS). This post-descent illness is a physiological response to the reversal of the body’s adaptations to a low-oxygen environment.
The Body’s Response to High Altitude
When an individual spends time at high altitude, where oxygen pressure is lower, the body initiates acclimatization to ensure tissues receive enough oxygen. A primary adjustment is the hypoxic ventilatory response, which increases the depth and rate of breathing. This sustained hyperventilation helps raise the oxygen concentration in the blood.
The kidneys also adjust blood chemistry by increasing the excretion of bicarbonate to make the blood less alkaline. Over a longer period, hematological adaptations occur, including an increase in the hormone erythropoietin, which stimulates the bone marrow to create more red blood cells. These changes collectively improve the blood’s oxygen-carrying capacity.
Understanding Post-Descent Illness
HADAS manifests when the body struggles to switch off the acclimatization processes established at elevation. The symptoms often mirror those of acute mountain sickness but occur following the opposite environmental change. Individuals typically begin to feel unwell within 12 to 72 hours after arriving at a significantly lower altitude.
The common presentation includes persistent headaches, profound fatigue, and general malaise. Sleep disturbances are frequently reported, involving insomnia or, conversely, excessive sleepiness. Other symptoms can include mild dizziness, nausea, difficulty concentrating, and a feeling of fogginess.
These symptoms indicate the body’s regulatory systems are adjusting to the sudden return to a normal oxygen environment. The severity of the illness is often proportional to the height and duration of the previous altitude exposure, as well as the speed of the descent. The syndrome is not life-threatening like severe forms of altitude sickness, but it significantly impairs recovery and daily function.
The Physiology of De-Acclimatization
The underlying cause of post-descent symptoms is the failure of the body’s systems to immediately reverse high-altitude adaptations. The respiratory control center in the brainstem maintains an exaggerated ventilatory drive, having become highly sensitive to low oxygen and high carbon dioxide (CO2) levels. This continued hyperventilation at sea level causes the body to expel too much CO2, leading to a temporary chemical imbalance.
This lower-than-normal CO2 level, or hypocapnia, causes blood vessels in the brain to constrict, contributing to persistent headaches and lightheadedness. Furthermore, long-term acclimatization increases red blood cell mass, which thickens the blood. The sudden return to higher oxygen saturation can cause a temporary “oxygen shock,” overwhelming the body’s regulatory systems.
The rapid change in atmospheric pressure also plays a role in the discomfort. Pressure changes affect fluid dynamics, which, combined with shifts in blood volume and viscosity, contribute to the general feeling of being unwell. The physiological mechanisms that were a survival advantage at high altitude become a temporary liability at sea level.
Managing Symptoms After Descent
Managing the symptoms of post-descent illness focuses on supporting the body through the final stages of re-acclimatization. The most effective initial strategy is to prioritize rest and minimize physical or mental exertion for the first few days upon returning to lower elevation. Avoiding strenuous activity allows the respiratory and circulatory systems to gradually normalize.
Hydration is also important, as the high-altitude environment often causes dehydration, and maintaining fluid balance can help the body adjust to the changes in blood chemistry. For persistent headaches, over-the-counter non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, are often effective in reducing discomfort.
If symptoms are severe, include significant mental confusion, or persist for more than a week, it warrants a consultation with a healthcare professional. While HADAS is usually self-limiting, prolonged or intense symptoms may require medical evaluation to rule out other potential causes or complications. Allowing for a gradual return to normal activity and supporting basic bodily functions facilitates a faster and smoother recovery.