Functional involvement of central nervous system at high altitude

Acute mountain sickness is a common discomfort experienced by unacclimatized persons on ascent to high altitude. We tested the hypothesis that exposure to high altitude affects cortical excitability using transcranial magnetic stimulation. We specifically analyzed the motor cortex excitability in normal subjects at high altitude and in a control condition near sea level. Mean resting motor threshold (RMT) was significantly higher at high altitude than at sea level (69.3 ± 10.4 versus 56.3 ± 10.9%; P = 0.042). Mean short intracortical inhibition (SICI) was significantly lower at high altitude than at sea level (percentage of test motor-evoked potential = 79.3 ± 19.8 versus 28.7 ± 17.5%; P = 0.0004). Symptoms of acute mountain sickness correlated with resting motor threshold changes induced by high altitude (R ² = 0.53, P = 0.037). SaO₂ correlated with SICI changes induced by high altitude (R ² = 0.45, P = 0.036). We suggest that high altitude deeply changes cortical excitability by affecting both inhibitory and excitatory circuits and that this is reflected in acute mountain sickness symptoms.