High altitude headache. Lessons from headaches at sea level.

There is little known about high altitude headache, except that it is an important and serious problem that often heralds the onset of acute mountain sickness. We do know that the brain itself is an insensate organ except for its meninges which contain sensory axons projecting from the trigeminal nerve. These nerve fibers travel in proximity to meningeal blood vessels and constitute an important component of the trigeminovascular system. Signals generated at high altitude which may activate the trigeminovascular system can arise from brain, blood or the blood vessel wall, include protons, neurotransmitters and other potential noxious agents which can discharge or sensitize small unmyelinated fibers. Brain edema and raised intracranial pressure may cause headache by compressing brain structures leading to displacement and stretching of the pain-sensitive intracranial structures. Small hemorrhage may irritate and discharge these fibers chemically. Furthermore, high altitude seems capable of decreasing the threshold of response to sensory stimulation. Therefore, headache can be attributed to activation of a common pathway, the trigeminovascular system by both chemical or mechanical stimulation.