Have you been extremely tired after a week of arduous work, perhaps including several night shifts, when you would rather spend a lazy Sunday morning in bed? I am talking about one of those days when you take a 15-minute nap after brunch that turns into a nice 5-hour long afternoon sleep. And, when you wake up to find the dusk sun painting the sky rose-gold, you may have experienced a tingling sensation in the forehead or blurry vision. It is not a headache yet, but enough to warrant taking a pain killer pill with a tall glass of water. While experts debate on whether oversleeping can trigger migraines, one thing is clear: the odd tingling skin sensation, or odd visual disturbances, or auditory sensitivity can foreshadow the upcoming headache. One of the hypotheses explaining these strange feelings is that it is a bodily representation of an unusual spreading wave of reduced brain activity, called the spreading depolarization (SD). SDs may reach certain parts of the brain cortex responsible for bodily sensations and alter their function, resulting in a slightly modified perception. SDs are caused by unusual fluctuations in brain oxygen consumption and blood supply. The life events that trigger SDs range extremely from benign to severe. Whether it is for a benign reason, such as an afternoon sleep, or it is for a stressful flight, or it is for a serious injury following stroke or a traumatic accident, SDs can occur spontaneously in different scenarios.
Interestingly, SDs are very common. They have been documented in the majority (if not all) mammals and many invertebrates. Some bring forward the hypotheses that SDs may serve an evolutionary role of protecting the brain from excessive activity.
For example, SDs are known to occur in migraineurs, manifesting themselves as the migraine “aura,” a phenomenon that causes a strange tingling in the skin of patients, or visual tinting or slight sound distortion. In this case, some scientists hypothesize that the SDs could occur pre-migraine as a compensatory attempt of the brain to shut itself down and preserve energy, or to signal to its body that it needs rest and sleep to avoid the migraines caused by the excessive exposure to environmental stimuli. That might be the case as migraines, while debilitating themselves, once gone, do not seem to cause any apparent lasting injury in the brain with many patients being able to continue with their lives after some rest and taking pain killers. As such, “auras” may serve as an indicator that helps to take measures required to prevent migraines. The problem arises when migraines occur without any apparent external reason, too frequently and start to interfere with daily functioning. In such cases, additional clinical intervention is often required to target headaches.
On the other hand, research in animal models and complex study techniques in severely brain injured patients show that SDs can trigger release of noxious substances that further damage the brain, killing the cells and leading to an unfavorable outcome. It is also possible that a distinction should be made between SDs in otherwise healthy tissue and brain tissue at risk due to a recently sustained injury or underlying medical conditions, such as atherosclerosis, diabetes, oxygen deprivation, etc. In our lab we strive to understand the line that separates the two and find out what exactly makes the SDs so noxious in some patients.
One explanation as per why some SDs cause “aura,” while others underly additional brain damage is whether the vasculature can supply enough oxygen for the brain to return itself to the normal activity state once SD has occurred. It is possible that in critical conditions, following brain trauma for example, the demand of the tissue for oxygen by far exceeds its supply, resulting in additional injury. Interestingly, pure oxygen treatment has been found useful for critical brain conditions and in management of severe migraines. The question that still remains is what the purpose of an “aura” following an afternoon sleep is?
