subject: Hypoxic-Ischemic Encephalopathy [print this page] Hypoxic-Ischemic Encephalopathy Hypoxic-Ischemic Encephalopathy
Hypoxic-Ischemic Encephalopathy is a broad medical term referring to the dysfunction in the brain caused by hypoxic or ischemic injury. Hypoxia refers to the denial of oxygen while ischemia refers to the denial of blood. Ischemia would include oxygen deprivation as well as the deprivation of glucose and nutrients that blood provides. Put another way, the brain needs constant support in order to stay alive. When brain cells are denied essential support from oxygen, fuel or other nutrients, parts of the brain will die, causing changes to the brain's ability.
The Importance of Blood
Blood provides the brain with oxygen, energy and essential supplies. This is what the cells of the body need to work. Oxygen is particularly important because of the speed at which the body uses it and the speed at which it needs to be replenished as it is used. If the cells are denied oxygen or fuel long enough, they will die. When this happens in the brain, it is called HIE.
After you are born, the blood gets oxygen from the lungs, goes back to the heart to be pumped all over your body, including your brain. Cells need glucose and fuel to burn too. You get this from the food you eat. Babies in the womb don't eat or breathe, so before a baby is born, oxygen and essential nutrients are passed from the mother, through the placenta, through the umbilical cord on the way to the baby's cells.
Hypoxic-Ischemic Encephalopathy occurs when the brain is denied these essential deliveries from the blood for any reason. It could be caused by a lack of oxygen within the system or decreased blood flow to the brain. Babies are unique in the sense that they depend on another person's body to deliver the life-sustaining oxygen and fuel to them. When something goes wrong with this system, the baby is at risk immediately.
How the body deals with a lack of oxygen
Your brain monitors your blood like a computer. A lack of oxygen issues an alert to the brain, triggering defense mechanisms. If there is less oxygen in the same amount of blood, the brain needs to make up for that by getting more blood in less time. So the brain will increase cerebral blood flow (CBF). In other words, the brain will demand more blood for itself. The heart will beat faster, directing more blood to the brain, heart and essential organs. The brain releases signals to tell the body to increase blood pressure to get things going in this emergency.
Adults experiencing this sequence will do a lot better than a newborn baby can. This is because we have the ability to self-regulate our brain's blood flow even if our blood pressure continues to rise. There is a certain range of blood pressure that we can deal with automatically, but when blood pressure goes outside of these limits, we lose our ability to deal with the situation. Blood pressure cannot be increased forever and the brain needs to figure out another plan. In newborns, there is a smaller blood pressure window before they lose their auto-regulation. While it is unknown exactly what pressure becomes dangerous in a newborn baby, the blood pressure limit is lower than it is in an adult.
As blood pressure rises beyond this tipping point, the brain gives up on monitoring its own blood flow. Blood pressure drops and blood flow to the brain decreases. The brain isn't getting what it needs to survive from the blood. It goes into survival mode, decreases its temperature, and tries to make it through the deprivation. This is where the damage to the brain begins to take place. Cells begin to die, a process known as necrosis.
After oxygen returns
When the oxygen or fuel supply is restored within the system, the problem isn't over. In fact, a lot of the damage is just beginning. The brain and the body have been through hell and there is no quick fix. "Reperfusion" injuries occur as circulation returns. The injured cells need repair and the dead cells need to go. White blood cells are like first-aid responders within the blood. The injured tissue calls for help and the body's immune system causes inflammation. This inflammation combines with a new flow of oxygen that results in further damage to the cells. This extra damage triggers more inflammation which leads to further damage, and so on. The damage can even lead to blood flow blockages that can cause more ischemia, even though the initial suffocating event is long over.
Another process, apoptosis, results in further systematic cell death over an extended period of time after the initial event. In the brain, this recovery process is followed by another failure in the cells, a "delayed phase of neuronal injury".
