Ischemia Can Cause Secondary Brain Damage
To understand the role of an increase in intracranial pressure after head trauma, it is also important to understand its related condition of ischemia. Brain ischemia is the reduction of blood to flow to brain tissue. Another related term is hypoxia, insufficient oxygen supply to brain will typically accompany ischemia, but can also happen if the blood oxygen levels are low. While ischemia is typically associated with stroke, when there has been a blood clot cutting off flow to the brain, in trauma, it is principle cause of complications in severe brain injury.
What happens is that the skull has a certain capacity to contain fluids, like a pressure cooker. The normal brain would be like the cooker, before the lid is fastened down and the heat is applied. But unlike a pot, the normal head has a equilibrium of flow of fluids, principally blood, in and out of it. After trauma, some neuropathology may occur that reduces the flow of fluids out of the head, trapping more and more fluid inside the skull. As the skull has no more capacity to expand than the pressure cooker, the result is an increase in pressure inside the brain. The brain’s internal blood pressure is intracranial pressure (meaning within the “cranium” – the skull). ICP is the abbreviation that is used far more than the actual term.
When there has been an increase in ICP, the flow to some blood vessels will be restricted. The smaller the blood vessel, the more the flow becomes restricted as ICP goes up. The capillaries, the smallest blood vessels in the brain, are the vessels which are responsible for carrying the oxygen and glucose (the brain’s fuel) to the brain cells, where the microscopic fire created as these oxidize, fuels the brain.
More importantly after a brain injury, the capillaries are also responsible for removing the fumes from this microscopic fire. Even before brain cells begin to die from lack of oxygen, they will die from failure to circulate the toxic fumes away from the cell.
Primary versus Secondary Brain Damage – Intracranial Pressure
Keep in mind the distinction between primary and secondary brain injury. The primary injury is what happens as the direct result of the blow/mechanical forces applied to both the skull and the brain. Examples of a primary injury would be a brain bleed or bruise. The secondary injury is the events which occur over time which as a result of the pathology that stems from the primary injury. For example, as a blood vessel bleeds, it creates an increase in ICP, resulting in a reduced blood flow into and away from cells, causing further damage.
Other Causes of Secondary Brain Injury
Increased ICP is not the only cause of secondary damage. In addition, it can be very harmful if the brain temperature increases (like in a fever). In fact, lowering the body temperature at times has been shown to reduce brain damage, such as in near drowning situations. It is critical that heart rate and overall blood pressure is maintained at normal levels. It is also important to make sure that the breathing apparatus and lungs are working properly to keep the proper oxygenation of the blood. At times, super saturation of the blood with oxygen, such as in hyperbaric oxygen chambers, may prove beneficial.