FOCAL BRAIN DAMAGE  [ back to Glossary Index ]
Focal Brain Damage is caused by a bruise ("contusion") to a specific portion of the cortex (the gray matter at the surface of the brain where the the cell bodies of nerve cells are packed) or by a space occupying hematoma (blood clot). The same person can have both types of focal damage from trauma. Focal damage tends to disrupt specific brain functions (like seeing, speaking or walking) rather than to affect all major brain functions. Contusions typically result from the fragile brain slamming against the hard inside of the skull or by "gliding" over and catching against the bony ridges in the skull. These ridges, which exist to hold and support the brain, were not designed to protect it from trauma. In fact, during a high speed impact to the head, these bony ridges are like knives which cut the brain.

Hematomas can form inside or outside the membranous coverings of the brain. The brain is covered with three layers known as meninges. The pia matter is a thin, translucent covering which adheres directly to the brain. The arachnoid is a spidery net of tissue over the pia. The dura matter (Latin for "tough mother") is a tough, leathery membrane over the subarachnoid which adheres to the skull on its top side. A blow to the head which ruptures the middle meningeal artery will cause an epidural hematoma, usually in a spherical, golf ball shape, which expands quickly between the dura and skull (the "extradural area"). The victim will be initially conscious, then suddenly within an hour or so will show fixed, dilated pupils and drop unconscious to the ground from crushing pressure on his brain. A blow to the head which ruptures one of the bridging veins (which lead from the cortex to the dura) will leak blood into the space between the dura and subarachnoid and cause an acute subdural hematoma (ASDH). ASDH can also result from severe contusion to the fronto-temporal lobes which then bleed below the dura. On CT scan ASDH appears in an eliptical, boomerang shape because of how the liquid blood fans out. An ASDH is classified as an "intradural" clot.

If large enough, epidural or subdural hematomas can kill if not surgically removed in time by cutting off arterial blood flow, herniating the brain through the hole for the spinal cord or damaging the brain stem structures. If these are removed quickly and there is minimal diffuse damage, a person who was rendered unconscious, even for a period of days, can make a remarkable recovery. The third type of clot, which occurs with a high speed impact the head, is known as an "intracerebral hematoma," which is highly serious, because it involves bleeding deep inside the brain in the white matter area such as the corpus callosum or in islands of gray matter like the basal ganglia which control smooth movement. Blood is toxic to the brain. Once active bleeding has stopped, scavenger cells in the brain called hemosiderian macrophages will start ingesting the clotted blood like little pacmen, and eventually the blood will disappear - something called "resolution of the hematoma." CT scan will pick up only active bleeding. If done too early (before the bleeding starts) or too late (after it stops) it will be negative. MRI can pick up areas of damage where bleeding occurred earlier, where the brain has developed scar tissue or formed a cyst filled with cerebro-spinal fluid. However, both types of scans will only show the structure of the brain not disturbance of function.