How can doctors tell extent of brain damage?

There are three cardinal neuropathologic dimensions that are considered in the diagnosis. They are the a) distribution b) severity and c) types of pathology.

Focal Lesions: severity factors depend on location size (bigger lesions worse than small), depth (deep lesions worse than superficial). The pathologic process may involves hemorrhage, infarct, tumor, abscess, trauma and secondary effects (edema, mass effect, neuronal degeneration.

Multifocal Lesions: severity depends on factors same as that for focal lesions. In addition it also depends on number (the more lesions the worse), simultaneous Vs stages (simultaneous the worse) and unilateral Vs bilateral (bilateral worse). The pathologic process commonly involved are grossly same as that for focal lesions.

Diffuse Lesions : severity factors depend on density or quantity (the more the worse) , locations ( structures and neural elements involved ). The pathologic processes involved here are traumatic hypoxic, ischemic, inflammatory, metabolic, as well as degenerative secondary processes.

More over it has been known for a long time that when particular areas of brain are damaged there are characteristic effects on its function. For instance damage to the side of the brain half way from the front to the back (parietal area ) results in weakness of the arm or leg on the opposite side. Damage to the left side tends to impair speech. Injury behind the forehead ( frontal area ) results in changes in behavior and loss of restraint and insight. When there is localized injury to the brain perhaps caused by an open or penetrating injury, only one of these functions may be affected. However most head injuries are closed injuries caused by acceleration with multiple areas of brain damaged. Some or perhaps many of brain functions may be affected, some more severely than others.