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Concussion, with loss of function, is only the mildest effect of CNS trauma, whereas contusion or bruising and laceration with tissue disruption cause morphologic lesions with degrees of cell death, hemorrhage, edema, and neutrophil and macrophage infiltration. Microglial then astrocytic activation, proliferation, and hypertrophy all occur in adjacent viable tissue. The site of major tissue damage may not be directly related to the site of traumatic impact on the calvaria or vertebral column. Parenchymal and subarachnoid bleeding is common, but subdural and epidural hematoma formation is less frequent in domestic animals than in man. Hematomas, cerebral abscesses and other space‐occupying lesions, create local pressure and edema that can cause brain swelling and herniation of parts of the brain (Figures 4.7, 4.9, and 4.10).³⁰ Destroyed tissue is removed by activated microglia, but if it is more extensive, it is removed by infiltrating monocytic macrophages. This results in pools of lipid‐laden macrophages with net‐like cytoplasm—gitter cells. Such large lesions may heal to form an astroglial scar, but more so a fluid‐filled cavity lined by astrocytes. Secondary neuronal fiber degeneration will occur along fiber tracts that are separated from their cell bodies by the injury.

Figure 4.10 This is a caudal view of the occipital lobes and section through the midbrain of a neonatal foal that suffered birth asphyxia and demonstrated bizarre behavior and somnolence prior to developing intractable seizures. Diffuse forebrain swelling that resulted from the cerebral neuronal necrosis caused by the asphyxia is evident by the herniated ventral portions of the occipital lobes of the cerebrum (arrows). Additionally, softening and necrosis (malacia) is present in both rostral colliculi (arrowheads) that likely also resulted from the effects of oxygen deprivation on these sensitive regions of gray matter.

Heat, cold, chemicals, and ionizing irradiation can cause immediate cell death with massive vascular breakdown and tissue infarction. Some delayed effects, particularly on the maintenance of myelin, may be noted with irradiation.