Pathology

Grossly, an abscess begins as a circumscribed purulent encephalitis (Fig. 6.3). Within a few days, the center of the inflammation liquefies. This acute stage of abscess formation is associated with a vasogenic edema that produces increased intracranial pressure and herniation of the hippocampus and cerebellar tonsils. During this stage, the abscess may rupture into the ventricles, producing a pyocephalus, and it may extend into the sub-arachnoid space, producing a purulent leptomeningitis.

Histologically, the purulent necrotic tissue is gradually surrounded by highly vascular granulation tissue (see Fig. 6.3). This, in 2 to 3 weeks, forms a thick col-lagenous capsule around the purulent center. Meanwhile, outside the capsule, astrocytes proliferate, separating the abscess from the healthy tissue.

Multiple disseminated embolic microabscesses are usually encountered in chronically ill, debilitated elderly individuals. Acute bacterial endocarditis caused by staphylococcus is a major source of infection. Clinically, the disease presents as a septic encephalopathy. The abscesses, a few millimeters in size, are usually situated at the cortical-white matter junction.

FIGURE 6.3

Metastatic cerebral abscesses from the lung, caused by Streptococcus pneumoniae in a 38-year-old man, a chronic alcoholic. A. A subacute abscess is situated deep in the parietal white matter. Its center contains liquefied pus, and the wall contains purulent hemorrhagic necrotic tissue surrounded by massive edema. Pus from the ruptured abscess covers the ependyma of lateral ventricles. B. Macrosection of a second abscess, situated in the frontal lobe at the junction of cortex and white matter; it is surrounded by a thin capsule of granulation tissue (cresyl violet).

FIGURE 6.3

Metastatic cerebral abscesses from the lung, caused by Streptococcus pneumoniae in a 38-year-old man, a chronic alcoholic. A. A subacute abscess is situated deep in the parietal white matter. Its center contains liquefied pus, and the wall contains purulent hemorrhagic necrotic tissue surrounded by massive edema. Pus from the ruptured abscess covers the ependyma of lateral ventricles. B. Macrosection of a second abscess, situated in the frontal lobe at the junction of cortex and white matter; it is surrounded by a thin capsule of granulation tissue (cresyl violet).

Subdural abscess (empyema), a collection of pus on the undersurface of the dura, is usually associated with infections of the ear, facial sinuses, and with thrombophlebitis of the dural sinuses.

Epidural abscess, a collection of pus on the outer surface of the dura, is associated with osteomyelitis of the skull and infection of adjacent structures.

Spinal epidural abscess, a complication of back injury, skin infection, or osteomyelitis of the vertebrae, is a surgical emergency. Dural abscesses may be difficult to diagnose. MRI and CT scan are most useful in confirming the diagnosis.

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