Aseptic thrombosis of intracranial venous sinuses and cerebral veins is discussed in Chap. 33 on cerebrovascular disease; aspects related to intracranial pressure are discussed in Chap. 29, on CSF circulation.
With the exception of a small proportion of cases (approximately 10 percent) in which infection is introduced from the outside (compound fractures of the skull, intracranial operation, bullet wounds), brain abscess is secondary to bacteremia and a bacterial focus elsewhere in the body. Purulent pulmonary infections (abscess, bronchiectasis) and bacterial endocarditis account for the largest number of brain abscesses in the modern era. A decreasing proportion of brain abscesses in the current era is related to disease of the paranasal sinuses, middle ear, and mastoid cells. Of abscesses originating in the ear, about one-third lie in the anterolateral part of the cerebellar hemisphere; the remainder occurs in the middle and inferior parts of the temporal lobe. The sinuses most frequently implicated are the frontal and sphenoid, and the abscesses derived from them are in the frontal and temporal lobes, respectively.
Otogenic and rhinogenic abscesses reach the nervous system by direct extension, in which the bone of the middle ear or nasal sinuses becomes the seat of an osteomyelitis, with penetration of the dura and leptomeninges, infection may spread along the major intracranial veins. Thrombophlebitis of the pial veins and dural sinuses, by infarcting brain tissue, renders the latter more vulnerable to invasion by infectious material. The close anatomic relationship of the lateral (transverse) sinus to the cerebellum explains the frequency with which this portion of the brain is infected via the venous route. The spread along venous channels also explains how an abscess may sometimes form at a considerable distance from the primary focus in the middle ear or paranasal sinuses.
As mentioned, the majority of brain abscesses is metastatic, that is, hematogenous. These are usually traceable to bacterial endocarditis or to a primary septic focus in the lungs or pleura, as indicated earlier. Occasional cases are associated with infected pelvic organs, skin, tonsils, abscessed teeth, and osteomyelitis of noncranial bones.
Individuals with a congenital cardiac defect with right-to-left shunt or pulmonary arteriovenous malformation (including Osler-Weber-Rendu malformations) that permits infected emboli to bypass the pulmonary circulation and reach the brain, are particularly vulnerable to brain abscess. In approximately 20 percent of cases, the source cannot be ascertained.
Metastatic abscesses from hematogenous spread are usually situated in the distal territory of the middle cerebral arteries (Fig. 31-2), and they sometimes may be multiple, in contrast to otogenic and rhinogenic abscesses. Also, almost all deep cerebral abscesses have a systemic source. It should also be noted that the clinical and radiologic features of a solitary abscess mimic those of a brain tumor. Small and miliary abscesses may progress to large ones.