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This chapter is concerned mainly with the bacterial infections of the central nervous system (CNS), i.e., bacterial meningitis, septic thrombophlebitis, brain abscess, epidural abscess, and subdural empyema. The granulomatous infections of the CNS, notably tuberculosis, syphilis and other spirochetal infections, and certain fungal infections, are also discussed in some detail. In addition, consideration is given to sarcoidosis, a granulomatous disease of uncertain etiology, and to the CNS infections and infestations caused by certain rickettsias, protozoa, worms, and ticks.

A number of other infectious diseases of the nervous system are more appropriately discussed elsewhere in this book. Viral infections of the nervous system, because of their frequency and importance, are allotted a chapter of their own (Chap. 33). Diseases caused by bacterial exotoxins—diphtheria, tetanus, botulism—are considered with other toxins that affect the nervous system (Chap. 43). Leprosy, which is essentially a disease of the peripheral nerves, is described in Chap. 46, and trichinosis, mainly a disease of muscle, in Chap. 48.

These infections reach the intracranial structures by one of two pathways, either by hematogenous spread (emboli of bacteria or infected thrombi) or by extension from cranial structures (ears, paranasal sinuses, osteomyelitic foci in the skull, penetrating cranial injuries, or congenital sinus tracts). In a number of cases, infection is iatrogenic, being introduced in the course of cerebral or spinal surgery, the placement of a ventriculoperitoneal shunt, or, rarely, by a lumbar puncture needle. Increasingly, infection is nosocomial, i.e., acquired in-hospital; in urban hospitals, nosocomial meningitis is now as frequent as the non–hospital-acquired variety (Durand et al).

Surprisingly little is known about the mechanisms of hematogenous spread and animal experiments involving the injection of virulent bacteria into the bloodstream have yielded somewhat contradictory results. In most instances of bacteremia or septicemia, the nervous system seems not to be infected; yet sometimes a bacteremia caused by pneumonia or endocarditis is the only apparent predecessor to meningitis. With respect to the formation of brain abscess, the resistance of cerebral tissue to infection is notable. Direct injection of virulent bacteria into the brain of an animal seldom results in abscess formation. In fact, this condition has been produced consistently only by injecting culture medium along with the bacteria or by causing necrosis of the tissue at the time bacteria are inoculated. In humans, infarction of brain tissue because of arterial occlusion (thrombosis or embolism) or venous occlusion (thrombophlebitis) appears to be a common and perhaps necessary antecedent.

The mechanism of meningitis and brain abscess from infection of the middle ear and paranasal sinuses is easier to understand. The cranial epidural and subdural spaces are practically never the sites of blood-borne infections, in contrast to the spinal epidural space, where such infections are almost always introduced by the hematogenous route. Furthermore, the cranial bones and the dura mater (which essentially constitutes the inner periosteum of the skull) protect the cranial cavity against ...

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