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 ...