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U N I T 1 0
Nervous System
Infections
Infections of the brain may be classified according to the
structure involved—meninges (meningitis) or parenchy-
mal brain tissue (encephalitis). They also may be classi-
fied by the type of invading organism: bacterial, viral,
or other. In general, pathogens enter the CNS from the
bloodstream or by direct invasion through a skull frac-
ture, or, rarely, by contamination during surgery or lum-
bar puncture. Local extension from an adjacent structure
(e.g., infected sinus, tooth, middle ear) may also occur.
Meningitis
Meningitis is an inflammation of the pia mater, the
arachnoid, and the CSF-filled subarachnoid space.
Inflammation spreads rapidly because of CSF circulation
around the brain and spinal cord. The inflammation usu-
ally is caused by an infection, but chemical meningitis
can occur. There are two types of acute infectious men-
ingitis: acute purulent meningitis (usually bacterial) and
acute lymphocytic (usually viral) meningitis.
1
Factors
responsible for the severity of meningitis include viru-
lence of the pathogen, host factors, brain edema, and the
presence of permanent neurologic sequelae.
Acute Bacterial Meningitis
Acute bacterial meningitis, which has a high potential
for morbidity and mortality, is an inflammatory pro-
cess of the leptomeninges and CSF within the subarach-
noid space.
1,4,42,43
Most cases of bacterial meningitis are
caused by
Streptococcus pneumoniae
(pneumococcus)
or
Neisseria meningitidis
(meningococcus), except in
neonates, who are more commonly infected by group
B streptococci. The development of effective vaccines
against
Haemophilus influenzae and S. pneumoniae
has resulted in a profound decline in bacterial men-
ingitis among children in the United States.
44
Among
adults, however, the incidence of meningitis has not
changed. Epidemics of meningococcal meningitis occur
in settings such as colleges and the military, where
young people reside in close contact with each other.
Other pathogens in adults are gram-negative bacilli
and
Listeria monocytogenes.
The very young and the
very old are at highest risk for pneumococcal meningi-
tis. Risk factors associated with contracting meningitis
include head trauma with basilar skull fractures, otitis
media, sinusitis or mastoiditis, neurosurgery, dermal
sinus tracts, systemic sepsis, or immunocompromise.
In the pathophysiologic process of bacterial meningi-
tis, the microorganisms replicate and undergo lysis in the
CSF, releasing endotoxins and cell wall fragments that
cause inflammation, characterized by a cloudy, purulent
exudate (Fig. 37-17). Thrombophlebitis of the bridging
veins and dural sinuses or obliteration of arterioles by
inflammation may develop, causing vascular congestion
and infarction in the surrounding tissues. Ultimately, the
meninges thicken and adhesions form. These adhesions
may impinge on the cranial nerves, giving rise to cranial
nerve palsies, or may impair the outflow of CSF, causing
hydrocephalus.
The most common symptoms of acute bacterial men-
ingitis are sudden onset of headache, fever, and stiffness
of the neck (nuchal rigidity), sometimes accompanied
■■
A hemorrhagic stroke is caused by the
spontaneous rupture of an intracerebral vessel
with bleeding into the brain.The most common
predisposing factors are advancing age and
hypertension.
■■
A subarachnoid hemorrhage involves bleeding
into the subarachnoid space. Most subarachnoid
hemorrhages are the result of a ruptured cerebral
aneurysm or arteriovenous malformations.
■■
Arteriovenous malformations are congenital
abnormal communications between arterial
and venous channels that result from failure in
the development of the capillary network in the
embryonic brain.The vessels in the arteriovenous
malformations may enlarge to form a space-
occupying lesion, become weak and predispose
to bleeding or divert blood away from other parts
of the brain.
SUMMARY CONCEPTS
(continued)
FIGURE 37-17.
Purlent meningitis. A creamy exudate opacifies
the leptomeninges in bacterial bacterial meningitis.The
superficial veins are engorged and may develop thrombosis
and the arteries on the surface of the brain may also develop
thrombosis leading to infarcts. (From Fuller GN, BouldinTW.
The nervous system. In: Rubin R, Strayer DS, eds. Rubin’s
Pathology: Clinicopathologic Foundations of Medicine. 6th ed.
Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams &
Wilkins; 2012:1318.)
