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U N I T 1 0
Nervous System
BrainTumors
Primary brain tumors account for 2% of all cancers in
the United States.
4,47
Metastasis to the brain from other
sites is more common. Central nervous system tumors
are the second most common malignancy in children
and adolescents, with the incidence rate being highest in
infants and children 5 years of age or younger.
48
The etiology of brain tumors is largely unknown.
Several CNS tumors are associated with rare genetic
conditions, most commonly the autosomal dominant
disorder neurofibromatosis 1 (see Chapter 6). Although
a large number of studies have examined the relation-
ship between environmental and occupational factors,
only high-dose radiation is a proven risk factor.
49,50
Irradiation given to treat intracranial and extracranial
cancers, including prophylactic irradiation for leuke-
mia, increases the incidence of brain tumors.
Types ofTumors
The term
brain tumor
refers to a collection of intracra-
nial neoplasms, each with its own histology, site of ori-
gin, treatment, and prognosis.
1,4
For most neoplasms, the
term
malignant
is used to describe the tumor’s lack of
cell differentiation, its invasive nature, and its ability to
metastasize. However, the terms
benign
and
malignant
do not apply to brain tumors in the same sense as they
do to tumors in other parts of the body. In the brain, even
a well-differentiated and histologically benign tumor
may grow and cause death because of its location. Also,
tumors in the brain are rarely benign because surgery
rarely cures. Most histologically benign tumors infil-
trate the normal brain tissue, preventing total resection
and allowing for tumor recurrence. Furthermore, brain
tumors seldom metastasize, except within the CNS itself.
Brain tumors can be divided into three basic types:
primary intracranial tumors of neuroepithelial tissue
(e.g., neuroglia, neurons), primary intracranial tumors
that originate in the cranial cavity but are not derived
from the brain tissue itself (e.g., meninges, primary
CNS lymphoma, pituitary gland tumors [discussed in
Chapter 32]), and metastatic tumors.
1,4,50
Collectively,
neurogliomas of astrocytic origin are the most common
type of primary brain tumor in adults, and medulloblas-
tomas the most common type in children. Whatever the
type, brain tumors commonly present with symptoms
related to a disruption in neuronal activity (seizures),
signs of increased ICP (progressive headache, nausea
and vomiting, drowsiness, visual abnormalities), focal
motor or sensory deficits, and cognitive dysfunction.
7,50
Neuroglial Tumors
Neuroglial tumors, which are the most common form of
primary brain tumors, derive from astrocytes, oligoden-
drocytes, and ependymal cells.
1,4,51
Gliomas are divided
into four grades: grades I and II are low-grade tumors,
whereas grade III and IV are high-grade tumors.
Astrocytomas.
There are two types of astrocyte tumors:
infiltrating and noninfiltrating astrocytomas. Infiltrating
astrocytomas account for 80% of adult primary brain
tumors.
1
They occur most commonly in the fourth
through the sixth decade. Although they usually are
found in the cerebral hemispheres, they also can occur
in the cerebellum, brain stem, or spinal cord. The most
common presenting signs and symptoms are seizures,
headaches, and focal neurologic deficits related to the
location of the tumor. Infiltrating astrocytomas are char-
acterized by a spectrum of histologic differentiation that
includes diffuse astrocytomas (grade II); anaplastic astro-
cytomas (grade III); and the least differentiated and most
aggressive, glioblastomas (grade IV).
1
The grade I designation is reserved for the noninfil-
trating pilocytic astrocytomas, which are distinguished
from other astrocytomas by their cellular appearance
and their benign behavior. Typically, they occur in chil-
dren and young adults and usually are located in the
cerebellum, but they also can be found in the floor and
walls of the third ventricle, in the optic chiasm and
nerves, and occasionally in the cerebral hemispheres.
These tumors grow slowly and, in the cerebellum par-
ticularly, may be treated by resection.
Oligodendrogliomas.
Oligodendrogliomas are tumors
of the oligodendrocytes or their precursors.
1
They rep-
resent approximately 5% to 15% of neuroglial tumors
and are most common in the fourth and fifth decades
of life. The tumors are found mostly in the cerebral
hemispheres, with a predilection for white matter. The
prognosis of persons with oligodendrogliomas is less pre-
dictable than for persons with infiltrating astrocytomas.
SUMMARY CONCEPTS
■■
Infections of the CNS may be classified according
to the structures involved (the meninges
[meningitis] or brain parenchyma [encephalitis])
and the type of organism causing the infection
(bacteria or virus).The damage caused by
infection may predispose to hydrocephalus,
seizures, or other neurologic defects.
■■
Except in neonates, most cases of bacterial
meningitis are pneumococcal or meningococcal.
The most common symptoms are sudden onset
of headache, fever, and stiffness of the neck
(nuchal rigidity).
■■
Viral meningitis manifests in much the same way
as bacterial meningitis, but the course is less
severe and symptoms resolve spontaneously.
■■
Encephalitis is usually a viral infection of the brain.
In addition to fever, headache, and nuchal rigidity,
patients often experience neurologic disturbances.
