C h a p t e r 3 7
Disorders of Brain Function
917
weight, it receives one sixth of the resting cardiac output
and accounts for 20% of the body’s oxygen consump-
tion.
2
By definition,
hypoxia
denotes a deprivation of
oxygen with maintained blood flow, whereas
ischemia
represents a situation of greatly reduced or interrupted
blood flow. The cellular effects of hypoxia and ischemia
are quite different, and the brain tends to have different
sensitivities to the two conditions. Hypoxia interferes
with the delivery of oxygen, whereas ischemia interferes
with the delivery of oxygen and glucose as well as the
removal of metabolic wastes.
Hypoxia
usually is seen in conditions such as expo-
sure to reduced atmospheric pressure, carbon monox-
ide poisoning, severe anemia, and failure of the lungs
to oxygenate the blood. Contrary to popular belief,
hypoxia is fairly well tolerated, particularly in situations
of chronic hypoxia. Neurons are capable of substantial
anaerobic metabolism and are fairly tolerant of pure
hypoxia, in which case it produces listlessness, drowsi-
ness, and impaired problem solving. Unconsciousness
and convulsions may occur when hypoxia is sudden and
severe. However, the effects of severe hypoxia on brain
function seldom are seen because the condition rapidly
leads to cardiac arrest and ischemia.
Ischemia
is seen in conditions of low blood flow.
Cerebral ischemia can be focal, as in a stroke due to
cerebral artery occlusion, or global, as in cardiac
arrest.
3
Cerebral artery occlusion leads to focal isch-
emia, and if sustained, to infarction (death) of brain
tissue in the distribution of the affected vessel. The
location of the infarct and extent of tissue damage that
results is determined by modifying variables, of which
collateral blood flow is the most important. The collat-
eral circulation may even provide sufficient blood flow
to the borders of the focal ischemic region to maintain
a low level of metabolic activity, thereby preserving tis-
sue integrity.
Within the brain, certain regions and cell popula-
tions are more susceptible than others to ischemic
injury
4
(Fig. 37-1). Areas of the brain located at the
border zones between the overlapping territories sup-
plied by the major cerebral arteries, sometimes called
the
watershed areas,
are extremely vulnerable to
ischemia. During events such as severe hypotension,
these distal territories undergo a profound lowering
of blood flow, predisposing to ischemia and infarc-
tion of brain tissues. As a consequence, areas of the
cortex that are supplied by the major cerebral arter-
ies usually regain function on recovery of adequate
blood flow, whereas infarctions may occur in the
watershed strips, resulting in focal neurologic deficits.
Laminar necrosis
refers to short, creeping segments of
necrosis that occur within and parallel to the cerebral
cortex, in areas supplied by the penetrating arteries
Anterior cerebral artery
Middle cerebral artery
Watershed
zone of infarction
Short penetrating
arteries
Laminar necrosis
Necrosis in
Sommer sector
of hippocampus
Necrosis of
Purkinje cells
of cerebellum
FIGURE 37-1.
Consequences of
global ischemia. A global insult
induces lesions that reflect the
vascular architecture (watershed
infarcts, laminar necrosis) and the
sensitivity of individual neuronal
systems (pyramidal cells of the
Sommer section, Purkinje cells).
(FromTrojanowsi JQ, Kenyon L.The
central nervous system. In: Rubin R,
Strayer DS, eds. Rubin’s Pathology:
Clinicopathologic Foundations of
Medicine, 5th ed. Philadelphia, PA:
Wolters Kluwer Health | Lippincott
Williams &Wilkins; 2008:1191. Courtesy
of Dmitri Karetnikov, artist.)
Neurocognitive Disorders
Alzheimer Disease
Pathology
Manifestations
Diagnosis and Treatment
Other Types of Dementia
Vascular Dementia
Frontotemporal Dementia
Wernicke-Korsakoff Syndrome
Huntington Disease
