balance, posture, and muscular coordi-
nation. Both of these regions operate at
a subconscious level.
The brain and spinal cord are encased
in 3 layers of membranes, known as
meninges. Cerebrospinal fluidcirculates
between the layers and within the 4 main
brain cavities called ventricles. This fluid
helps to nourish andcushion the brain.
The brain receives about 20 per cent of
the blood from the heart’s output.
brain abscessA collection of pus, sur-
rounded by inflamed tissues, within the
brain or on its surface. The most com-
mon sites are the frontal and temporal
lobes of the cerebrumin the forebrain.
Brain abscesses may occur after a
head injury, but most cases result from
the spread of infection from elsewhere
in the body, such as the middle ear or
sinuses. Another cause is an infection
following a penetrating brain injury.
Multiple brain abscesses may occur as a
result of blood-borne infection, most
commonly in patients with a heart-valve
infection (see endocarditis). Symptoms
include headache, drowsiness, vomiting,
visual disturbances, fever, seizures, and
symptoms, such as speech disturbances,
that are due to local pressure. Treatment
is with antibiotic drugsand surgery. A
craniotomymay be needed to open and
drain the abscess. Untreated, brain abs-
cesses can cause permanent damage or
can be fatal. Despite treatment, scarring
can cause epilepsyin some cases.
brain damageDegeneration or death
of nerve cells and tracts within the brain
that may be localized to a particular
area of the brain or diffuse. Diffuse
damage most commonly results from
prolonged cerebral hypoxia(which may
occur in a baby during a difficult birth),
cardiac arrest, respiratory arrest, or
causes such as poisoning or status
epilepticus(prolonged convulsions). The
damage may also occur gradually due
to environmental pollutants such as lead
or mercury compounds (see Minamata
disease) or if nerve-cell poisons build up
in the brain, as in untreated phenylketo-
nuria. Other possible causes include
brain infections such as encephalitis.
Localized brain damage may occur as
a result of a head injury, stroke, brain
tumour, or brain abscess. At birth, a
raised blood level of bilirubin (in haem-
olytic disease of the newborn) causes
local damage to the basal gangliadeep
within the brain. This leads to a condi-
tion called kernicterus. Brain damage
that occurs before, during, or after birth
may result in cerebral palsy.
Damage to the brain may result in dis-
abilities such as learning difficultiesor
disturbances of movement or speech.
Nerve cells and tracts in the brain and
spinal cord cannot repair themselves
once they have been damaged, but some
return of function may be possible.
brain deathThe irreversible cessation
of all functions of the brain, including
the brainstem. (See also death.)
brain, disorders ofDefects and disor-
ders of the brain, which may have one of
numerous causes including infection, inj-
ury, brain tumour, or a lack of blood or
oxygen (hypoxia). Because the brain is
encased in the skull, any space-occupy-
ing tumour, brain abscess, or haematoma
creates raised pressure, which impairs
the function of the whole brain. Brain
disorders that are localized in a small
region may affect a specific function
such as speech (see aphasia). More
often, damage is more diffuse and the
symptoms can be varied and numerous.
Some brain disorders are congenital
due to genetic or chromosomal disor-
ders, as in Down’s syndrome. Structural
defects that arise during the develop-
ment of the fetus in the womb include
hydrocephalus and anencephaly.
Reduced oxygen supply may occur at
birth, causing cerebral palsy. Later in
life, cerebral hypoxia can result from
choking or from arrest of breathing and
heartbeat. From middle age onwards,
cerebrovascular disease is the most
important cause of brain disorder. If an
artery within the brain becomes blocked
or ruptures, leading to haemorrhage,
the result is a stroke. The brain may also
be damaged by a blow to the head see
head injury).
Infection within the brain (enceph-
alitis) may be due to viral infection.
Infection of the membranes surrounding
the brain (meningitis) is generally due
to bacterial infection. Creutzfeldt–JakobBRAIN ABSCESS BRAIN, DISORDERS OF
B