CHOROID PLEXUS
C
or as part of a generalized inflamma-
tion
affecting
the
whole
eye
(see
uveitis
) .
The
condition
is
often
the
result of infections such as
toxocariasis
or
toxoplasmosis
,
and, more rarely, of
sarcoidosis
,
syphilis
,
and
histoplasmosis
,
although it sometimes has no obvious
cause. Chorioditis is painless but causes
blurring of vision.
Treatment
of
chorioditis
includes
corticosteroid drugs
to reduce the inflam -
mation
and
antibiotic drugs
for
any
causative infection.
STRUCTURE OF CHOROID
The choroid lines the inside of the
eyeball. It thickens around the lens
to form the ciliary body. Muscles
stretching between the choroid and
the lens contract to adjust the shape
of the lens in focusing.
choroid plexus
A network of thin-walled blood vessels
in the
eye
or
brain
.
The choroid plexus
of the eye supplies blood to the
retina
.
In the brain, the choroid plexus lines
the
ventricles
(cavities)
and produces
cerebrospinal fluid
.
Christmas disease
A rare genetic bleeding
disorder in
w hich there is deficient production of
one of the proteins in blood that is
needed
for
blood
coagulation
(see
blood clotting
) .
Christmas disease has similar features
to
haemophilia
.
However, the deficient
proteins are different. In Christmas dis-
ease the deficiency is of a protein called
factor IX, while in haemophilia the
deficiency is of the protein factor VIII.
chromium
A metallic element that plays a vital role
in the activities of several
enzymes
(sub-
stances that control the rate of chemical
reactions) in the body
Chromium is needed in only minute
amounts (see
trace elements
) .
In excess,
it is toxic. It causes inflammation of the
skin, and, if inhaled, damages the nose.
Chrom ium fumes may increase the risk
of
lung cancer
.
chromosomal abnormalities
Variations from normal in the number
or structure of
chromosomes
contained
in a person’s cells. In most cases, the
chromosomal abnormality is present in
all of the body’s cells. The possible
effects range from virtually none to a
lethal condition, depending on the par-
ticular type of abnormality
CAUSES
The cause of a chromosomal abnormal-
ity is generally a fault in the process of
chromosome
division,
either
during
the formation of an egg or sperm, or
during the first few divisions of a fertil-
ized egg. Occasionally a parent passes
on an abnormal arrangement of his or
her own chromosomes.
TYPES
Chromosomal abnormalities are classi-
fied according to whether they involve
the
44
autosomes (pairs of very similar
chromosomes) or the two
sex chromo-
somes
(X and Y). A whole extra set of
chromosomes per cell is called poly-
ploidy; this is lethal in early pregnancy
AUTOSOMAL ABNORMALITIES
These abnormalities cause physical and
mental defects of varying severity Some
types of autosomal abnormality, known
as trisomy, involve the presence of an
extra chromosome on one of the 22
pairs of autosomes. The most common
trisomy is
Down’s syndrome
,
w hich is
caused by the presence of three num -
ber 21 chromosomes.
Sometimes, part of a chromosome is
missing, as in
cri du chat syndrome
.
In
translocation
,
a part of a chromosome is
joined to another, causing no ill effects
in the person but a risk of abnormality
in his or her children.
SEX CHROMOSOME ABNORMALITIES
Normally, a female has two X chromo-
somes and a male has an X and a Y.
Abnormalities may occur if there are
missing or extra sex chromosomes.
In
Turner’s syndrome
,
a girl is born
with only a single X chromosome in
her cells instead of the normal comple-
ment
of two. The
condition
causes
physical abnormalities, defective sexual
development, and infertility.
In boys, one or more extra X chro-
mosomes causes
Klinefelter’s syndrome
.
This condition results in defective sex-
ual development and infertility.
The presence of an extra X chromo-
some
in women
or
of an
extra Y
chromosome in men normally has no
physical effect but increases the risk of
m ild learning difficulties.
DIAGNOSIS
If suspected, chromosomal abnormali-
ties can be diagnosed by
chromosome
analysis
in
early
pregnancy,
using
amniocentesis
or
chorionic villus sampling
.
Because chromosomal abnormalities
affect every one of a person’s cells, no
cure is possible. Many disorders caused
by
autosomal
chromosome
defects
result in early death. Others, such as
Down’s syndrome, are compatible with
survival but cause physical and mental
disability. Horm onal or surgical treat-
ment, or a combination, can help to
correct
some
of the
developmental
defects
caused
by
Klinefelter’s
and
Turner’s syndromes.
Anyone w ho has a child or other
family member affected by a chromo-
somal
abnormality
may
wish
to
consider
genetic counselling
to establish
the risk of his or her future children
being affected by the condition.
chromosome analysis
Study of the
chromosomes
in cells to
find out if a
chromosomal abnormality
is
present or to establish its nature.
WHY IT IS DONE
Some pregnancies are associated with a
higher-than-average risk of the baby
having
a
chromosomal
abnormality.
Risk factors include an older mother,
the birth of a previous child with a
chromosomal defect,
or a defect or
translocation
(rearrangement)
in
the
mother’s or the father’s chromosomes.
In many cases, there are no identifiable
risk factors, and so all pregnant women
are offered a preliminary blood test
(see
antenatal screening
)
to
identify
those at high risk of a fetal abnormality.
If the fetus is at high risk, a sample of
cells is taken for chromosome analysis.
If a serious abnormality, such as
Down’s
syndrome
,
is identified, the parents w ill
be offered termination of the pregnan-
cy (see
abortion, induced
)
and
genetic
counselling
to assess the risk of a subse-
quent pregnancy being affected.
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