or as part of a generalized inflamma-
result of infections such as
and, more rarely, of
although it sometimes has no obvious
cause. Chorioditis is painless but causes
blurring of vision.
to reduce the inflam -
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.
A network of thin-walled blood vessels
The choroid plexus
of the eye supplies blood to the
In the brain, the choroid plexus lines
A rare genetic bleeding
w hich there is deficient production of
one of the proteins in blood that is
Christmas disease has similar features
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.
A metallic element that plays a vital role
in the activities of several
stances that control the rate of chemical
reactions) in the body
Chromium is needed in only minute
it is toxic. It causes inflammation of the
skin, and, if inhaled, damages the nose.
Chrom ium fumes may increase the risk
Variations from normal in the number
or structure of
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
The cause of a chromosomal abnormal-
ity is generally a fault in the process of
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.
Chromosomal abnormalities are classi-
fied according to whether they involve
autosomes (pairs of very similar
chromosomes) or the two
(X and Y). A whole extra set of
chromosomes per cell is called poly-
ploidy; this is lethal in early pregnancy
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
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
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.
a girl is born
with only a single X chromosome in
her cells instead of the normal comple-
of two. The
physical abnormalities, defective sexual
development, and infertility.
In boys, one or more extra X chro-
This condition results in defective sex-
ual development and infertility.
The presence of an extra X chromo-
chromosome in men normally has no
physical effect but increases the risk of
m ild learning difficulties.
If suspected, chromosomal abnormali-
ties can be diagnosed by
chorionic villus sampling
Because chromosomal abnormalities
affect every one of a person’s cells, no
cure is possible. Many disorders caused
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
Anyone w ho has a child or other
family member affected by a chromo-
the risk of his or her future children
being affected by the condition.
Study of the
in cells to
find out if a
present or to establish its nature.
WHY IT IS DONE
Some pregnancies are associated with a
higher-than-average risk of the baby
Risk factors include an older mother,
the birth of a previous child with a
or a defect or
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
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
is identified, the parents w ill
be offered termination of the pregnan-
to assess the risk of a subse-
quent pregnancy being affected.