toxocariasisor toxoplasmosis, more rarely
by sarcoidosis, syphilis, and histoplasmo-
sis. It sometimes has no obvious cause.
Treatment includes corticosteroid drugs
for the inflammation, and antibiotic
drugsfor any causative infection.
choroid plexusA network of thin-
walled blood vessels in the eyeor brain.
The choroid plexus of the eye supplies
blood to the retina. In the brain, the
choroid plexus lines the ventriclesand
produces cerebrospinal fluid.
Christmas diseaseA rare genetic
bleeding disorder in which there is defi-
cient production of one of the proteins
in blood needed for blood coagulation
(see blood clotting). Christmas disease
has similar features to haemophilia.
chromiumA metallic element that has
a vital role in the activities of several
enzymes in the body. Chromium is
required only in minute amounts (see
trace elements). In excess, chromium is
toxic and produces inflammation of the
skin and, if inhaled, damages the nose
and may increase the risk of lung cancer.
chromosomal abnormalitiesVariations
from normal in the number or structure
of chromosomescontained in a person’s
cells. The cause 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 fer-
tilized egg. Chromosomal abnormalities
are classified according to whether they
involve the 44 autosomesor the 2 X and Y
sex chromosomes. A complete extra set
of chromosomes percell is called poly-
ploidy and is lethal.
Autosomal abnormalities cause physi-
cal and mental defects of varying
severity. Some types of autosomal
abnormality, known as trisomy, consist
of an extra chromosome on 1 of the 22
pairs of autosomes. The most common
trisomy is Down’s syndrome. Some-
times, part of a chromosome is missing,
as in cri du chat syndrome. In transloca-
tion, a part of a chromosome is joined
to another, causing no ill effects in the
person but a risk of abnormality inhis or
her children.
Sex chromosome abnormalities include
Turner’s syndrome,in which a girl is
born with a single X chromosome in herchoreoathetosisA condition in which
the jerky, uncontrolled movements char-
acteristic of choreaare combined with
the slower, continuous writhing move-
ments of athetosis. Choreoathetosis
occurs in children with cerebral palsy
and as a side effect of certain drugs.
choriocarcinomaA rare cancerous tu-
mourthat develops from placentaltissue
in the uterus, usually as a complication
of a hydatidiform mole(a noncancerous
tumour) but sometimes after a normal
pregnancy or a miscarriage. Untreated,
it destroys the walls of the uterus and
may spread to the vagina and vulva and,
eventually, to the liver, lungs, brain, and
bones. Successful treatmentrelies on
early diagnosis.
If a woman has a hydatidiform mole,
she is screened regularly after treatment
using ultrasound scanning and tests to
measureblood and urine levels of the
hormone human chorionic gona-
dotrophin (HCG). High levels of HCG
are associated with choreocarcinoma.
Treatment is with anticancer drugs.
chorionOne of the 2 membranes that
surround the embryo. The chorion lies
outside the amnion, has small finger-
like projections called the chorionic
villi, and develops into the placenta.
chorionic villus samplingA method
of diagnosing genetic abnormalities in
a fetususing a small sample of tissue
taken from the chorionic villiat edge of
the placenta. Because the cells have the
same chromosome makeup as those in
the fetus, they can be used to detect
genetic abnormalities. Chorionic villus
sampling (CVS) is usually performed in
the first 3 months of pregnancy in
womenwho are at a higher-than-normal
risk of having a child with a chromoso-
mal disorder, such as Down’s syndrome,
or a genetic disease, such as thalassaemia.
Chromosome analysisof the villi cells
takes place in the laboratory. CVS
slightly increases the risk of miscarriage.
choroidA layer of tissue at the back of
the eye, behind the retina. The choroid
contains many blood vessels that supply
nutrients and oxygen to the retinal cells
and to surrounding tissues in the eye.
choroiditisInflammation of the choroid.
It is often caused by infections such asCHOREOATHETOSIS CHROMOSOMAL ABNORMALITIESC