CHOREOATHETOSIS
CHROMOSOMAL ABNORMALITIES
C
choreoathetosis
A condition in which
the jerky, uncontrolled movements char-
acteristic of
chorea
are 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.
choriocarcinoma
A rare cancerous
tu-
mour
that develops from
placental
tissue
in the uterus, usually as a complication
of a
hydatidiform m ole
(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 treatment relies on
early diagnosis.
If a woman has a hydatidiform mole,
she is screened regularly after treatment
using
ultrasound scanning
and tests to
measure blood and urine levels of the
hormone
human
chorionic
gona-
dotrophin (HCG). High levels of HCG
are associated with choreocarcinoma.
Treatment is with
anticancer drugs
.
chorion
One 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 sampling
A method
of diagnosing genetic abnormalities in
a
fetus
using a small sample of tissue
taken from the chorionic
villi
at 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
women who are at a higher-than-normal
risk of having a child with a chromoso-
mal disorder, such as
Do'wn's syndrome,
or a genetic disease, such as
thalassaemia
.
Chrom osom e analysis
of the villi cells
takes place in
the laboratory.
CVS
slightly increases the risk of
miscarriage
.
choroid
A 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.
choroiditis
Inflammation of the
choroid.
It is often caused by infections such as
toxocariasis
or
toxoplasmosis
, more rarely
by
sarcoidosis
,
syphilis
, and
histoplasmo-
sis.
It sometimes has no obvious cause.
Treatment includes
corticosteroid drugs
for the inflammation, and
antibiotic
drugs
for any causative infection.
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
and
produces
cerebrospinal fluid
.
Christmas
disease
A
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
haem ophilia
.
chromium
A 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 elem ents
). 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 abnormalities
Variations
from normal in the number or structure
of
chromosom es
contained 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
autosomes
or the 2 X and Y
sex chromosomes. A complete extra set
of chromosomes per cell 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 syndrom e
. In
transloca-
tion,
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 include
Turner's syndrome,
in which a girl is
born with a single X chromosome in her
128
previous page 126 BMA Illustrated Medical Dictionary read online next page 128 BMA Illustrated Medical Dictionary read online Home Toggle text on/off