the temporary blockage of a small
blood vessel in the eye by emboli(parti-
cles of solid matter such as cholesterol
or clotted blood). These are carried in
the bloodstream from diseased arteries
in the neck or, rarely, the heart. Attacks
may be infrequent or they may occur
many times a day. This symptom indi-
cates an increased risk of strokeand
requires medical investigation.
ambidexterityThe ability to perform
manual skills equally well with either
hand because there is no definite hand-
edness. Ambidexterity is an uncommon
and often familial trait.
amblyopiaA permanent defect of vis-
ual acuity in which there is usually no
structural abnormality in the eye. In
many cases, there is a disturbance of
the visual pathway between the retina
and the brain. The term is also some-
times applied to toxic or nutritional
causes of decreased visual acuity, as in
tobacco–alcohol amblyopia.
Amblyopia will develop if there is a
marked discrepancy between the images
received by the brain from each eye
while vision is developing during early
childhood. The most common cause is
squint. Failure to form normal retinal
images may also result from congenital
cataract, and severe, or unequal, focus-
ing errors, such as when one eye is
normal and there is an uncorrected
large degree of astigmatismin the other.
Toxic and nutritional amblyopia may
result from damage to the retina and/or
the optic nerve.
To prevent amblyopia due to squint,
patching (covering up the good eye to
force the deviating eye to function prop-
erly) is the usual treatment. Surgery to
place the deviating eye in the correct
position may be necessary. Glasses may
be needed to correct severe focusing
errors. Cataracts may be removed surgi-
cally. After the age of 8, amblyopia
cannot usually be remedied.
ambulanceA vehicle for transporting
sick or injured people that is staffed by
trained personnel who can provide emer-
gency treatment during the journey.
ambulatory ECGIn ambulatory ECG
(electrocardiography), a wearable device
called a Holter monitoris used to record
the electrical activity of the heart by
means of electrodes attached to the
chest. The monitor is usually worn for
24 hours or longer and detects intermit-
tent arrhythmias(abnormal heart rates
and rhythms). The wearer can press a
button on the monitor to mark the
recording whenever symptoms occur.
The recording can later be analysed
to see if the periods of arrhythmia
coincide with the symptoms.
amelogenesis imperfectaAn inherited
condition of the teeth in which the
enamel is either abnormally thin or is
deficient in calcium. Affected teeth may
be pitted and discoloured (see dis-
coloured teeth) and more susceptible to
dental caries(tooth decay) and wear.
amenorrhoeaThe absence of mens-
trual periods. Primary amenorrhoea is
defined as failure to start menstruating
by the age of 16. Secondary amenor-
rhoea is the temporary or permanent
cessation of periods in a woman who
has menstruated regularly in the past.
The main cause of primary amenor-
rhoea is delayed puberty. The delay may
not indicate a disorder, but, rarely, it may
result from a disorder of the endocrine
system, such as a pituitary tumour,
hypothyroidism, an adrenal tumour, or
adrenal hyperplasia. Another rare cause
of delayed puberty is Turner’s syndrome.
In some cases, menstruation fails to
take place because the vagina or the
uterus has been absent from birth, or
because there is no perforation in the
hymen to allow blood to escape.
The most common cause of temporary
secondary amenorrhoea is pregnancy.
Periods may also cease temporarily after
a woman has stopped taking oral con-
traceptives. Secondary amenorrhoea may
also result from hormonal changes due
to stress, depression, anorexia nervosa,
or certain drugs. Another possible
cause is a disorder of the ovary such as
polycystic ovary (see ovary,polycystic)
or an ovarian tumour. Amenorrhoea
occurs permanently following the meno-
pauseor after a hysterectomy.
amfetamine drugsA group of stimu-
lant drugsused mainly in the treatment
of narcolepsy(a rare disorder character-
ized by excessive sleepiness).AMBIDEXTERITY AMFETAMINE DRUGS
A