AVITAMINOSIS
A
lungs, and the intestines. When pressure
drops during ascent, the volume of gas
in these cavities increases and usually
escapes freely. On descent, the gas vol-
ume decreases as pressure outside the
body rises. Unless preventative mea-
sures are taken, this may cause pain
and, rarely, damage (see
barotrauma).
There is increasing concern about the
risk of developing deep vein thrombosis
(see
thrombosis, deep vein)
during air
travel. The condition may be caused by
long periods of sitting in one position
or compression of the tissues, both of
which occur during long-haul flights.
The accelerative forces experienced
by civil aircraft passengers are mild,
even during take-off and landing, and
no medical precautions are necessary.
Military aircraft pilots, on the other
hand, may experience severe accelera-
tions and must wear special suits and
use a reclined seat to prevent pooling of
blood in the feet, which would cause
immediate loss of consciousness.
Motion sickness
usually causes fewer
problems during air travel than during
road or sea travel. Passengers who are
prone to motion sickness may benefit
from taking an anti-motion sickness
preparation.
Air travel allows the rapid crossing of
several time zones within a short period
of time, which can affect sleep-waking
cycles, causing
jet-lag.
AVIATION MEDICINE SPECIALISTS
Most large airlines have doctors who are
specially trained in aviation medicine
who are responsible for the healthcare
of the airline staff. The doctors also
give advice on the transportation of sick
passengers, the provision of training
and equipment to deal with illness
during flight, and the maintenance of
airline hygiene.
avitaminosis
See
hypovitaminosis.
Avloclor
A brand name for
chloroquine,
a drug
that is used to prevent and treat
malaria
.
Avloclor is also used in the treatment of
some
rheumatic
disorders,
such
as
rheumatoid arthritis.
avulsed tooth
A tooth that has become completely
dislodged from its socket following an
injury. If the tooth is kept clean and
moist (ideally by being stored in milk,
saliva, or contact-lens solution), is not
otherwise washed,
and treatment is
sought immediately, reimplantation (see
reimplantation, dental)
may be possible.
avulsion
The tearing away of a body structure
from its point of attachment. Avulsion
may be due to an injury, for example
excessive contraction of a
tendon
may
avulse a small piece of bone at its
attachment point. Avulsion may also be
performed as part of a surgical proce-
dure, as in the surgical removal of
varicose veins.
axilla
The medical name for the armpit.
axis
The second cervical
vertebra
in the
human
spine.
The axis is attached by a
pivot joint to the
atlas,
the topmost ver-
tebra, which in turn is attached to the
base of the skull. The pivot joint allows
the head to turn to either side.
axon
The thin, elongated part of a
neuron
(nerve cell) that conducts nerve impuls-
es. Many axons in the body are covered
with a fatty
myelin
sheath.
Ayurvedism
See
Indian medicine.
azathioprine
An
immunosuppressant drug
used to treat
severe
rheumatoid arthritis
and
other
autoimmune disorders
(in which the
im-
mune system
attacks the body’s own
tissues). The drug is also used to prevent
organ rejection after
transplant surgery
.
Azathioprine may be injected or given
in
tablet form.
Side
effects
include
increased susceptibility to infection.
azelaic acid
A
topical
(applied to the skin) drug that
is used to treat mild to moderate
acne.
azithromycin
A macrolide
antibiotic drug
used to treat
infections of the skin, chest, throat, and
ears. Azithromycin is also used to treat
genital infections due to chlamydia (see
chlamydial infections).
azoospermia
The absence of sperm from semen, caus-
ing
infertility
in males. Azoospermia may
be caused either by a congenital (pre-
sent from birth) disorder or by one that
develops later in life. It can also occur
following a
vasectomy
.
CAUSES
Congenital azoospermia may be due to
a
chromosomal abnormality
such as
Kline-
felter’s syndrome
(the presence of an
extra sex chromosome); failure of the
testes to descend into the scrotum;
absence of the vasa deferentia (ducts
that carry sperm from the testes to the
seminal vesicles, where it is stored prior
to
ejaculation);
or
cystic fibrosis
(a
genetic disease of the lungs and pan-
creas that may also cause defects of the
vasa deferentia).
In some males, azoospermia may be
the result of hormonal disorders affect-
ing the onset of puberty. Another cause
is blockage of the vasa deferentia, which
may follow a
sexually transmitted infec-
tion, tuberculosis,
or surgery on the groin.
Azoospermia can also be the result of
damage to the testes. This can follow
radiotherapy,
treatment
with
certain
drugs (for example,
anticancer drugs),
prolonged exposure to heat,
or the
effects
of occupational
exposure
to
toxic chemicals. In some cases, produc-
tion of sperm ceases permanently for
no known reason.
TREATMENT AND OUTLOOK
If the cause is treatable (with hormones
to bring on puberty or surgery to
unblock ducts closed by infection, for
example), sperm production may restart.
However, in some cases the testes will
have been permanently damaged.
AZT
The abbreviation for azidothymidine,
the former name for
zidovudine.
aztreonam
An
antibiotic drug
used to treat some
types of
meningitis
and infections by
certain
types
of bacteria,
including
P
seudomonas.
azygous
A term meaning not paired. Azygous
describes a structure such as the heart,
which does not have a twin organ on
the opposite side of the body. The azy-
gous
vein
drains
blood
from
the
abdomen and chest and travels along
the right side of the spine.
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