amyotrophic lateral sclerosisSee
motor neuron disease.
amyotrophyShrinkage or wasting away
of a muscle, leading to weakness. Amy-
otrophy is usually due to poor nutrition,
reduced use of the muscle (as when a
limb is immobilized for a long period),
or disruption of the blood or nerve sup-
ply to the muscle (as can occur in
diabetes mellitusor poliomyelitis).
anabolic steroidsSee steroids,anabolic.
anabolismThe manufacture of complex
molecules, such as fatsand proteins,
from simpler molecules by metabolic
processes in living cells. (See also
catabolism; metabolism.)
anaemiaA condition in which the con-
centration of the oxygen-carrying pigment
haemoglobinin the blood is below nor-
mal. Haemoglobin molecules are carried
inside red blood cells and transport
oxygen from the lungs to the tissues.
Normally, stable haemoglobin concen-
trations in the blood are maintained by
a balance between red-cell production
in the bone marrow and red-cell des-
truction in the spleen. Anaemia may
result if this balance is upset.
Anaemia is not a disease but a feature
of many different disorders. There are
various types, which can be classified
into those due to decreased or defective
red-cell production by bone marrow (see
anaemia,aplastic; anaemia,megaloblastic;
anaemia,iron-deficiency) and those due
to decreased survival of the red cells in
the blood (see anaemia,haemolytic).
The severity of symptoms depends on
how low the haemoglobin concentra-
tion has become. Slightly reduced
levels can cause headaches, tiredness,
and lethargy. Severely reduced levels
can cause breathing difficulty on exer-
cise, dizziness, angina, and palpitations.
General signs include pallor, particular-
ly of the skin creases, the lining of the
mouth, and the inside of the eyelids.
Anaemia is diagnosed from the symp-
toms and by blood tests (see blood
count; blood film). A bone marrow biop-
symay be needed if the problem is with
red blood cell production.
anaemia, aplasticA rare but serious
type of anaemiain which the red cells,
white cells, and platelets in the blood
are all reduced in number. Aplastic
anaemia is caused by a failure of the
bone marrowto produce stem cells, the
initial form of all blood cells.
Treatment of cancer with radiotherapy
or anticancer drugs can temporarily
interfere with the cell-producing ability
of bone marrow, as can certain viral
infections and other drugs. Long-term
exposure to insecticides or benzene
fumes may cause more persistent aplas-
tic anaemia, and a moderate to high
dose of nuclear radiation is another
recognized cause. An autoimmune dis-
orderis responsible in about half of
all cases. Aplastic anaemia sometimes
develops for no known reason.
A low level of red blood cells may
cause symptoms common to all types
of anaemia, such as fatigue and breath-
lessness. White-cell deficiency increases
susceptibility to infections; platelet defi-
ciency may lead to a tendency to bruise
easily, bleeding gums, and nosebleeds.
The disorder is usually suspected from
blood-test results, particularly a blood
count, and is confirmed by a bone marrow
biopsy. Blood and platelet transfusions
can control symptoms. Immunosuppres-
sion is used to treat anaemia due to an
autoimmune process.Severe persistent
aplastic anaemia may be fatal unless a
bone marrow transplantis carried out.
anaemia, haemolyticA form of anaemia
caused by premature destruction of red
cells in the bloodstream (haemolysis).
Haemolytic anaemiascan be classified
according to whether the cause of haem-
olysis is inside or outside the red cells.
When haemolysis is due to a defect
inside the red cells, the underlying
problem is abnormal rigidity of the cell
membrane. This causes the cells to
become trapped, at an early stage of
their life-span, in the small blood vessels
of the spleen, where they are destroyed
by macrophages (cells that ingest for-
eign particles). Abnormal rigidity may
result from an inherited defect of the
cell membrane (as in hereditary sphero-
cytosis), a defect of the haemoglobinin
the cell (as in sickle-cell anaemia), or a
defect of one of the cell’s enzymes. An
inherited deficiency of the glucose-6-
phosphate dehydrogenase enzyme (seeAMYOTROPHIC LATERAL SCLEROSIS ANAEMIA, HAEMOLYTIC
A