Drugs such as donepezil and rivastig-
mine work by blocking the action of
acetylcholinesterase, the enzyme in the
brain responsible for the breakdown of
acetylcholine. This raises acetylcholine
levels, and, in half of all patients, the
drugs slow the rate of progression of
dementia. They have no effect on
dementia due to other causes, such as
stroke or head injury, however. Com-
mon side effects include nausea,
dizziness, and headache. Rarely, diffi-
culty in passing urine may occur.
acetylcysteineA drug used in the treat-
ment of paracetamoloverdose and as a
mucolytic drugto loosen sputum. When
the drug is taken in large doses, vomiting
or rash may occur as rare side effects.
achalasiaA rare condition of unknown
cause in which the muscles at the lower
end of the oesophagusand the sphinc-
ter (valve) between the oesophagus and
the stomach fail to relax to let food into
the stomach after swallowing. As a
result, the lowest part of the oesopha-
gus is narrowed and becomes blocked
with food, while the part above widens.
Symptoms include difficulty and pain
with swallowing and pain in the lower
chest and upper abdomen.
A barium swallow (a type of barium
X-ray examination) and gastroscopymay
be performed to investigate achalasia.
Oesophageal dila-
tation allows the
oesophagus to be
widened for long
periods. Surgery to
cut some of the
muscles at the sto-
mach entrance may
be necessary.
Achilles tendon
The tendon that
raises the heel.
The Achilles ten-
don is formed from
the calf muscles
(gastrocnemius,
soleus, and plan-
tar muscles) and
is attached to the
calcaneus(heel-bone).Minor injuries to
the Achilles tendon are common and
can result in inflammation (tendinitis).Violent stretching of the tendon can
cause it to rupture; in such cases, surgi-
cal repair may be necessary.
achlorhydriaAbsence of stomach acid
secretions. This may be due to chronic
atrophic gastritisor to an absence or
malfunction of acid-producing parietal
cells in the stomach lining. Achlorhydria
may not produce symptoms but is asso-
ciated with stomach cancer, however,
and is a feature of pernicious anaemia
(see anaemia,megaloblastic).
achondroplasiaA rare genetic disor-
der of bone growth that leads to short
stature. The condition is caused by a
dominant gene (see genetic disorders)
but often arises as a new mutation. The
long bones of the arms and legs are
affected mainly. The cartilage that links
each bone to its epiphysis (the growing
area at its tip) is converted to bone too
early, preventing further limb growth.
Those affected have short limbs, a well-
developed trunk, and a head of normal
size except for a protruding forehead.
aciclovirAn antiviral drugthat can be
taken by mouth, used topically, or given
intravenously to reduce the severity of
viral infections including herpes simplex
and herpes zoster. Local adverse reac-
tions commonly occur after topical use.
Other side effects are uncommon but
can include nausea and vomiting.
acidA substance defined as a donor of
hydrogen ions (hydrogen atoms with
positive electrical charges). Acid molec-
ules, when mixed with or dissolved in
water, split up to release their con-
stituent ions; all acids release hydrogen
as the positive ion. (See also acid–base
balance; alkali.)
acid–base balanceA combination of
mechanisms that ensures that the body’s
fluids are neither too acidnor too alka-
line (alkalisare also called bases).
The body has three mechanisms for
maintaining normal acid–base balance:
buffers, breathing, and the activities of
the kidneys. Buffers are substances in
the blood that neutralize acid or alka-
line wastes. Rapid breathing results in
the blood becoming less acidic; slow
breathing has the opposite effect. The
kidneys regulate the amounts of acid or
alkaline wastes in the urine.ACETYLCYSTEINE ACID–BASE BALANCEA
Calf
muscleAchilles
tendonACHILLES TENDONHeel