the enamel, it spreads into the dentine
and permits bacteria to enter the pulp.
(See also caries, dental.)
decay, dentalSee caries, dental.
decerebrateThe state of being without
a functioning cerebrum, the main con-
trolling part of the brain. It occurs if the
brainstemis severed, which effectively
isolates the cerebrum.
deciduous teethSee primary teeth.
decompression sicknessA hazard of
divers and of others who work in or
breathe compressed air or other gas
mixtures. Decompression sickness is
also called “the bends”, and it results
from gas bubbles forming in the tissues
and impeding the flow of blood. At
depth, divers accumulate inert gas in
their tissues from the high-pressure gas
mixture that they breathe (see scuba-
diving medicine). Problems can usually
be avoided by allowing the excess gas
in their tissues to escape slowly into
the lungs during controlled, slow ascent
or release of pressure. If ascent is too
rapid and pressure falls too quickly, gas
can no longer be held within a tissue.
Resulting bubbles may block blood ves-
sels, causing symptoms such as skin
itching and mottling and severe pain in
and around the larger joints. Symptoms
of nervous system impairment (such as
leg weakness or visual disturbances) are
particularly serious, as is a painful, tight
feeling across the chest.
Divers with decompression sickness
are immediately placed inside a recom-
pression chamber. Pressure within the
chamber is raised, causing the bubbles
within the tissues to redissolve. Subse-
quently, the pressure in the chamber is
slowly reduced, allowing the excess gas
to escape safely via the lungs. If treated
promptly, most divers with the “bends”
make a full recovery. In serious, untreated
cases, there may be long-term prob-
lems, such as paralysis.
decompression, spinal canalSurgery
to relieve pressure on thespinal cord or
a nerve root emerging from it (see micro-
discectomy). Pressure may have various
causes, including a disc prolapse, a
tumour or abscess of the spinal cord, or
a tumour, abscess or fracture of the verte-
brae. Any of these conditions can causeweakness or paralysis of the limbs and
loss of bladder control.
To treat major disc prolapses and
tumours, a laminectomy(removal of the
bony arches of 1 or more vertebrae) to
expose the affected part of the cord or
nerve roots may be performed. Recovery
after treatment depends on the severity
and duration of the pressure, the success
of the surgery in relieving the pressure,
and whether any damage is sustained
by the nerves during the operation.
decongestant drugs Drugs that are
used to relieve nasal congestioncom-
monly in people with upper respiratory
tract infections. They work by narrowing
blood vessels in the membranes lining
the nose. This action reduces swelling,
inflammation, and the amount of mucus
produced by the lining. Common drugs
include ephedrine, oxymetazoline, and
phenylephrine. Small amounts of these
drugs are present in many over-the-
counter cold remedies. Taken by mouth,
decongestant drugs may cause tremor
and palpitations. Adverse effects are
unlikely with nose drops, but if taken for
several days they become ineffective
and symptoms may then recur or wors-
en despite continued treatment.
decubitus ulcerSee bedsores.
deep vein thrombosisSee thrombo-
sis, deep vein.
defaecationThe expulsion of faeces
from the body via the anus.
defence mechanisms Techniques used
by the mind to cope with unpleasant or
unwelcome emotions, impulses, experi-
ences, or events. Repression of emotions
surrounding a particular event or refus-
ing to accept an event (denial) are both
defence mechanisms.
defibrillationAdministration of one or
more brief electric shocks to the heart,
usually via 2 metal plates, or paddles,
placed on the chest over the heart. It is
performed to return a heart’s rhythm to
normal in some types of arrhythmia
(irregular or rapid heartbeat), such as
atrial fibrillationor ventricular fibrilla-
tion. Defibrillation can be carried out as
an emergency procedure to treat ven-
tricular fibrillation, which is a cause of
cardiac arrestand most commonly occurs
after a heart attack (see myocardialDECAY, DENTAL DEFIBRILLATIOND