stroke Damage to part of the
by an interruption to its blood supply.
The interruption is most often due to the
blockage of a cerebral artery by a blood
clot, which may have formed within the
or may have
been carried into the artery in the circu-
lation from a clot elsewhere in the body
Stroke may also result
from localized haemorrhage due to rup-
ture of a blood vessel in or near the brain.
The incidence of stroke rises with age
and is higher in men. Certain factors in-
crease the risk. The most important are
by association, factors such as
that contribute to these disorders). Other
risk factors are
, a damaged
, and a recent
; these can cause clots in the
heart which may migrate to the brain.
Symptoms usually develop abruptly
and, depending on the site, cause, and
extent of brain damage, may include
headache, dizziness, visual disturbance,
and difficulty in swallowing. Sensation,
movement, or function controlled by the
damaged area of the brain is impaired.
Weakness or paralysis on one side of the
, is a common
effect of a serious stroke. A stroke that
affects the dominant cerebral hemisphere
may cause disturbance of language (see
). About a third of major strokes
are fatal, a third result in some disabil-
ity, and a third have no lasting ill effects
transient ischaemic attack)
In some cases, urgent treatment may
improve the chances of recovery.
CT scanning, chest X-rays, blood tests,
may be used to
investigate the cause and extent of brain
damage. If a stroke is proven by scan to
be due to thrombosis,
may be given.
given if there is an obvious source of an
embolism, such as atrial fibrillation or a
narrowed carotid artery. In some cases,
antiplatelet agents such as
given. In most cases, attention to hydra-
tion and pressure areas, and good nursing
care, are the most important influences
lost movement or sensation;
may help language disturbances.
stroma The tissue that forms an organ's
framework, as distinct from the functional
) and the fibrous
outer layer that holds the organ together.
strongyloidiasis An infestation of the
intestines by the parasitic worm
It is widespread in
the tropics. Strongyloidiasis is contract-
ed in affected areas by walking barefoot
on soil contaminated with faeces. Larvae
penetrate the soles, migrating via the
lungs and throat to the intestine. Here
they develop into adults and produce
larvae. Most larvae are passed in the fae-
ces, but some enter the skin around the
anus to begin a new cycle. A person
may be infested for more than 40 years.
The larvae cause itching and red weals
where they enter the skin. In the lungs
they may cause
Heavy intestinal infestation may cause
swelling of the abdomen and diarrhoea.
Occasionally, an infected person with
reduced immunity dies of complica-
tions, such as
Treatment with an
, kills the worms.
strontium A metallic element occurring
in various compounds in certain miner-
als, seawater, and marine plants.
A radioactive variety, strontium 90, is
produced during nuclear reactions and
may be present in nuclear fallout. Stron-
tium 90 accumulates in bone, where the
it emits may cause
. Other forms of
radioactive strontium have been used
to diagnose and treat bone tumours.
strychnine poisoning Strychnine is a
poisonous chemical found in the seeds
of Strychnos species (tropical trees and
shrubs). Its main use is as an ingredient
in some rodent poisons; most cases of
strychnine poisoning occur in children
who accidentally eat such poisons.
Symptoms begin soon after ingestion
and include restlessness, stiffness of the
face and neck, increased sensitivity of
hearing, taste, and smell, and
, followed by alternating episodes of
seizures and floppiness. Death may occur
The victim is given intravenous injec-
tions of a