myocardial infarctionor adrenal failure.
Treatment depends on the cause. In the
absence of serious disease, low blood
pressure is associated with decreased
risk from cardiovascular disordersand
stroke. Symptomless hypotension does
not require treatment.
hypothalamusA region of the brain,
roughly the size of a cherry, situated
behind the eyes and the thalamus.
The hypothalamus controls the sym-
pathetic nervous system (part of the
autonomic nervous system). In response
to sudden alarm or excitement, signals
are sent from higher regions of the
brain to the hypothalamus, initiating
sympathetic nervous system activity.
This causes a faster heartbeat, widening
of the pupils, an increase in breathing
rate and blood flow to muscles.
Other nerve cells in the hypothalamus
are concerned with the control of body
temperature, thirst, and appetite for food.
The hypothalamus is also involved in
regulating sleep, motivating sexual be-
haviour, and determining mood and
emotions. It indirectly controls many
endocrine glandsthrough its influence
on the pituitary gland.
Disorders of the hypothalamus are us-
ually due to an intracerebral haemorrhage
or a pituitary tumour. They have diverse
effects, ranging from hormonal disorders
to disturbances in temperature regula-
tion, and increased or decreased need
for food and sleep.
hypothermiaA fall in body temperature
to below 35°C. Most cases occur in sick,
elderly people exposed to low tempera-
tures. The body loses its sensitivity to
cold as it ages, becoming less able to re-
verse a fall in temperature. Babies also
have an increased risk of hypothermia
because they lose heat rapidly and can-
not easily reverse a fall in temperature.
A person suffering from hypothermia
is usually pale and listless. The heart-
rateis slow, the body is cold, and the
victim is often drowsy and confused. In
severe hypothermia, breathing becomes
slow and shallow, the muscles are stiff,
the victim may become unconscious,
and the heart may stop beating.
Hypothermia is a medical emergency.
Treatment varies according to the age ofthe victim. A young person may be placed
in a warm bath. An elderly victim is usu-
ally warmed gradually by being covered
with layers of heat-reflecting material in a
room temperature of 25°C. When hypo-
thermia is life-threatening, victims may
be admitted to an intensive care unit for
controlled warming. This may be done by
withdrawing blood from the circulation,
warming it, and returning it to the body.
hypothermia, surgicalThe deliberate
reduction of body temperature to prolong
the period for which the vital organs can
safely be deprived of their normal blood
supply during open heart surgery. Cold
reduces the rate of metabolism in tissues
and thus increases their tolerance to lack
of oxygen. Cooling may be achieved by
continuously instilling cold saline at
about 4°C into the open chest cavity.
hypothyroidismThe underproduction of
thyroid hormonesby an underactive thy-
roid gland. Most cases are caused by an
autoimmune disordersuch as Hashimoto’s
thyroiditis. More rarely, hypothyroidism
results from the removal of part of the
thyroid gland to treat hyperthyroidism.
Symptoms include tiredness and leth-
argy. There may also be muscle weakness,
cramps, a slow heart-rate, dry skin, hair
loss, a deep and husky voice, and weight
gain. A syndrome calledmyxoedema, in
which the skin and other tissues thicken,
may develop. Enlargement of the thyroid
gland may also occur (see goitre). If the
condition occurs in childhood, it may
retard growth and normal development.
The disorder is diagnosed by measur-
ing the level of thyroid hormones in the
blood. Treatment consists of replace-
ment therapy with the thyroid hormone
thyroxine; usually for life.
hypotoniaAbnormal muscleslackness.
Normally, a muscle that is not being
used has a certain inbuilt tension, but
in a number of disorders affecting the
nervous system (such as Huntington’s
disease) this natural tension is reduced.
hypotonia in infantsExcessive limpness
in infants, also known as floppy infant
syndrome. Hypotonic babies cannot hold
their limbs up against gravity and so tend
to lie flat with their arms and legs splayed.
Hypotonia may be caused by Down’s
syndromeor hypothyroidismand may beHYPOTHALAMUS HYPOTONIA IN INFANTSH