is characterized by reduced sweating
and is accompanied by dry, wrinkled
skin, sparse hair, small, brittle nails,
and conical teeth. Other causes of
hypohidrosis include exfoliative der-
matitisand some anticholinergic drugs.
hypokalaemiaA deficiency of potassium
in the blood. Hypokalaemia is usually
caused by excess fluid loss due, for
example, to severe diarrhoea, but which
may be the result of treatment with
diuretic drugs.
hypomaniaA mild degree of mania.
hypoparathyroidismInsufficient pro-
duction of parathyroid hormoneby the
parathyroid glands. A deficiency of this
hormone results in low levels of calcium
in the blood (hypocalcaemia).
The most common cause of hypopara-
thyroidism is damage to the parathyroid
glands during surgery. Occasionally, the
parathyroid glands are absent from
birth, or they may cease to function for
no apparent reason.
A low blood calcium level may cause
tetany. Occasionally, seizuressimilar to
those of an epileptic attack may occur.
The condition is diagnosed by blood
tests. To relieve an attack of tetany, cal-
cium may be injected slowly into a vein.
To maintain the blood calcium at a nor-
mal level, a lifelong course of calcium
and vitamin Dtablets is necessary.
hypophysectomyThe surgical removal
or destruction (by means of a radioac-
tive implant) of the pituitary gland. This
may be performed to remove pituitary
tumours or to treat some cancers of the
breast, ovary, or prostate gland, the
growth of which is stimulated by hor-
mones secreted by the pituitary gland.
hypopituitarismUnderactivity of the
pituitary gland, resulting in inadequate
production of 1 or more pituitary hor-
mones. The effects depend on which
hormones are affected. Possible causes
are a pituitary tumour, an abnormality
affecting the hypothalamus, or injury to
the pituitary gland. Hypopituitarism may
also follow surgery or radiotherapyof
the pituitary gland. Treatment involves
replacing the deficient hormones.
hypoplasiaThe failure of an organ or a
body tissue to develop fully and to reach
its normal adult size.
hypoplasia, enamelA defect in tooth
enamel (see enamel, dental), usually
due to amelogenesis imperfecta. It may
also be caused by vitamin deficiency,
injury, or infection of a primary tooth
that interferes with enamel maturation.
hypoplastic left-heart syndromeA
very serious form of congenital heart
disease (see heart disease, congenital).
The baby is born with a poorly formed
left ventricle, often associated with
other heart defects. The aortais mal-
formed and blood can reach it only via a
duct (the ductus arteriosus) that links
the aorta to the pulmonary artery.
At birth, the baby may seem healthy.
However, within a day or 2 the ductus
arteriosus naturally closes off and the
baby collapses, becoming pale and
breathless. In most cases, hypoplastic
left-heart syndrome cannot be treated
surgically, and most affected babies die
within a week. A few infants have been
treated with heart transplants.
hyposensitizationA preventive treat-
ment of allergyto specific substances,
such as grass pollens and insect venom.
Hyposensitization involves giving grad-
ually increasing doses of the allergen so
that the immune systembecomes less
sensitive to that substance. The treat-
ment, which may need to be repeated
annually for a few years, carries the risk
of anaphylactic shock.
hypospadiasA congenitaldefect of the
penis, in which the opening of the ure-
thrais on the underside of the glansor
shaft. In some cases, the penis curves
downwards, a condition that is known
as called chordee. Hypospadias can usu-
ally be corrected by surgery.
hypotensionThe medical term for low
blood pressure. In its most common form,
known as postural hypotension, symp-
toms occur after abruptly standing or
sitting up. Normally, blood pressure in-
creasesslightly with changes in posture;
in people with postural hypotension, this
normal increase fails to occur. Postural
hypotension may be a side effect of
antidepressant drugsor antihypertensive
drugs. It may also occur in people with
diabetes mellitus. Acute hypotension is
a feature of shock, and may be caused
by serious injury or a disease such asHYPOKALAEMIA HYPOTENSION
H