cause of infertility, especially when other
disorders of the sperm are also present.
Normally, there are more than 20 mil-
lion sperm per millilitre of semen. A low
sperm count can be due to various dis-
orders, including orchitis, undescended
testis (see testis, undescended), and,
infrequently, a varicocele(varicose vein
of the testis). Smoking, alcohol abuse,
stress, and some drugs may cause tem-
porary oligospermia. Treatment is for
the underlying cause. If the cause is
unknown, gonadotrophin hormonesmay
be prescribed. (See also azoospermia.)
oliguriaThe production of low quanti-
ties of urinein proportion to the volume
of fluid taken in. The condition may be
caused by excessive sweating; in some
cases, it is a sign of kidney failure.
olive oilAn oil, obtained from the fruit
of the olive tree OLEA EUROPAEA, that
may be used to soften earwax or to treat
cradle capin babies.
-omaA suffix denoting a tumour, which
may be cancerous or noncancerous, as
in lipomaand carcinoma.
omentumA double fold of fatty mem-
brane hanging in front of the intestines.
omeprazoleA drug that is used to treat
peptic ulcer, reflux oesophagitis, and
Zollinger–Ellison syndrome. Adverse ef-
fects include rashes, headache, nausea,
diarrhoea, and constipation.
omphaloceleAn alternative name for
exomphalos.
onchocerciasisA tropical disease, also
called river blindness, caused by the
worm ONCHOCERCA VOLVULUS. The dis-
ease is a type of filariasistransmitted by
simulium flies. The worms’ dead larvae
can lead to blindness if they cause an
allergic reaction in or near the eyes.
Treatment is with anthelmintic drugs.
oncogenesGenes found in every cell that
control growth, repair, and replacement.
Abnormalities of oncogenes are known
to be a factor in the development of can-
cerous cells.Mutationsin oncogenes,
resulting from damage by carcinogens,
can cause a cell to grow unrestrainedly
and infiltrate and destroy normal tissues
(see cancer). Factors known to cause
cancer include ultravioletlight, radioac-
tivity, tobacco, alcohol, asbestos, some
chemicals, and certain viruses.oncologyThe study of the causes, devel-
opment,characteristics, and treatment
of tumours, particularly cancers.
ondansetron A serotonin antagonist drug
used to control nausea and vomiting
following an operation or induced by
radiotherapy or anticancer drugs. It is
taken as tablets or suppositories, or given
by injection. Side effects may include
constipation, headache, and hiccups.
onychogryphosis Abnormal thicken-
ing, hardening, and curving of the nails
that occurs mainly in elderly people.
Onychogryphosis may be associated
with fungal infectionor poor circulation.
onycholysisSeparation of the nail from
its bed: a feature of many skin condi-
tions, including psoriasisand dermatitis.
oophorectomyRemoval of the ovaries,
usually to treat ovarian cystsor cancer
(see ovary, cancer of). A partial oophor-
ectomy may be performed to preserve
ovarian function in women under 40. In
a hysterectomy, both ovaries may be
removed if disease has spread from the
uterus, or to prevent ovarian cancer from
developing in the future. The ovaries
may be removed as part of the treat-
ment for breast cancerif growth of the
tumour depends on hormones produced
by the ovary. If both ovaries are removed
before the menopause, hormone replace-
ment therapy (HRS) may be needed.
-opathyA suffix that denotes a disease
or disorder, as in neuropathy(a disor-
der of the peripheral nerves).
open heart surgeryAny operation on
the heartin which it is stopped tem-
porarily and its function taken over by a
mechanical pump. The main forms of
open heart surgery are correction of con-
genitalheart defects (see heart disease,
congenital), surgery for narrowed or leaky
heart valves (see heart-valve surgery),
and coronary artery bypass surgery. Once
the pump is connected, the heart is
opened, and the defects repaired. Surgi-
cal hypothermia is used to keep the
heart cool and help prevent damage to
the heart muscle from lack of oxygen
(see hypothermia, surgical).
operableA term applied to a condition
that is suitable for surgical treatment,
such as an accessible noncancerous
tumour. (See also inoperable.)OLIGURIA OPERABLEO