accompanied by pain on moving the
eyes. Vision usually improves within 6
weeks, but some optic nerve fibres will
be damaged. Recurrent attacks usually
lead to permanent loss of visual acuity.
Most cases are thought to be due to
demyelination of the optic nerve fibres in
multiple sclerosis. The condition may also
result from inflammation or infection of
tissues around the optic nerve. Cortico-
steroid drugsmay help to restore vision,
but seem to have little effect on long-
term outcome. (See also optic atrophy.)
optometryThe practice of assessing
visionto establish whether glasses or
contact lenses are needed to correct a
visual defect, as carried out by an
optometrist. Disorders of the eye may
require treatment by an ophthalmolo-
gist. (See also ophthalmology;optician.)
oralConcerning the mouth.
oral contraceptivesA group of oral
drug preparations containing one or
more synthetic female sex hormones,
taken by women in a monthly cycle to
prevent pregnancy. “The pill” commonly
refers to the combined or the phased pill,
which both contain an oestrogen drug
and a progestogen drug, and the mini-
pill, which contains only a progestogen.
Oestrogen pills include ethinylestradiol;
progestogens include levonorgestrel and
norethisterone. When used correctly, the
number of pregnancies among women
using oral contraceptives for one year is
less than 1 per cent. Actual failure rates
may be 4 times higher, particularly for
the minipill, which has to be taken at
precisely the same time each day.
Combined and phased pills increase
oestrogen and progesterone levels. This
interferes with the production of two
hormones, luteinizing hormone (LH)
and follicle-stimulating hormone(FSH),
which in turn prevents ovulation. The
minipill works mainly by making the
mucus lining of the cervixtoo thick to
be penetrated by sperm.
Oestrogen-containing pills offer protec-
tionagainst uterine and ovarian cancer,
ovarian cysts, endometriosis, and iron-
deficiencyanaemia. They also tend to
make menstrual periods regular, lighter,
and relatively pain-free. Possible side
effects include nausea, weight gain,
depression, swollen breasts, reduced sex
drive, increased appetite, leg and abdom-
inal cramps, headaches, and dizziness.
More seriously, there is a risk of throm-
bosiscausing a strokeor a pulmonary
embolism. These pills may also aggravate
heart disease or cause hypertension,
gallstones, jaundice, and, very rarely, liver
cancer. All oral contraceptives can cause
bleeding between periods, especially
the minipill. Other possible adverse
effects of the minipill include irregular
periods, ectopic pregnancy, and ovarian
cysts. There may be a slightly increased
long-term risk of breast cancer for
women taking the combined pill.
Oestrogen-based pills should generally
be avoided in women with hypertension,
hyperlipidaemia, liver disease, migraine,
otosclerosis, or who are at increased risk
of a thrombosis. They are not usually pre-
scribed to a woman with a personal or
family history of heart or circulatory dis-
orders, or who suffers from unexplained
vaginal bleeding. The minipill or a low-
oestrogen pill may be used by women
who should avoid oestrogens. Combined
or phased pills may interfere with milk
production and should not be taken
during breast-feeding. Certain drugs
may impair the effectiveness of oral
contraceptives. (See also contraception.)
oral hygieneMeasures to keep the
mouth and teeth clean and reduce the
risk of tooth decay (see caries, dental),
gingivitisand other gum disorders, and
halitosis. Oral hygiene includes regular,
thorough toothbrushingand flossing (see
floss, dental) to remove plaque. Disclosing
agentshelp to reveal build-up of plaque.
Dentures are brushed on all surfaces and
soaked in cleansing solution.
Professional treatment to remove cal-
culusand stubborn plaque by scaling
and polishing is usually carried out by a
dentist or dental hygienist during a rou-
tine check-up. In periodontal disease,
treatment may be needed more often.
oral rehydration therapySee rehy-
dration therapy.
oral surgeryThe branch of surgery
that treats deformity, injury, or disease
of the teeth, jaws, and other parts of the
mouth. Procedures include the extrac-
tion of impacted wisdom teeth (seeOPTOMETRY ORAL SURGERY
O