nitrofurantoinAn antibacterial drugthat
is used in the treatment of urinary tract
infection. Nitrofurantoin should be taken
with food to reduce the risk of stomach
irritation, abdominal pain, and nausea.
More rarely, breathing difficulty, numb-
ness, and jaundice occur.
nitrogenA colourless, odourless gas
that makes up 78 per cent of the Earth’s
atmosphere. Although nitrogen gas
cannot be utilized by the body, com-
pounds of nitrogen, such as amino
acids, are essential to life.
nitrous oxide (N 2 O)A colourless gas,
sometimes called laughing gas. Nitrous
oxide is used with oxygen to provide
analgesia(pain relief) and light anaes-
thesia (see anaesthesia, general).
Adverse effects of nitrous oxide and
oxygen may include nausea and vomit-
ing during the recovery period.
nitsThe eggs of lice. Both head lice and
pubic lice produce eggs, which they stick
to the base of hairs. Nits measure only
about 0.5 mm in diameter. They are
light brown when newly laid, and white
when hatched. (See also lice; pubic lice.)
nocardiosisAn infection caused by a
fungus-like bacterium present in soil.
The infection, acquired through inhala-
tion, usually starts in the lung and
spreads via the bloodstream to the brain
and tissues under the skin. Nocardiosis
is rare except in people with immuno-
deficiency disordersor those already
suffering from another serious disease.
The infection causes a pneumonia-like
illness, with fever and cough. It fails to
respond to short-term, antibiotic treat-
ment, and progressive lung damage
occurs. Brain abscesses may follow.
Treatment is with sulphonamide drugs,
often in conjunction with other antibac-
terial drugs, for example trimethoprim.
nocturiaThe disturbance of sleep at
night by the need to pass urine.
A common cause of nocturia in men is
enlargement of the prostate gland (see
prostate, enlarged), which obstructs the
normal outflow of urine and causes
the bladderto empty incompletely. In
women, a common cause is cystitis
(inflammation of the bladder), in which
irritation of the bladder wall increases
its sensitivity so that smaller volumes
of urine trigger a desire to urinate. Other
causes of nocturia include diabetes melli-
tus, heart failure (reduced pumping
efficiency), chronic kidney failure, and
diabetes insipidus.
nocturnal emissionEjaculation that
occurs during sleep, commonly called a
‘wet dream’. Nocturnal emission is nor-
mal in male adolescents.
nodeA small, rounded mass of tissue.
The term most commonly refers to a
lymph node, a normal structure in the
lymphatic system. (See also nodule.)
noduleA small lump of tissue. A nodule
may protrude from the skin’s surface or
form deep under the skin. Nodules may
be either hard or soft.
noiseAny sound, particularly one that is
disordered and irregular, that is unwan-
ted or interferes with the ability to hear.
(See also noise-induced hearing loss.)
noise-induced hearing loss Hearing
loss caused by prolonged exposure to
excessive noise or by brief exposure
to intensely loud noise.
Exposure to a sudden, very loud noise,
usually above 130 decibels, can cause
immediate and permanent damage to
hearing. Normally, muscles in the middle
earrespond to loud noise by altering
the position of the ossicles (the chain of
bones that pass vibrations to the inner
ear), thus damping down the intensity
of the noise. If these protective reflexes
have no time to respond, the full force
of the vibrations is carried to the inner
ear, severely damaging the delicate hair
cells in the cochlea. Occasionally, loud
noises can rupture the eardrum.
More commonly, noise damage occurs
over a period of time by prolonged expo-
sure to lower levels of noise. Any noise
above 85–90 decibels may cause dam-
age, with gradual destruction of the hair
cells of the cochlea, leading to perma-
nent hearing loss. Prolonged exposure
to loud noise leads initially to a loss of
the ability to hear certain high tones.
Later, deafness extends to all high fre-
quencies, and the perception of speech
is impaired. Eventually, lower tones are
also affected.
Sounds at 85–90 decibels or above may
cause pain and temporary deafness. Pro-
longed tinnitus(ringing or buzzing inNITROFURANTOIN NOISE-INDUCED HEARING LOSS
N