Webster's New World Medical Dictionary

hydrocephalus, communicating
Hydrocephalus
in which there is no obstruction to the flow of the
cerebrospinal fluid (CSF). Specifically, there is no
obstruction within the ventricular system of the
brain or where the CSF passes into the spinal canal.
Communicating hydrocephalus is due to overpro-
duction of CSF or failure of the brain to reabsorb
CSF normally.
hydrocephalus, congenital
Hydrocephalus that
is present at birth. See also 
hydrocephalus.
hydrocephalus, normal pressure
Hydro-
cephalus that occurs because of a gradual blockage
of CSF drainage pathways in the brain. Although the
ventricles enlarge, intracranial pressure remains
within normal range. Abbreviated NPH. NPH can
occur as a complication of brain infection or bleed-
ing (hemorrhage). In some patients, no predispos-
ing cause can be identified. NPH is characterized by
memory loss (dementia), gait disorder (ataxia),
urinary incontinence, and a general slowing
of activity. See also 
hydrocephalus; hydrocephalus
ex-vacuo.
hydrocephalus ex-vacuo
Hydrocephalus that
occurs when there is damage to the brain caused by
stroke or injury, in which there may be an actual
shrinkage of brain substance. The CSF pressure
itself is normal. See also 
hydrocephalus; hydro-
cephalus, normal pressure.
hydrocephaly
See 
hydrocephalus.
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hydrocephaly
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hydrogen breath test
A test that uses the meas-
urement of hydrogen in the breath to diagnose sev-
eral conditions that cause gastrointestinal
symptoms. Large amounts of hydrogen may be pro-
duced by anaerobic bacteria in the colon when
there is a problem with the digestion or absorption
of food in the small intestine that allows more unab-
sorbed food to reach the colon, or when the colonic
bacteria move back into the small intestine.
Hydrogen produced by the bacteria is absorbed into
the blood and travels to the lungs where it is exhaled
in the breath and measured. The diagnosis of lac-
tose intolerance is one of the most common indica-
tions for use of a hydrogen breath test.
hydronephrosis
Distention of the kidney with
urine. Hydronephrosis is caused by obstruction of
urine outflow (for example, by a stone blocking the
ureter).
hydrops fetalis
A serious and potentially fatal
condition of a fetus characterized by gross edema
(swelling) from abnormal accumulation of fluids in
the body. Hydrops fetalis has both immune-related
and non-immune-related causes. Immune-related
hydrops fetalis can be due to hemolytic disease such
as Rh blood group incompatibility, in which antibod-
ies crossing the placenta from the mother destroy
the red blood cells of the fetus. Non-immune-related
hydrops fetalis can have many causes but is most
commonly a result of cardiac abnormalities in the
fetus. See also 
Rh incompatibility.
hydroxyapatite
An essential ingredient of nor-
mal bone and teeth that makes up bone mineral and
the matrix of teeth and gives them their rigidity. See
also 
hydroxyapatite crystal disease.