DIABETES INSIPIDUS (DI)
The quantity of water retained in the body is closely controlled by
several mechanisms, one of the most important of which is a hormone (a
chemical secreted into the blood by an endocrine gland) secreted by the
posterior pituitary gland, a structure which lies deep within the brain.
This is called anti-diuretic hormone (ADH) because its function is to control
the loss of water as urine from the kidneys. The amount of ADH secreted
by the posterior pituitary is determined by sensors in the brain which
monitor the concentration of the blood. If the blood becomes too concentrated
more ADH is secreted and the kidneys retain water, if it is too dilute
less ADH is secreted and the kidneys release more urine. Adults on average
drink and pass about a litre and a half of fluid each day and children
proportionately less. If their fluid intake increases the blood concentration
falls, the secretion of ADH is reduced and more urine is passed, if their
fluid intake reduces the opposite happens.
Cause:
If for any reason the function of the posterior pituitary gland fails,
the secretion of ADH is inadequate and the kidneys lose too much water.
DI can occur spontaneously without any apparent cause and is then referred
to as Òidiopathic DIÓ. It may also be genetically determined
or can result from destruction of the posterior pituitary by disease, a
head injury or even a tumour. The gland can be shown well on CT or MR scanning
of the brain and a scan is always needed in investigation. The increased
loss of urine (ÔpolyuriaÕ) is reflected in greatly increased
thirst and fluid intake (polydipsia) but eventually it may become impossible
to take in enough water and the person with this condition will become
dehydrated.
Symptoms:
The presenting symptoms of DI are thus excessive urine output and excessive
thirst. Depending on the degree to which ADH secretion is lost the symptoms
can be quite mild or so great that some affected children virtually give
up eating and lose a lot of weight. If fluid is not immediately available
they will drink from flower vases, lavatory cisterns, puddles or anywhere
else. In general affected children remain well but if they become dehydrated
they may seem obviously ill. When these symptoms are first noticed many
parents naturally assume this is a behavioural problem and try to restrict
the child's drinking. This is obviously very upsetting for a child with
severe thirst but is fortunately seldom harmful.
Investigations:
To assess the concentrating power of the kidneys it is usually necessary
to deprive the child of fluid for some hours, measuring the concentration
of the blood and the urine. DI is one of the most important reasons for
failure to concentrate the urine normally. There are other causes, some
of them much more common, including diabetes mellitus, in which the excessive
urine production is due to glucose in the urine, and excessive drinking
for psychological reasons, which is seldom seen in children.
Treatment:
ADH is available but is destroyed in the stomach so can only be given
by injection. Fortunately similar and even more effective chemical compounds
have been developed which are well absorbed through the lining of the nose
and adequately absorbed by mouth. The most effective of these is DDAVP
or Desmopressin. This is available as a fluid which can be measured into
a small tube and blown up the nose (easier than it sounds) which allows
exact adjustment of the dose. Most children require a dose from 5-20micrograms
each 12 hours to control water balance fully. There is also available a
spray, Desmospray, which delivers a standard dose of 10mcg and tablets,
Desmotabs, which contain 200mcg.
Outlook:
This clearly depends on the cause of the problem but in general DI is
a permanent condition. However, it is virtually always possible to restore
normal fluid intake and output with DDAVP and so relieve the symptoms completely.
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