Central
Diabetes InsipidusThe central DI can be inborn from a hypothalamic
harmatoma where hypothalamic hormone production is diminished. Not
infrequently it can also be acquired as Ref. 5 reviewed. This reference showed
that MRI studies can be useful to follow these patients. The reasons for this
can be quite varied and include metastasizing tumors, trauma (basal skull fracture),
brain sarcoidosis, brain tuberculosis, cerebral aneurysms and destructive hypothalamic
lesions from meningitis or encephalitis. Finally
there is a rare genetic abnormality of the vasopressin gene on chromosome 20,
which is responsible for an autosomal dominant form of a familial diabetes insipidus.
However, apart from the cases mentioned above, most cases still remain unexplained
as to why they occur. Symptoms: The two cardinal symptoms
as mentioned in the introduction chapter on DI are polydipsia (excessive
thirst and fluid intake) and polyuria (frequent trips to the bathroom
with immense amounts of urine). The urine is very diluted and if osmolality is
measured, it is less than 200 mOsm per liter. The patient has to go to the bathroom
at nights as well (nocturia) contrary to psychogenic water drinking. Diagnosis:
The simplest way to diagnose central DI is in the hospital setting by way
of the water deprivation test. This test consists of
a period of fluid deprivation, which in normal persons would lead to urine concentration. However,
in central DI patients the urine osmolality never exceeds that of the plasma osmolality.
When vasopressin is give by injection, the normal person would only increase
the urine osmolality by an additional 5%, whereas the central DI would concentrate
it now by more than 50%. In contrast, a patient with nephrogenic DI would not
show any response of the urine osmolality to the vasopressin injection. Treatment:
Hormone replacement therapy with a vasopressin analogue is the treatment of choice.
In the past vasopressin had to be given several times per day by needle subcutaneously.
This was necessary as the regular vasopressin wears off in 6 hours or less.
However, all of this changed with the arrival of desmopressin acetate, or DDAVP.
This
is an acronym for a complicated chemical name of an arginine derivative of vasopressin
that lasts for between 12 and 24 hours and can be given by nasal spray (Desmopressin
nasal spray, brand names: DDAVP, Stimate, Minirin). Each puff delivers 10 micrograms
of DDAVP and most people need only one puff in the morning and one puff in the
evening. It is well tolerated and normalizes the life of a patient with central
DI. It is also availabel by injection. Side effects can be high blood pressure
and hyponatremia (when overdosed, see Ref. 10). Prognosis:
As long as the central DI is not due to an incurable tumor, life expectancy with
the use of the DDAVP nasal spray for such a patient is normal.
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