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08.07.05 (3 years ago)
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Diabetes insipidus is a well known complication of tuberclous meningitis, and for diagnosis, water deprivation test is done.
Water deprivation test: Involves fluid restriction followed by AVP administration. In normals, dehydration causes maximum AVP release, which produces maximal urine concentration. Thus, administration of exogenous AVP will have no further effect on Urine osmolality.
Method: All fluids are withheld until patient is sufficiently dehydrated to provide a potent stimulus for AVP secretion (Plasma osmolality > 295 mOsm/kg). Deprivation lasts 4-18 hours (4-6 hours [start at 6 a.m.] in patients with Urine Output > 10 L/d, longer for milder polyuria [start at 10 p.m.-midnight], and 16+ hours for patients with primary polydipsia, with hourly measurements of body weight and Uosm, until 2-3 consecutive samples vary by less than 30 mOsm/kg (or < 10%) or until patient loses 5% of body weight. At this point, AVP is measured and 5 U AVP or 1 µg of DDAVP (use DDAVP if patient pregnant) is injected SC, and Uosm is measured 30 and 60 minutes later. Posm is measured before starting fluid deprivation, at the end of fluid deprivation, and after AVP administration. If severe weight loss or dehydration occurs, prompt measures to restore hydration should be taken.
Interpretation: 95% sensitivity/specificity if performed properly.
a. If dehydration does not increase Uosm above Posm despite evident dehydration, primary polydipsia can be excluded. If neither Uosm nor Posm increases during dehydration and body weight does not decrease according to the urinary volume, surreptitious drinking should be suspected.
b. Normal individuals and those with dipsogenic DI display Uosm > Posm following fluid restriction, and a minimum (< 10%) increase in Uosm after AVP injection. Test is not diagnostic of disease if Uosm > Posm (see below).
c. In central DI, Uosm remains below Posm after dehydration, and after AVP injection, Uosm increases by > 50%.
d. In nephrogenic DI, Uosm remains below Posm, and after AVP injection, Uosm increases by < 50%. Uosm may also increase < 50% in partial central DI. The level of AVP at the end of dehydration is elevated in nephrogenic DI (10-20 pg/mL); also, there is no increase in urinary aquaporin-2 levels after DDAVP is given.
e. In partial (central or nephrogenic) DI (and dipsogenic DI), dehydration may lead to Uosm > Posm (Uosm/Posm = 1-1.4 may indicate partial central DI). Response to AVP is not informative in this case. Further testing must be done to distinguish partial central vs. partial nephrogenic DI vs. dipsogenic DI
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