Diabetes insipidus (DI) is characterized by the decreased ability of the kidneys to concentrate urine.
- Antidiuretic hormone (ADH) is the primary determinant of free water balance
- ADH is produced in posterior pituitary and acts on the V2 receptors of the collecting tubules of the kidney
- ADH alters the permeability of the collecting tubes to control the free water excretion
DI can be due to different distinct mechanisms (Figure 117-1).
Figure 117-1. Mechanisms of Diabetes Insipidus
- Rare disease with prevalence of 3 per 100,000 population
- No significant sex gender difference
- Adults—Polyuria, polydipsia, nocturia, and craving for ice water
- Children—Anorexia, growth defects, enuresis, sleep disturbance, fatigue, difficulty at school
- Infants—Irritability, chronic dehydration, growth retardation, neurologic disturbance, and hyperthermia
- If thirst mechanism is intact—
- Hydronephrosis and distended bladder due to excessive urinary volume
- If no access to free water or damage to hypothalamic thirst center then—
- Hypernatremia, dehydration, hypertonic encephalopathy, obtundation, coma, seizure, subarachnoid hemorrhage, and intracerebral hemorrhage
- Diabetes mellitus
- Cushing syndrome
- Psychogenic polydipsia
- No single diagnostic laboratory test
- 24-hour urine output of less than 2 liters rules out DI
- Hyperuricemia can be seen as urate clearance is reduced due to reduced V1 stimulation
- MRI of the pituitary and hypothalamus should be done to rule out mass lesions
- In T1-weighted MRI, the normally present bright spot in the sella is lost in most DI patients
- Water deprivation test is the gold standard for diagnosing DI (see Figure 117-2)
- Check baseline Na+; do not permit oral intake, measure volume and osmolality of each voided urine sample; weigh patient
- When two consecutive urine osmolality do not vary by more than 10% and the patient has lost 2% of weight, check Na+, urine osmolality and serum vasopressin levels. Then give 2 mg of desmopressin if needed
Figure 117-2. Diagnosis of Diabetes Insipidus
- Goal: to prevent nocturnal enuresis and to control polydipsia
- General: Avoid dehydration by drinking fluids to match the urine output and by providing intravenous fluid replacement with hypo-osmolar fluid
Treatment of Diabetes Insipidus
- Has antidiuretic and vasopressor activity
- Continuous low-dose infusion preserves organs after brain death
- Antivasopressin antibodies can be formed
- Caution: can cause myocardial ischemia, therefore concurrent use of nitrates is advisable in susceptible patients
- Synthetic analog of vasopressin
- Has markedly reduced pressor activity and is 2000 times more specific for antidiuresis...
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