diabetes%20insipidus
DIABETES INSIPIDUS
Diabetes insipidus is a polyuric disease characterized by excretion of a large volume of hypotonic urine and hypernatremia. It is due to the absence of antidiuretic hormone.
Central (hypothalamic or neurohypophyseal) diabetes insipidus is the inability to secrete & produce vasopressin in the neurohypophyseal system. It is due to damage to the pituitary gland & hypothalamus, may be due to diseases, head injuries, neurosurgery, infection or genetic or autoimmune disorders.
In nephrogenic diabetes insipidus, there is inappropriate renal response to vasopressin. Kidney function may be impaired by drugs & by chronic disorders like polycystic kidney disease, sickle cell disease, kidney failure, partial ureteral block, hypokalemia, hypocalcemia, low protein diet & genetic disorders.
Primary polydipsia have abnormal increase in fluid intake.

Diagnosis

The diagnosis of DI requires osmotic stimulation of arginine vasopressin (AVP) secretion & measuring the adequacy of AVP secretion by either direct measurement of plasma AVP levels or indirect assessment by urine osmolality

History

Medical History & Physical Exam

  • History of head trauma or previous surgery, etc
  • Exclude possible mimics
    • DM
    • Intrinsic renal disease
  • Physical exam may be normal
    • Check for signs of dehydration &/or manifestations of hyperosmolality

Laboratory Tests

DI, by determining presence of true hypotonic polyuria, is suggested if both of the following are present:

  • Urine output >2.5-3 L/24 hr (>40 mL/kg/24 hr) in adults or >100 mL/kg/24 hr in infants
  • Urine osmolality <200 mosmol/kg or urine specific gravity <1.01 (when serum osmolality is >310 mosmol/kg)

Other lab tests should be performed:

  • Blood glucose, blood urea nitrogen (BUN), creatinine, serum electrolytes, urinalysis

Alternative Diagnosis

  • The table below compare central neurogenic DI w/ other conditions such as syndrome of inappropriate ADH secretion (SIADH) & cerebral salt-wasting syndrome based on laboratory parameters:
    Lab Tests Central Neurogenic DI SIADH Cerebral Salt-Wasting Syndrome
    Serum urea nitrogen Increased Normal or low Increased
    Serum sodium High Low Low
    Serum osmolality High Low Low
    Urinary sodium Normal or decreased Normal or increased Increased
    Urinary osmolality Decreased Increased Increased
    Urinary output Increased Decreased Decreased
    Urinary specific gravity Very dilute Dark & concentrated Dark & concentrated
  • Source: John CA, Day MW. Central neurogenic diabetes insipidus, syndrome of inappropriate secretion of antidiuretic hormone, and cerebral salt-wasting syndrome in traumatic brain injury. Crit Care Nurse. 2012 Apr;32(2):e1-7.

Fluid Deprivation Test

Fluid deprivation should not be done if patient has severe fluid loss (eg >4.5 L urine/24 hr): Proceed directly to Desmopressin administration

  • Consists of closely observed 7-hr fluid fast
    • May be shortened to 4 hr for infants
  • Follow w/ administration of Desmopressin
  • Measure urine osmolality after fluid deprivation & after Desmopressin administration

Interpretation:

Urine Osmolality (mmol/kg)After Fluid Deprivation Urine Osmolality (mmol/kg)After Desmopressin Diagnosis
<300 <300 Nephrogenic DI
<300 >750 Central DI
>750 >750 Primary polydipsia
300-750 <750 CDI, NDI or primary polydipsia

Plasma Vasopressin Response to Increased Plasma Osmolality

  • This test is useful when fluid deprivation test is inconclusive
    • Used to confirm central DI
  • Increased plasma osmolality is induced by hypertonic 5% saline infusion given over 2 hr at 0.05 mL/kg/hr
    • Or until plasma osmolality of 300 mmol/kg is achieved
  • Not usually used in children
    • Limited availability of vasopressin assay
    • Irritability to hypertonic saline administration

Measure Vasopressin After Period of Fluid Restriction

  • This test is useful when fluid deprivation test is inconclusive
    • Used to confirm nephrogenic DI
  • Urine osmolality & plasma vasopressin are measured after a period of fluid restriction
  • Not usually used in children
    • Limited availability of vasopressin assay

Trial of Low-Dose Desmopressin

  • This test is useful in younger children & when there is no facility to measure plasma vasopressin
  • Observe patient for ~4 days
    • Measure wt, plasma Na, urine volume & osmolality
  • Then give small doses SC or IM daily doses of Desmopressin x 7-10 days
    • Continue measurements
  • Central DI: Improvement, less thirst, decrease in urine output, Na normalizes; nephrogenic DI: Thirst & polyuria persist; primary polydipsia: Thirst persists & hyponatremia develops
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