Polyuria resident survival guide

Overview

• Polyuria is defined as urine output more than 2 L/24 hours, or 30 ml/kg/24 hours. There are 3 pathophysiologic causes of polyuria: increased thirst (idiopathic, psychogenic polydepsia, hypothalamic disease, and medications), central diabetes insipidus (DI) (decreased secretion of arginine vasopressin (AVP)), and nephrogenic diabetes insipidus (DI) (renal resistance to AVP).

• Nocturnal polyuria (NP), characterized by overproduction of urine at night (more than 20%-33% of total 24-hour urine volume depending on age). It can be caused by intake, urological, nephrological, hormonal, sleep, and cardiovascular factors.

Causes

Common causes

The most common causes of polyuria are: psychogenic polydipsia, diabetes insipidus (central and nephrogenic), chronic kidney disease and uncontrolled diabetes mellitus.

Approach to polyuria

Polyuria ❑ 24-hour urine volume >3L ❑ 24-hour urine volume >50 ml/kg

Urine Osmolality >300mosmol

Urine Osmolality <300mosmol

Solute diuresis ❑ Glucose ❑ Mannitol ❑ Contrast media ❑ High protein intake ❑ Diuretics ❑ Medullary cystic disease ❑ Resolving ATN ❑ Resolving obstruction

Water diuresis ❑ Primary polydipsia ❑ Diabetes inspidous

Water restriction test OR administration of hypertonic saline 0.05 mL/kg/min for 2 h

Water restriction test ❑ Overnight fluid restriction should be avoided ❑ Recommend the patient to stop drinking 2-3 hours before coming to clinic ❑ Meaure urine volume every hour ❑ Measure urine osmolality every hour ❑ Measure plasma sodium concentration every 2 hours ❑ Measure plasma osmolality every 2 hours

Test endpoints in adults: ❑ Urine osmolality reaches normal value (above 600 mosmol/kg) ❑ The urine osmolality is stable for 2 or 3 successive hourly measurements despite a rising plasma osmolality ❑ Plasma osmolality >295-300 mosmol/kg ❑ Plasma sodium is 145 or higher

Treatment

Cranial diabetes insipidus:

• Cranial diabetes insipidus (DI) can be managed by desmopressin orally or Intranasal (rarely used).
• Partial DI can be treated with a single nocturnal dose to prevent sleep loss due to nocturia, but complete DI requires 2-4 daily doses.

Nephrogenic diabetes insipidus:

• Withdrawal of lithium therapy usually leads to reversal of lithium-induced diabetes insipid (DI). It can persist for years after lithium withdrawal, usually indicating that the patient has developed interstitial nephritis secondary to lithium.
• Thiazide diuretics reduce urine output by up to 50%, and indomethacin has also been used. Results are frequently unsatisfactory, treatment is directed at sufficient fluid intake to replace urinary losses.