Definition: Polyuria is defined as urine output >2000ml per 1.73M2 /24 hr or more than 2.5-3 ml/kg/24 hrs. Accurate measurement of 24 hours intake of fluids and quantity of urine passed should be done to establish diagnosis of polyuria.

Common causes of polyuria :

• Polydipsia or excessive fluid intake, which can be psychogenic.
 
• Solute diuresis e.g. glucosuria in diabetes mellitus, calcium in hypercalciuria, sodium chloride in Barter's syndrome and nephronopthisis, sodium bicarbonate in renal tubular acidosis, Fanconi's syndrome or cystinosis.
 
• Vasopressin or ADH deficiency as in central diabetes insipidus due to hypothalamic or pituitary disorders.
 
• Resistance to ADH action at the site of renal tubules as in nephrogenic diabetes insipidus
 
• Chronic renal failure with concentration defect.
 
• Structural malformations of kidney like obstructive uropathy, hydronephrosis, chronic pyelonephritis with reflux nephropathy etc.

Clinically polyuria due to pathological condition can present with irritability, failure to thrive, fever due to dehydration, excessive thirst, mental retardation, and seizures due to hypernatremia. Older children present with nocturnal enuresis or bedwetting.

Diagnostic tests should be done in a stepwise manner and practical approach is as follows:

• Establish presence of polyuria.
                         
 
• Morning sample of urine to be tested for sugar (Diabetes Mellitus), Specific Gravity (less than 1005 - Diabetes insipidus - DI, 1010 - chronic renal failure).
                    

Plasma and urinary osmolality are important investigations to decide further work up

If urine/plasma osmolality is less than 1 -water deprivation test should be done to differentiate polydipsia from diabetes insipidus. If urine/plasma osmolality is more than 1.5 - solute diuresis is considered.

IV Urinary calcium, sodium chloride excretion, BUN, S. creatinine, arterial blood PH and TCo2 or bicarbonate for diagnosis of chronic renal failure and renal tubular acidosis and Fanconi's syndrome.

Water deprivation test :- A 12 hours overnight fast is kept during which water is withheld (8pm - 8am) 8am urine sample is collected for quantity, specific gravity and osmolality. Wt. loss upto 5% may be found. Hourly urine sample is tested for specific gravity & osmolality and serum sodium and plasma osmolality estimated after 2-3 hours.

Interpretation

• If urinary osmolality increases 2 times after water deprivation i.e. >750 mosm/kg- polydipsia
 
• If urinary osmolality <750 mosm/kg - D.I. Pitressin or vasopressin test should be done to differentiate central from nephrogenic DI.
 
• If urine Osmolality increases to 500 mosm/kg and urine/plasma osmolality is or more- polydipsia

Serum sodium levels may reach upto 160-170 meq/l in diabetes insipidus and dehydration may occur, then the test should be stopped.

Vasopressin or pitressin test to differentiate Central from Nephrogenic DI.

Inj Pitressin in oil 10mcg for infants and 20 mcg for children is given subcutaneously or by nasal insufflation.

In central DI - urinary osmolality and specific gravity increases within 2-4 hours and urine volume falls.

In nephrogenic DI - urinary osmolality, specific gravity and output does not show dramatic improvement.

Special imaging studies

• CT/MRI of brain for signal in post pituitary, to detect cysts, tumors, hydrocephalus craniopharyngiomas, histiocytosis as secondary causes of central DI.
 
• Ultrasonography of KUB with MCU for diagnosis of obstructive uropathy, reflux nephropathy, hydronephrosis, cystic kidney disease etc.
 
• Genetic studies for diagnosis of inherited types of nephrogenic DI, distal RTA, nephronophthisis, cystinosis etc.

Last created on 06-03-2002
Last updated on 01-07-2006